UNIVERSITY OF NAIROBI
DEPARTMENT OF MECHANICAL
ENGINEERING
FINAL YEAR PROJECT SUBMITTED IN PARTIAL
FULFILMENT
TITLE: STUDY OF WOOD ASH GLAZES IN
KENYA
AUTHORS: KIPLAGAT HENRY F18/1818/2006
KAMIRA NYUTU F18/1826/2006
GITONGA ANTONY F18/1833/2006
SUPERVISOR: DR. KAMAU GACHIGI
i
DECLARATION
This work is original to the best of our knowledge and has not been presented for
examination or any other purpose to this or any other university. This report is
based on our own results findings and is our original work in this study.
Signed ……………………..
Date……………………………..
KIPLAGAT HENRY
Signed ……………………
Date……………………………..
KAMIRA JOSEPH NYUTU
Signed…………………….
Date……………………………….
GITONGA ANTONY NDIRANGU
This work has been presented for examination under the supervision as University
Supervisor
Signed ……………………….
Date……………………………
Dr. KAMAU GACHIGI
ii
DEDICATION
This project is a dedication to our parents, thank you for supporting and believing
in us. To our siblings and friends, for your undying encouragement and motivation.
To all those who made it possible for us to go through the education system, for
your guidance and inspiration. God bless you all.
iii
ACKNOWLEDGEMENTS
“I can do all things through Christ who strengthens me” Philippians 4:13. We are
grateful to the Lord Almighty for making this possible. You have been with us
through the journey and for that we are truly grateful.
We would also like to express our gratitude to the following people for their
invaluable guidance, assistance and inspiration in the course of this project;
1. Dr. Kamau Gachigi, (Mechanical Engineering Department, University of
Nairobi) our project supervisor for his guidance and inspiration while
overseeing the progress of our project.
2. Mr. Njue, Mr. Kimani, Mr. Kogi and the mechanical engineering technical
staff for their assistance in the workshop.
3. Mr. Mutua and Mr. Chumo of the Department of Geology and Minerals
(Madini), for their immense assistance while running tests on samples in
their laboratories.
4. Mr. Rono (Design Department, University of Nairobi) for his assistance in
preparation of samples.
5. Mrs. Nancy Muthuli and Mr. Wanjalla of the physics department for their
assistance in the science workshop, Chiromo.
iv
ABSTRACT
This project was a study on wood ash glazes. Specific types of wood used in this
study were cypress, bamboo, blue gum and pine. The preparation of ash and firing
of the ash to glaze were discussed. Chemical composition of the ash and feldspar
was done to establish the elements present and analyzing the glaze basing on the
firing process and elements in the sample.
Elemental analysis on the wood ash and feldspar was done using XRF and AAS
tests. This provided information on the elements present in samples and a guide to
possible melting temperatures of the ash on firing. The elements gave a prediction
of possible glaze features like color and texture. Individual elements in the ash
imparted different properties on the glazes.
The two methods employed in firing the wood ash were electric furnace and
oxyacetylene gas firing. Electric furnace firing was done at 1160˚C when firing the
mixture of ash and feldspar and 1100˚C when firing the ash alone. However the
ash did not mature to glaze at these temperatures. Oxy acetylene flame was
adjusted to give oxidizing flame above 1400˚C which fired the mixture of ash and
feldspar to glaze.
On analyzing the glazes Blue gum gave a glazes white in color with specs of green
and black regions, Cypress were observed to have shades green in color not evenly
distributed which is attributed to uneven mixing of the ash and powdered feldspar,
matured Bamboo glaze had the major color as white with very faint shade of bluegreen, Pine glaze obtained were white in color and glossy in appearance, Feldspar
glaze was white in color and glossy in appearance.
v
LIST OF FIGURES
Fig 2.2 (a) Percentage of the total forested area in Kenya.
Fig 3.1.1 Open air kiln
Fig 3.1.2 Electric furnace
Fig 3.1.4 Atomic absorption spectrometer
Fig 3.1.5 Crucibles
Fig 3.1.6 Density balance
Fig 4.1 (a) XRF results for wood ash
Fig 4.1 (b) XRF results for the soil samples
vi
LIST OF TABLES
Table 2.1.1 Chemical composition of various ashes
Table 4.2 (a) Results from AAS test
Table 4.2 (b) Table of conversion factors
Table 4.2 (c) Table of quantity of metallic elements
vii
LIST OF GRAPHS
Graph 4.1.1 (a) Quantity of oxides in blue gum
Graph 4.1.1 (b) Quantity of oxides in cypress
Graph 4.1.1 (c) Quantity of oxides in pine
Graph 4.1.1 (d) Quantity of oxides in Bamboo
Graph 4.2.2 (a) Quantity of metallic elements in Blue gum
Graph 4.2.2 (b) Quantity of metallic elements in Cypress
Graph 4.2.2 (c) Quantity of metallic elements in Pine
Graph 4.2.2(d) Quantity of metallic elements in Bamboo
Graph 4.3 (a) Bamboo soil sample
Graph 4.3 (b) Blue gum soil sample
viii
ABBREVIATIONS
Mn
-Manganese
MnO
-Mangangese Oxide
BaO
-Barium Oxide
Si
-Silicon
SiO2
-Silicon dioxide
Al
-Aluminum
Al2O3
-Aluminium Oxide
Ca
-Calcium
CaO
-Calcium oxide
Mg
-Magnesium
MgO
-Magnesium oxide
Na
-Sodium
Na2O
-Sodium oxide
K
-Potassium
K2O
-Potassium Oxide
Ti
-Titanium
TiO2
-Titanium dioxide
Fe
-Iron
Fe203
-Iron Oxide
Nm
-Nanometer
ix
Ma
-Miliampere
HF
-Hydrofluoric acid
HCl
-Hydrochloric acid
Gm
-Grammes
AAS
-Atomic Absorption Spectroscopy
XRF
-X-ray fluorescence
KeV
-Kilo electron volts
KV
-Kilo volts
LED
-Light emitting diodes
Kw
-Kilowatts
AC
-Alternating Current
W
-Watt
M
-Meter
K
-Kelvin
Kg
-Kilogram
KJ
-Kilojoule
P2O5
- Phosphorous pentoxide
ZnO
-Zinc Oxide
XRD
- X-ray diffraction
XRF
-X-ray fluorescence
AAS
- Atomic absorption spectroscopy
x
Table of Contents
DECLARATION ................................................................................................................................................ i
DEDICATION ................................................................................................................................................. iii
ACKNOWLEDGEMENTS ................................................................................................................................ iv
ABSTRACT...................................................................................................................................................... v
LIST OF FIGURES ........................................................................................................................................... vi
LIST OF TABLES ............................................................................................................................................ vii
LIST OF GRAPHS ......................................................................................................................................... viii
ABBREVIATIONS ........................................................................................................................................... ix
1.0
1.1
INTRODUCTION ............................................................................................................................. 1
OBJECTIVES ............................................................................................................................... 5
2.0. LITERATURE REVIEW .......................................................................................................................... 6
2.1 ASH .................................................................................................................................................. 6
2.2 WOOD IN KENYA [3] ....................................................................................................................... 7
2.3 CHEMISTRY OF WOOD [4] ............................................................................................................... 8
2.4 WOOD ASH GLAZES ......................................................................................................................... 9
2.5 CLASSIFICATION OF GLAZES ........................................................................................................... 9
2.4
PROPERTIES OF SOME OXIDES IN WOOD ASH GLAZES [5] ........................................................ 12
2.5 WOOD ASH GLAZE PREPARATION................................................................................................ 17
3.0 EXPERIMENTAL SET UP ..................................................................................................................... 18
3.1 APPARATUS ................................................................................................................................... 18
3.2 EXPERIMENTAL PROCEDURE......................................................................................................... 25
3.3 FIRING THE ASH TO GLAZE ............................................................................................................ 29
4.0 RESULTS AND ANALYSIS .................................................................................................................... 33
4.1
5.0
5.1
TABLE OF XRF RESULTS ........................................................................................................... 33
DISCUSSION................................................................................................................................. 42
ANALYSIS OF ELEMENTS PRESENT IN SAMPLES ...................................................................... 43
6.0 CONCLUSION ..................................................................................................................................... 46
REFERENCES ................................................................................................................................................ 48
xi
1.0
INTRODUCTION
Glaze is a layer or coating of a vitreous substance which has been fired to fuse to a
ceramic object to color, strengthen, decorate or waterproof it.
Glazes can be divided and sub divided into many groups. The principal way of
defining a glaze type is by the temperature of firing, i.e. Earthenware (9801100oC), Mid temperature or High-firing Earthenware (1100-1200oC) and
Stoneware (1200-1300oC).It is also possible to take a glaze which would fit in to
any of the above categories and define them by their appearance, i.e. Transparent,
Opaque, Matt, Semi-matt, Shiny etc.
Sometimes a glaze will be defined by a material used in the recipe which gives a
particular effect. A dolomite glaze will always be satin matt stoneware, and glazes
which use prefixes such as Tin, Zircon and Titanium will be white opaque glazes.
Practically all matt glazes are opaque. They can also be characterized by their
surface quality, groups of glazes such as ash glazes, and specialty glazes such as
crystalline glaze. The glaze has glasslike and pooling (the builds up of glaze)
characteristics which put emphasis on the surface texture of the piece being glazed.
Ash glaze, as the name suggests, is a glaze derived from ashes. The use of ash as a
glaze ingredient has long historical antecedents; it has been an integral part of the
evolution of pottery making in China, and from there played a similar role in the
development of high fired pottery in Japan and through other places in Asia.
As a glaze ingredient ash has the value of bringing a complex array of minerals to
the mix in a very simple manner. It is primarily a source of calcium and silica,
1
providing flux and glass matrix to the melt that forms our glazes, as well as
variable amounts of alkalis (potash, K2O and soda Na2O), magnesium and other
minerals such as iron and manganese which contribute to the eventual fired color
of the glaze. When the glaze is mostly made up of ash, the final result is mostly
dark brown to green. The pots with these glazes resemble the earth in color and
texture. As the ash percentage decreases, the artist has more control on the color
and the final glaze color differs from light to dark shades of brown or green.
The earliest such ash glazes can be traced back to the Shang period in China (c.
1500 B.C.), and it is thought they were produced accidentally, the result of whitehot wood ash being carried through the kiln with the draft of the fire and settling
onto the pots, where the searing white heat melted it to a glass .It was probably
before the end of Shang period and certainly during the Zhou dynasty(1066221BC)that pots were produced that carried a true glaze, one that had been
deliberately applied prior to the pot being placed into the kiln. These earliest of
glazes were probably mixtures of wood ash and clay.
It wasn’t until the development of Yue kilns during the eastern Han period that the
glaze technology took further step forward and the potters began to better
understand the contributions of the various ingredient materials to the glaze melt.
The result was glazes that were better balanced in their oxides and were therefore
less prone to running, resulting in a thicker and more uniform covering. It is
possible that introduction of a siliceous stone into the clay and ash or clay, ash and
limestone mixture contributed to this new superior finish. The stone, probably
feldspar was high in silica but contained some of the fluxing oxides within itself.
During the 8th and 9th centuries the ash glaze was placed over an iron-bearing slip
and then oxidized to produce a straw-colored finish reminiscent of lower fired lead
2
glaze. Wood ash remained an important glaze flux for many centuries, certainly
until the beginning of the 17th century, but as time went by the glazes showed less
and less evidence of an ash content.
Almost three thousand years later, wood ash remains an important and immensely
popular feature of pottery glaze making. For the modern potter, the satisfaction of
working with ash glazes comes from following an ancient tradition as well as from
using materials that occur naturally, hence the ash is mainly mixed with other
materials giving the different ash glazes recipes. Ash glaze recipes can be as
simple as classic ash(half ash and half clay),or basic ash(33% each of ash, feldspar
and clay) .Traditional ash glazes look good on stoneware since they benefit from
the iron oxide in the clay body. Fake ash glazes and ash glazes with colorants other
than iron oxide benefit from the brightness of a porcelain clay body. Fake ash
glazes are those that contain large amounts of calcium oxides from sources other
than wood ash and they are indistinguishable from wood ash glazes and they are
sometimes preferred because whiting is more stable than wood ash [6].
The results of different wood ashes often vary dramatically, making it possible to
achieve a wide range of unique finishes. Even wood from the same species of tree
garnered just miles apart can produce subtly different results, since different trees
draw up different minerals from the soil. Iron, phosphate, calcium, silica and many
other elements are drawn into the tree throughout its life, surviving in the ash when
it is burnt. These differences are reflected in the characteristics of each glaze. Used
without an under-glaze, the differences between the glazes from different trees is
significant, but not great. The color of a natural ash glaze varies with the type of
wood used. Tree bark is known to produce particularly nice colors because of the
high mineral content of the bark.
3
Ash glazes are prepared by first sifting the ash. The ash is then washed if necessary
to remove the soluble alkalis which can be used in manufacture of soaps. It is then
sprayed on the earthenware before being fired into a glaze in a kiln.
Though ash glazes deliver superior tone and high quality texture, the ash is a
hazardous material to work with due to its caustic nature; corrosive to skin and
lung tissue [5]
Currently, ash glazes are usually used by artists as a decorative tool, but some still
use ash glaze dish-ware and in tile making .In Korea, the traditional ash glaze
composed of only ash and water is used to make functional pottery such as bowls,
cups and teapots. Koreans believe using ash glazed dishware is safer than using
plastic ware because of its lack of carcinogens, cancer causing materials.
This project is an attempt to prepare wood ash glazes from different trees namely;
bamboo, cypress, blue gum and pine ,combine them with feldspar while properly
establishing the elements present in the particular wood-feldspar mix and their
influence on the appearance of the fired glaze.
4
1.1
OBJECTIVES
The overall objective of this project was to prepare wood ash glazes from locally
available wood species. The specific objectives are as follows:
1. To prepare a wood ash glaze from bamboo, cypress, pine and blue gum.
2. To analyze the chemical composition of the different wood ash by
establishing the amount of each element in percentage
3. To analyze the chemical composition of potassium feldspar
4. To note the effects of the elements in the wood ash and potassium feldspar
and their interaction on the color of the fired glaze.
5
2.0. LITERATURE REVIEW
2.1 ASH
Ash is the powdery residue obtained after combustion of an object. Ash is formed
from mineral matter during combustion and gasification. During combustion, the
mineral ions oxidize and volatilize or form particulates. The carbon surface is
hotter than the gas or the interior of the particle, and the ash particles tend to form
on the carbon surface. As the carbon burns away, the ash particles are released. [1]
2.1.1 COMPOSITION OF ASH
Ash from various sources varies in composition depending on the actual
composition of the materials it was obtained from. However, ash can be said to be
composed of organic elements and mineral elements. The mineral elements occur
mostly in their oxide form due to the process of combustion. The most common
oxides found in ash from various sources are silica (SiO2), alumina (Al2O3),
calcium oxide (CaO), magnesium oxide (MgO) and manganese oxide (MnO). An
example of the chemical composition of ash is as follows.
SiO2
Al2O3 P2O5
Fe2O3
MnO
CaO
MgO
K2O
Na2O
Ignition loss
Wood ash
14.08 3.69
2.14
1.94
0.41
35.90
5.44
1.49
0.55
34.32
Isu ash
26.98 2.77
0.81
0.77
0.035
35.78
1.63
0.75
0.57
29.94
Straw ash
50.94 0.70
0.70
0.52
-
2.30
0.29
1.99
0.61
42.60
Fig 2.1.1 Chemical composition of various ashes. [2]
6
2.2 WOOD IN KENYA [3]
Kenya is lightly forested with around 1.7% of forest cover, but with an
additional 27% of other wooded land cover. The majority of closed forests
are upland broad leaved forests of either semi-deciduous or evergreen type.
The largest areas of upland forests occur on the main mountains, Mt Kenya,
Mt. Elgon, and the Aberdare range, and are generally dominated by Ocotea
spp. (camphor trees). Around 6% of Kenya’s forests are protected in the
country’s system of more than 20 national parks, sanctuaries and reserves.
The forests mainly act as the source of raw material used in wood and wood
product industries all over the country. Wood is an important fuels source in
Kenya. Generally more than 75% of the country’s domestic energy comes
from fuel wood and charcoal.
% of Total Forested Areas in Kenya
Coast , 22%
Rift Valley , 48%
Rift Valley
Western
Others
Eastern , 13%
Central
Eastern
Coast
Central , 12%
Others, 1%
Western , 4%
Fig 2.2 (a) Percentage
of the total forested area in Kenya [3]
7
2.3 CHEMISTRY OF WOOD [4]
Physical property values vary greatly and properties such as density,
porosity, and internal surface area are related to wood species whereas bulk
density, particle size and shape distribution are related to fuel preparation
methods. Density of dry wood and bark varies from 300 to 550 kg/m3.
Thermal property values such as specific heat, thermal conductivity, and
emissivity vary with moisture content, temperature, and degree of thermal
degradation but one order of magnitude. The carbon content of wood varies
from about 47 to 57 % due to varying lignin and extractives content. Mineral
content of wood is less than 1% but it can be over 10 times that value in
bark. The composition of mineral matter can vary between and within each
tree. Properties that need further investigation are the temperature
dependency of thermal conductivity and the thermal emissivity of ash.
The specific heat of wood charcoal can reasonably be assumed to be the
same as that of graphite. The specific heat of graphite varies from 0.715
kJ/kgK-1at 300K and 2.04 kJ/kgK-1 at 2000K.
Thermal conductivity of wood also increases with density, moisture content,
and temperature. Also, the thermal conductivity is approximately 1.8 times
greater parallel to the grain than in either the radial or tangential directions.
For a wide variety of wood species at room temperature, the average thermal
conductivity perpendicular to the grain is given by
k= S (0.1941 +0.4064M) +0.0184 (Wm-1K-1)
Where S is the specific gravity based on volume at the current moisture
content and weight when oven dry. The thermal conductivity of wood also
increases with temperature at the rate of approximately 0.2% per ˚C above
room temperature. Hence, thermal conductivity increases 10% for every 50K
increase in temperature.
8
2.4 WOOD ASH GLAZES
Wood ash glazes are glazes whose main recipe is ash from wood and products of
wood like saw dust. Discovery of the glazes made of the ashes of grass and wood
was the foundation of the ceramic development in East Asia, and the ash has been
the most important one among all the glaze raw materials. Wood ash glazes have a
superior tone and texture than from glazes made from other substitutes of ash for
example lime or talc. [2]
2.5 CLASSIFICATION OF GLAZES
Glazes can be categorized into various groups. However, the various categories are
closely inter-related to each other. Some of the ways to classify are:a) Temperature [8]
This is a classification of glazes according to the firing temperature
Low - fire: The low-fire range has historically been the most
commonly used firing range. This is because the elements that
produce color in a glaze can be nurtured at these temperatures,
which otherwise they would burn off or become unstable at
higher temperatures. The disadvantage of this type of glaze is
that glaze colors can appear harsh and raw-looking.
Mid- temperature: in this category, the glazes are fired at
relatively high temperatures of between 1165 to 1210 ˚C. This
range is popular with manufacturers of glazes due to the
availability of electric kilns that can comfortably reach this
range without severely decreasing the kiln’s and the kiln
elements’ lifespan.
9
Other advantages to firing in the mid-temperature range
include
- Glazes in this range are more durable than those fired at
lower temperatures.
- There is still a fairly extensive color range available.
High fire: Glazes in this category are fired at high temperatures
of 1260 to 1390 ˚C. This range includes the stoneware and
porcelains. Glazes fired at these temperatures are dense and
durable. However the color range is limited. Because of the
varying effects of oxidation and reduction on glaze colorants,
the few coloring oxides that are viable at this range can still
produce a rich, but much more limited variation.
b) Appearance
Glazes can also be classified according to their outward observable
appearance. Some of the expected variations in the appearance are:Transparency - this is the property of a glaze that allows it to
transmit light over the surface.
Opaque – this refers to the transparency of the glaze. Some of
the categories of opacity of a glaze are clear, transparent, semitransparent, semi-opaque and opaque.
Matte – in matte glazes, the surface of the glaze has
protruding crystals which are larger than 390 nm such that the
light’s wavelength is smaller than the protruding crystals. This
therefore causes the protruding crystals to scatter the light in
many or in all directions. Matte glazes are smooth to the
10
touch since the reduced contact area caused by the tiny,
regular bumps diminishes the friction forces between the
glaze surface and the point of contact. The most common
origin of matte glazes is the formation of crystals within the
glaze during the cooling phase after firing. Oxides commonly
employed to create matte glazes are MgO, CaO, BaO, Al2O3,
TiO2, ZnO and MnO. [7]
Color – glaze color is a combination of chemistry between
colorant and host, firing process and the amount and nature of
the colorant. It is possible to achieve very similar colors using
very different systems and very different colors by making
very small changes in a system.
Glossy – a glossy glaze is that which reflects light in a
coherent, mirror-like fashion so that you can see reflected
images. Glossy glazes are very smooth, smooth on the scale of
the wavelength of visible light (390nm – 750nm). Thus any
bumps, pits, or undulations on the glaze surface are smaller
than approximately 390 nm, so as far as the light is concerned
the surface is perfectly smooth. [5]
c) Material used in recipe [2]
Since various materials are used in the preparation of glazes, the
glazes can therefore be classified according to the major material
used in the recipe. These are:Wood ash glazes
Lime – zinc glaze
Synthetic ash glaze
11
2.4 PROPERTIES OF SOME OXIDES IN WOOD ASH GLAZES [5]
a) Silicon Dioxide, Silica (SiO2)
It normally forms the principle and often the only glass forming oxide in a glaze.
Lack of this oxide compromises structural stability and strength in the glaze.
Decreasing silica increases the melt fluidity and increased quantities of silica raises
the melting temperature, lowers expansion, increases hardness and gloss.
b) Aluminum Oxide, Alumina (Al2O3)
Alumina has a very high melting temperature and alumina ceramics can maintain
up to 90% of their strength above 1000 ˚C. Alumina controls the flow of the glaze
melt, preventing it from running off the ware. It is thus called an intermediate
oxide because it helps build strong chemical links between fluxes and silica. It is
second in importance to silica and combines with silica and basic fluxing oxides to
prevent crystallization and give body and chemical stability to a glaze.
It is the prime source of durability in glazes, which it achieves by improving the
tensile strength, lowering expansion, adding hardness and resistance to chemical
attack. Increased amounts of alumina prevent crystallization of glazes during
cooling because the stiffer melt resists free movement of molecules to form
crystalline structures.
Alumina is used in combination with chrome, manganese, and cobalt to achieve
pink colors.
c) Calcium Oxide, (CaO)
Calcium oxide is the principle flux in medium and high temperature glazes,
beginning its action around 1100 ˚C. Calcium oxide usually hardens a glaze and
12
makes it more scratch resistant and acid resistant. Its expansion is intermediate. At
higher temperatures, CaO contributed by wollastonite is more readily fusible than
that contributed by calcium carbonate.
Hardness, stability, and expansion of silica are improved with increased quantity of
CaO present.
CaO is not effective below cone 4 as a flux in glazes but in small amounts (less
than 10%) it can dissolve in earthenware glaze melts to add hardness and resistance
to leaching. In larger amounts, it increases the growth of crystals which can give
decorative effects to glossy glazes and produce matteness (30%). Glazes with high
quantity of CaO tend to crystallize. This occurs because of the high melt fluidity
imparted by calcium oxide at higher temperatures or also due to the readiness with
which calcium oxide forms crystals.
In oxidation, and with presence of Fe2O3, calcium oxide forms yellow crystalline
compounds. To form a glossy brown or black glaze, the glaze should be low in
CaO.
d) Magnesium Oxide, Magnesia (MgO)
Magnesium oxide has a cubic crystal structure, and like calcium oxide, MgO is
refractory at lower temperatures. Due to this, it can be used to increase opacity, act
as a matting agent, and to check glaze fluidity by preventing the tendency to
produce crystalline surfaces.
In high temperatures glazes it acts as a flux beginning action about 1170 ˚C
producing viscous melts of high surface tension and opaque and matte glazes. It
melting action drastically accelerates at high temperatures.
13
e) Manganese Oxide, (MnO)
Manganese monoxide exists only above 1080 ˚C where the dioxide form
disassociates to release its oxygen. Manganese is a colorant used in bodies and
glazes, producing glazes with black, brown and purple colors. Manganese is a
constituent in many igneous rocks, and thus occurs in much clay weathered from
these parent rocks. Smaller amounts are easily dissolved in most glaze melts,
however around the 5% threshold; the manganese will precipitate and crystallize.
In large amounts in a glaze (i.e. 20%), metallic surfaces are likely.
Above 1080 ˚C, half of the oxygen disassociates to produce MnO, a flux which
immediately reacts with silica to produce violet colors in the absence of alumina,
and brown colors in the presence of alumina. Manganese oxide is unaffected by
reduction. Manganese fuses and dissolves very well above 1200 ˚C in oxidation.
This means that if more than about 4% MnO is used, the oversupply will
precipitate on cooling leaving a network of crystals in a manner similar to iron in
high fire reduction. Speed of cooling, glaze fluidity, and amount of manganese will
all affect the results.
f) Sodium Oxide, Soda (Na2O)
Sodium oxide is a slightly more powerful flux than potassium. Sodium, potassium
and lithium all belong to the alkaline group in the periodic table. Sodium is a
useful flux over the entire temperature range from 900 to 1300 ˚C and its activity
increases further at higher temperature ranges. Sodium can begin to volatilize at
high temperatures.
Soda has a higher expansion than any other oxide and will promote crazing in
glazes lacking silica or alumina. Also it decreases tensile strength and elasticity
14
compared to other common bases. In addition, high soda glazes can often be
soluble and easily scratched.
Sodium Oxide gives strong color responses to copper, cobalt and iron elements
present in a glaze. Oxidation copper blues work best in high alkaline, low alumina
glazes. Increases in copper (4 – 6%) will move color toward turquoise.
g) Potassium Oxide (K2O)
Potassium oxide has a melting point of 707 ˚C and belongs to the alkaline group.
K2O promotes higher melt viscosity than Na2O. It is an important auxiliary flux in
high temperature glazes, and is considered a very stable and predictable oxide. Has
a relatively high expansion tends to contribute to crazing in higher amounts.
Alkaline dominant glazes will produce violet, purple, burgundy, red blue using
manganese dioxide to 2%.
h) Titanium Dioxide (TiO2)
Titanium dioxide also referred to as titania is a complex material because it
opacifies, variegates, and crystallizes glazes. It also modifies existing colors from
metals like Chromium, Manganese, Iron, Cobalt, Nickel and Copper. In amounts
below 1% titanium dioxide can dissolve completely in a glaze melt. In slightly
greater amounts it can give a bluish – white flush to transparent glazes (depending
on their amount of alumina).
Above 2% it begins to significantly alter the glaze surface and light reflectance
properties through the creation of minute crystals. This crystal mechanism gives
soft colors and pleasant opacity, and breaks up and mottles the surface. In the 2 –
6% range, it increasingly variegates the glaze surface. Large amounts (10 – 15%)
will tend to produce an opaque and matte surface if the glaze is not over fired.
15
They will also subdue color and can add sparkle to the surface. Although titania
will form a glass by itself, it is not highly soluble in silica melts, and it can act as a
modifier and within a narrow range it will combine with fluxes to make a glaze.
Minute amounts (i.e. 0.1%) can be used to intensify and stabilize colors. It can
alter and intensify existing color and opacity in a glaze. Glazes containing titanium
dioxide are phototropic and can change color slightly by the action of light. They
can also be thermotropic, in that they can change color when heated.
A phase diagram of alumina and titanium dioxide shows a eutectic at 80% Al2O3 at
1705 ˚C.
i) Phosphorus Pentoxide (P2O5)
Phosphorous pentoxide sublimates at 300 ˚C. It can act as a melt agent in middle to
high fire, but its power per unit added drops drastically beyond 5% additions. P 2O5
is a glass network former like silicon dioxide. Phosphoric glass tends to show as
bluish flush in glazes. It is not in any way a substitute for silica and does not enter
the silica chain, but remains as a separate colloidal presence in the silicate matrix.
P2O5 is known to influence the rate of nucleation and / or crystallization in lithium
dioxide and magnesium oxide low expansion glaze systems. It also combines with
certain oxides of iron to form colorless compounds, which suggests that P2O5 could
be used to allow the use of less pure materials in glazes and glass.
j) Iron (III) Oxide (Fe2O3)
Iron (iii) oxide has a co-efficient of linear expansion of 0.125 and a melting point
of 1350˚C. Iron compounds are the most common coloring agents in glazes. Iron
exhibits different characteristics with different kiln atmospheres, temperatures, and
16
firing cycles and with different glaze chemistries that it is among the most exciting
of all materials.
Chemically, iron is amphoteric (can react with acid and base) like alumina, but
generally Fe2O3 generally behaves as a refractory antiflux material in a glaze melt,
combining with alkalis. Oxidation iron-red glazes, for example, can have very low
alumina contents yet do not run off ware because the iron acts like alumina to
stabilize and stiffen the melt.
Fe2O3 is affected by a reducing atmosphere where it can act as a flux in both bodies
and glazes at high temperatures. Higher amounts of iron exhibit dramatically
increased fluidity.
Fe2O3 is the most natural state of iron oxide where it is combined with the
maximum amount of oxygen. In oxidation firing it remains in this form to typically
produce amber to yellow up to 4% in glazes.
2.5 WOOD ASH GLAZE PREPARATION
Properties and compositions of ashes from wood differ according to the kinds of
trees, and also the same kind of a tree gives different ash according to the location
it grows. Wood ash consists of lime mainly with some amount of silica, alumina,
magnesia, iron, manganese and phosphate. Glazes are prepared by mixing wood
ash with other materials like feldspar, kaolin or ferruginous clay. The practical
method of ash glaze preparation is where the wood ash and powdered feldspathic
stone are made and held separately, and later mixed in measurable proportions.
The amount of either component is varied so as to control the melting point of the
glazes. The higher the quantity of wood ash in the mixture, the lower the melting
point and there is also an increase brightness and transparency progressively.
17
3.0 EXPERIMENTAL SET UP
In this experiment, waste wood samples were used for production of ash glazes by
burning the wood to get ash which is then fired at high temperatures to melt and
form glaze. Various instruments were used at different stages of the experiment in
collecting data on elemental content of the ash and feldspar and in creating the
desired product (glaze).
3.1 APPARATUS
3.1.1 Wood burning kiln
This is an open air kiln constructed with bricks layered to provide spaces on the
walls. This is to ensure the kiln is well aerated for sufficient circulation of oxygen
needed for combustion.
The roof of the kiln has a long chimney to take away smoke making the
environment free of choking gases. The floor has a removable metal sheet to
handle the wood and the ash.
The wood samples were collected and cut into small pieces and piled on the metal
sheet the set on fire. After 30 to 50 minutes of burning the wood completely
reduced to ash which is collected ready to be used to make ash glazes.
18
Fig 3.1.1open air kiln
3.1.2 Electric furnace
Made in West Germany in 1991, the furnace type is Norbtherm AC electric
furnace, model N41/4. It is rated; 280V, 50 HZ, 23A, 15KW and it can attain a
maximum temperature 1280 ◦C in 2 hours.
The furnace is controlled by a precision microprocessor control program controller
C19, which does the job of measuring the temperature inside the furnace with help
of a digital microprocessor based thermocouple, displaying the temperatures and
plotting a graph of temperature against time. When set temperatures are attained
the program shuts down power supply to the heating elements.
The heating element is made of Nichrome 80% Nickel 20% which has high
resistance and forms a thin adherent coating of chromium oxide on heating for the
19
first time. This protects the wire beneath from oxidation breaking down or burning
out at high temperatures.
When electric current passes through the wire resistance to current flow generates
heat by Joule heating which is spread inside the furnace by radiation and
convection. The heating element is supported with rugged ceramic plates on the
side walls and the top of the furnace to provide excellent temperature uniformity in
the furnace.
It is constructed with high temperature ceramic fiber and lined with bricks on the
base to insulate it and prevent heat from reaching the furnace casing. It has a
resilient door gasket to contain atmosphere. Other safety instruments used in
handling the furnace include tongs crucibles gloves and fire proof jacket.
Fig.3.1.2 Electric furnace
20
3.1.3 XRF- X Ray Fluorescence
The XRF type is Philips XRF Minipal 2. It has specification 25 kV; 0.001Ma uses
Acetylene, Nitrous Oxide and air. It works on the principle that when a material is
exposed to short wavelength X-rays ionization of the component atom takes place.
Consisting of ejection of one or more electrons from the atom when the atom is
exposed to radiation of higher energy than their ionization potential.
When energetic enough it can expel tightly held electrons from inner orbitals. This
renders the electron structure unstable. Electrons from higher orbitals fall into
lower orbital releasing energy in form of photons. Thus material emits radiation
which is characteristic of the atoms present. When the sample is put into the
machine and X-rays are passed through it the emitted radiations are detected and
displayed on computer as percentage composition or plotted as graph of
wavelength versus count shows the abundance of the available elements. It’s an
NDT method as the sample is not destroyed. Unlike XRD method it does not detect
trace elements.
Fig.3.1.3 X-Ray fluorescence
21
3.1.4 Atomic Absorption Spectrometer –AAS
This is a machine that uses spectro-analytical procedure for quantitative and
qualitative determination of chemical elements or oxides by employing absorption
of optical radiation by free atoms in the gaseous state .It requires the preparation of
a standard sample by digesting the sample for 8 hours using Hydrofluoric acid
(HF), boric acid and Aquaragia. It uses flame atomization, where the flame is
principally the air - acetylene flame with a temperature of about 2300˚C and a
nitrous oxide (N2O)-Acetylene flame with a temperature of about 2700˚C; the
latter flame in addition provides a more reducing environment, being ideally
suitable for analytes with high affinity for oxygen.
Dissolved samples are used with flame atomizers. The sample is aspirated by a
pneumatic nebulizer, and then introduced into chamber where it is mixed with the
flame gases, radiation of specific wavelength passes through the analyte and
energy is absorbed by excited atoms, a ratio of energy detected during absorption
to energy when there is no absorption gives the concentration of the element in the
sample.
22
Fig.3.1.4 Atomic absorption spectrometer
3.1.5 Crucibles
The crucibles were also used to melt the ash, mixture of ash and feldspar and pure
feldspar in the furnace.
Fig 3.1.5.crucibles
23
3.1.6. Density balance
This pan has precision of 1000th a gram and it was used to weigh 0.1gm of the ash
to undergo the AAS test and also the mixing of ash with feldspar. The ash was.
Fig 3.1.6 Density balance
24
3.2 EXPERIMENTAL PROCEDURE
The various parameters that lead to the production of wood ash glazes were;
(1) Burning the wood samples to ash.
(2) Chemical analysis of the ash and soil sample.
(3) Firing the ash to glaze.
(4) Analysis of properties of the glaze.
3.2.1 COLLECTION AND BURNING THE WOOD TO ASH
Four samples were chosen for this project based on the availability and cost of
individual wood, these are; pine (pinus insularis), blue gum (Eucalyptus deglupta),
cypress (cupressus sempervirens) and bamboo (Bambusa vulgaris).
The parts selected were the stem for bamboo and waste wood for pine, cypress,
and blue gum.
Soil samples of the area where bamboo and blue gum was grown, were collected
for examination to establish the relationship between the soil mineral content and
ash elemental content.
The wood was gathered cut into small pieces and heaped in metal sheet then
inserted in a kiln, and then fire was lit to consume the wood oxidizing carbon and
making the ash. Care was taken so as to ensure no contamination of the ash with
any material (dirt, gravel) or mix different types of wood ash.
25
The ash obtained from the open air kiln still had some carbon so they were finally
ashed using an electric furnace to completely burn out carbon particles. Samples of
the ash obtained were weighed and put in jars.
3.2.2 CHEMICAL ANALYSIS OF THE ASH AND SOIL SAMPLES
The goal of chemical analysis was to obtain the elements present in the ash which
would give a guide on possible melting temperatures of the glaze, and observable
properties of the glaze for example color, texture, gloss and opacity .
Likewise soil sample was also tested to establish its chemical content, so as to
check if there is a relationship between the minerals in the ground to the elements
present in the ash of the wood samples.
3.2.2.1 WOOD ASH ANALYSIS
Two methods used to test the ash were X-Ray fluorescence (XRF) and Atomic
Absorption Spectroscopy (AAS).
(a) X-Ray fluorescence
This instrument was used to test the elements present and how abundant they
are occurred . It has a capacity of testing 12 samples at the same time. Samples
to be tested were put in small plastic containers and tighly sealed.The bottom of
these containers was made of transparent polyethene paper.X-rays were passed
through each sample with the machine set at 130kV and 12mA. On passing the
rays through the samples, energy was released in pulses with varying
strengths(count data) in form of a spectrum .The energies of x- rays are
characteristic of the element and the number is proportional to the abundance of
the element. The spectrum is used to interpret the elements present using a
26
graph of counts versus Energy (KeV) which shows the available elements
available at peaks of the graph.
The XRF was controlled by a computer programme and results were displayed
on the screen and later obtained in print form.
(b) Atomic absorption spectroscopy
This method used AAS machine designed to determine the amount
(concentration)of element in a sample.
It utilises the phenomena that atoms in the ground state absorb light of
characteristic wavelength passing through an atomic vapor layer of the element.
It consist of a light source , sample atomiser, a spectroscope, and a recording
system.
The standard solution was prepared in the following procedure:
(i) Digestion of sample (full assey)
-0.1000gm of each sample was accurately measured and transferred to plastic
bottles.
-1ml aquaragia was added (made from 1:3, Nitric acid : HCL) using an
automatic dispenser
-3mm of HF was added to attack silica
-the mixture was left to stand for 8 hours
-50mm of Boric acid (complete oxidation) was added.
27
-the mixture was left to stand for 30 minutes.
-46 mm of distilled water was added to make a total of 100mm solution
After the preparation of the solution, a specified light source lamp was fit into
the lamp housing and the instrument switched on.
-the lamp source was lite
-the wavelenght dial was adjusted and the slit –width was the set
-using supporting gas and combustible gas, mixture was ignited
-the gas flowrate and pressure was adjusted
Then the test solution was nebulized and its abundance measured .
3.2.2.2 SOIL SAMPLE
Soil is unconsolidated rock material that covers the earth’s surface and provides
nutrients and anchorage to plants.XRF test was done by placing the soil samples in
small plastic containers transparent at the bottom. On running the machine the
chemical composition of the soil is seen on computer screen and printed.
28
3.3 FIRING THE ASH TO GLAZE
The firing process was divided into three;
(a)Firing the wood ash to glaze.
(b) melting of feldspar and firing the mixture of ash with feldspar to glaze.
(c)oxy acetylene firing
(a)Firing wood ash to glaze
Glazes consist of four key components, each with their own function.
Glass formers, which the include silica and phosphorous as base material for
glass formation. Silica melts at high temperatures of about 1710˚C , which is
much too hot for ceramic kilns, and as such it cannot be used on its own.
Another component is fluxes, which lower the melting point of the silica,
making it usable to create glazes with easily available kilns. They can be
divided into alkaline fluxes and metallic oxides. Active fluxes allow glazes to
mature at low fire temperatures while other less active ones are useful in midrange and high-fire temperature.
Alumina, or aluminium oxide is also another vital component of glazes which
has the role of being an intermediate oxide by building a strong chemical link
between fluxes and silica.
The other important component of a glaze is colorants , which are in form of
metallic oxides that can also affect melting point and must thus be taken into
account.
29
In firing the wood ash, it was crushed using a pestle and motar , then sieved to
get rid of larger particles. The ash was then placed into crucibles.
The crucibles were then placed into the electric kilns, and temperatures were set
at 1100˚C and the furnace was switched on.
It took 2 hours to attain the set temperature after which it was left in that
temperature for 3hours then sample was removed using fireproof jacket and
gloves with aid of tongs.Then it was left to cool for some few minutes.
However the glaze had not matured at these temperatures.
(b) melting the feldspar and firing the ash-feldspar mixing to glaze
Feldspar was ground using a hand press to break it into small pieces. Then
further pulverised using a motar and pestle to make them as powdery as the ash
initially prepared.
A sample mass of feldspar was put on a crucible and onto furnace with a set of
maximum temperatuure of 1160◦C.
The mixing of the feldspar and ash wa done on gravimetric basis with mass
ratios of ash to feldspar : 1:1,1:2, 1:3, 1:4, of all the four ash samples.
This was done using the density balance , for 1:1 0.25gm of ash was mixed with
0.25gm of feldspar for subsequent ratios feldspar was increased twice, thrice
and four times for the 1:4 sample.
The mixtures were spread on small crucibles and fired at 1160◦C . On attaining
the maximum temparature it left at that temperature for 2 hours.
30
(c)oxy acetylene flame firing
Oxy acetylene flame are of three types termed neutral,carburizing and oxidizing
flames. The tpe of flame produced depends oupon the ratio of Oxygen to
Acetylene in gas mixture which leaves the torch tip.
Neutral flame-is produced when the ratio of oxygen to Acetylene gas leaving
the torch is almost one to one. It is termed neutral because it has no chemical
effect on the material heated. It does not oxidise the sample.
Carburizing flame-produced when the ratio of oxygen to acetylene is less than
one meaning oxygen is isufficient to cause complete combustion of the
acetylene.this flame causes increase of carbon content in the molten sample.
Oxidizing flame-produced when oxygen is in excess of the acetylene and it will
oxidise or burn the sample.
The flame chosen for this experiment is neutral flame. To avoid oxidation as it
occurs in oxidizing flame or increasing carbon in the sample which stains the
glaze.
Adjusting the flame
-the acetylene valve was opened and more gas added until the flame was just
about to separate at the tip of the nozzle.
-the oxygen pin valve was slightly opened till the flame turns blue
-the neutral flame is achieved by mixing egual parts of oxygen and acetylene
and is witnessed in the flame by adjusting the oxygen flow until the middle blue
section and inner whitish –blue parts merge into a single region.
31
The flame was to give a temperature between 1300˚C to achieve this the flame
was hang from a fixed position firmly held by a tripod stand and temperatures
mesured from the tip of the flame using a themocouple. Moving away until it
attains the required temperature. 25cm gave a good local temperature needed.
The samples put on brick lined table at 25cm away from the flame tip . starting
with feldspar it took it 20 minutes for the surface to start melting and flow was
noticed on the small crucible. This was repeated for other samples.
32
4.0 RESULTS AND ANALYSIS
The following were the results obtained from XRF and AAS tests.
4.1 TABLE OF XRF RESULTS
The results below indicate the percentage composition of oxides as obtained from
XRF test.
a) Wood ash
SAMPLE 786/11:BLUEGUM
compound
Al2O3
Concentration 3%
P2O5
CaO
MnO
Fe2O3
ZnO
1.0%
78.9%
6.32%
6.3%
4.3%
unit
SAMPLE 787/11: CYPRESS
compound
Al2O3
Concentration 3.00%
unit
SiO2
P2O5
K2O
CaO
MnO
Fe2O3
ZnO
4.80%
1.00%
11.8%
58.30%
0.64%
12.00%
7.70%
SAMPLE 788/11:BAMBOO
compound
Al2O3
P2O5
CaO
MnO
Fe2O3
ZnO
Co3O4
Concentration 3.00%
0.80%
79.20%
6.30%
6.20%
4.10%
0.08%
SiO2
K20
TiO2
MnO
Fe2O3
ZnO
49.50%
34.80%
0.38%
0.49%
7.45%
1.30%
unit
SAMPLE 788/11:PINE
compound
Al2O3
Concentration 6.10%
unit
Table 4.0 (a) XRF – results for blue-gum, cypress, bamboo, pine
33
b) Soil sample
BLUEGUM SOIL SAMPLE
compound
Al2O3
SiO2
K20
TiO2
MnO
Fe2O3
CaO
Concentration 19.0%
35%
2.9%
2.1%
1.9%
35.4%
2.8%
SiO2
K20
TiO2
MnO
Fe2O3
CaO
46.10%
3.30%
2.10%
1.30%
26.80%
1.30%
unit
BAMBOO SOIL SAMPLE
compound
Al2O3
Concentration 19.00%
unit
Table 4.0 (b) XRF – results for soil samples for bamboo and blue-gum
34
4.1.1. GRAPHICAL REPRESENTATION OF XRF RESULTS
ZnO
Fe2O3
MnO
CaO
P2O5
Al2O3
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
Graph 4.1.1 (a) Quantity of oxides in bluegum.
ZnO
Fe2O3
MnO
CaO
K2O
P2O5
SiO2
Al2O3
0.00%
10.00%
20.00%
30.00%
Graph 4.1.1(b)Quantity of oxides in cypress.
35
40.00%
50.00%
60.00%
70.00%
Co3O4
ZnO
Fe2O3
MnO
CaO
P2O5
Al2O3
0.00%
20.00%
40.00%
60.00%
80.00%
100.00%
Graph 4.1.1 (c) Quantity of oxides in pine.
ZnO
Fe2O3
MnO
TiO2
K20
SiO2
Al2O3
0.00%
10.00%
20.00%
30.00%
40.00%
Graph 4.1.1 (d) Quantity of oxides in Bamboo.
36
50.00%
60.00%
4.1.2 TABLE OF AAS RESULTS
The table below indicates the quantity in percentage of metallic oxides in
the four samples from AAS test.
Lab
No.
Specimen SiO2 Al2O3 CaO MgO Na2O K20 TiO2 MnO Fe203 LOI
786/11 Bluegum
7.81
2.62
20.5
3.00
0.34
6.10 0.01
0.90
2.50
56.22
787/11 Cypress
13.87 4.15
19.7
1.60
1.89
7.70 0.00
0.28
5.50
45.31
788/11 Bamboo
12.29 5.56
18.9
6.20
2.66
6.60 0.01
0.17
1.80
45.81
789/11 Pine
55.50 3.93
4.56
2.07
0.81
16.6 0.00
6.28
3.87
10.25
Table 4.2(a) Results from AAS test
The table below shows the conversion factors of oxides to their metallic elements,
which was used to obtain the percentage in quantity of elements in the samples.
OXIDE
ELEMENT
CONVERSION
FACTOR
MgO
Mg
0.6032
SiO2
Si
0.4672
Al2O3
Al
0.5291
CaO
Ca
0.7147
Na2O
Na
0.7419
K2O
K
0.8301
TiO2
Ti
0.5995
MnO
Mn
1/1.2913
Fe203
Fe
0.6994
Table 4.2 (b) Table of conversion factors
37
The table below indicates the quantity of different elements found in the samples
as obtained from the product of their respective quantities in oxides multiplied with
their respective conversion factors.
Lab
No.
Specimen Si
Al
786/11
Bluegum
3.649 1.39
787/11
Cypress
6.48
788/11
789/11
Ca
Mg
Na
K
14.651 1.81
0.252
2.196
14.08
0.97
Bamboo
5.742 2.942
13.51
Pine
55.50 3.93
4.56
Ti
Mn Fe
LOI
5.064 0.01
0.70
1.75
56.22
1.402
6.39
0.00
0.22
3.85
45.31
3.74
1.973
5.479 0.01
0.13
1.80
45.81
2.07
0.81
16.6
6.28
3.87
10.25
0.00
Table 4.2 (c) Table of quantity of metallic elements.
4.1.3. SAMPLE CALCULATION
Quantity of element = Conversion Factor x Quantity of Oxide
Taking Bluegum sample, its Manganese quantity is got by;
Quantity of Oxide = 0.90
Conversion factor = 1/1.2913
Quantity of element = 0.90 x 1/1.2913 = 0.70
38
4.1.4 GRAPHICAL REPRESENTATION OF AAS RESULTS
LOI
Fe203
MnO
TiO2
K20
Na2O
MgO
CaO
Al2O3
SiO2
0
10
20
30
40
50
60
Graph 4.2.2 (a) Quantity of metallic elements in Bluegum.
LOI
Fe203
MnO
TiO2
K20
Na2O
MgO
CaO
Al2O3
SiO2
0
10
20
30
40
Graph 4.2.2 (b) Quantity of metallic elements in Cypress.
39
50
LOI
Fe203
MnO
TiO2
K20
Na2O
MgO
CaO
Al2O3
SiO2
0
10
20
30
40
50
Graph 4.2.2 (c) Quantity of metallic elements in Pine.
LOI
Fe203
MnO
TiO2
K20
Na2O
MgO
CaO
Al2O3
SiO2
0
10
20
30
40
50
Graph 4.2.2 (d) Quantity of metallic elements in Bamboo.
40
60
4.1.5 GRAPHICAL REPRESENTATION OF SOIL COMPOSITION
CaO
Fe2O3
MnO
TiO2
K20
SiO2
Al2O3
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
Graph 4.3 (a) Bamboo soil sample.
CaO
Fe2O3
MnO
TiO2
K20
SiO2
Al2O3
0.00%
5.00% 10.00% 15.00% 20.00% 25.00% 30.00% 35.00% 40.00%
Graph 4.3 (b) Bluegum soil sample.
41
5.0 DISCUSSION
From the tabulated and graphical results of both the x-ray fluorescence and atomic
absorption spectrometry, it is observed that different metallic oxides occur in
varying quantities for the four samples. The oxides occurring in large quantities are
silica, alumina, calcium oxide and potassium oxide.
The sample of blue-gum had calcium oxide at 20.5% which has a melting point of
2500 to 2900˚C. Silica occurred at 7.81% and it has a melting point of 1700˚C.
Therefore the maturation temperature the ash would not exceed 1700˚ C. Cypress,
bamboo and pine had silica occurring as the highest element in quantity and thus
their maturation temperature was estimated at 1700˚ C.
The samples were then mixed with potassium feldspar to supplement the quantity
of silica, fluxes and alumina in order for the glaze to mature at temperatures that
could be achieved by locally available kilns. However, we encountered a problem
in that the kilns available could only attain a maximum temperature of 1160˚C.
We therefore opted to use an oxyacetylene torch which has maximum temperatures
of 3500˚C, and we heat the samples directly with the flame. The flame was an
oxidizing flame and had temperatures of above 1400˚C, which was the temperature
we could measure with the available equipment. These high temperatures however
had the disadvantage that they could have burnt off some of the color causing
oxides and thus reducing the range of colors we could obtain with the matured
glaze.
In addition, since the samples were in powder form and the flame was produced by
combustion of a mixture of gases ejected at relatively high pressure, the samples
42
were blown off the crucibles which adversely affected the samples that were
prepared in small quantities.
An oxidizing flame was preferred for use since it could achieve high temperatures
and kept the samples relatively clean by absence of for example soot particles
which would have been present if a carburizing flame were to be used. Trapped
carbon particles (soot), would have given the matured glaze a black color.
5.1
ANALYSIS OF ELEMENTS PRESENT IN SAMPLES
From the results of the x-ray fluorescence and atomic absorption spectroscopy for
the following samples, the expected results from the oxides present and the
observations made after maturation of the glaze are discussed below:
a) Blue gum
The oxide occurring in the largest quantity was calcium oxide at 20.5%, followed
by silica at 7.81% and potassium oxide at 6.1%. Due to the presence of calcium
oxide, the glaze was expected to be hard and scratch resistant. Also since it
occurred in relatively high quantities, it was expected to encourage the growth of
crystals which could give decorative effects to glossy glazes and also in
combination with adequate silica and preferably lower alumina form calcium
silicate crystal matte. The presence of zinc would increase the size of crystals.
In the sample of the ratio feldspar to wood ash 1:1, the matured glaze was observed
to have shades of brown, which can be attributed to the presence of the oxides
Fe2O3, CaO and K2O. For a sample of 1:4, it was observed to be highly crystalline
43
due to low amounts of alumina. It was majorly white in color with specs of green
and black regions.
b) Cypress
Calcium oxide in this sample was the most abundant occurring at 19.7%, followed
by silica at 13.87% by potassium oxide at 7.7%. The relatively high amount of
Fe2O3 was expected to give a greenish color in oxidation. However due to the
numerous colors that iron can exhibit, other colors like red, black, grey yellow or
maroon were also expected. Extra iron from melting was expected to precipitate
out during cooling to form crystals.
Four samples of cypress obtained of ratios of feldspar to wood ash of 1:1, 1:2, 1:3
and 1:4, were observed to have a shades of green color. The colored regions were
however not evenly spread due to uneven mixing of the ash and powdered
feldspar.
c) Bamboo
Bamboo had an abundance of CaO oxide at 18.9%, followed by silica at 12.29%
and magnesium dioxide at 6.60%. Due to presence of MgO, the matured glaze
when fired at high temperature (above 1700˚C) was expected to produce an opaque
and matte glaze. The presence of cobalt which is a classic and reliable colorant at
all temperatures was expected to give the glaze the matured glaze a blue color.
However the shade of blue could be affected in many ways by the presence of
different oxides.
The matured glaze samples had the major color as white with very faint shade of
blue-greenish occurring due to occurrence of CO3O4.
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d) Pine
This sample had silica occurring in large amounts of 55.5%, followed by
potassium oxide at 16.6% and Manganese oxide at 6.28%. At temperatures higher
than 1080˚C, MnO was expected to react with silica to produce brown colors in the
presence of alumina. Such high temperatures also had the probability of melting
the manganese oxide to produce a very metallic bronze- like surface. In addition,
the high quantity of silica was expected to produce a hard and glossy glaze.
From the matured glazes obtained, they were white in color. The surface was also
glossy in appearance.
e) Feldspar
Feldspar can form a glaze by itself given that it contains all the components
required in glaze formation per Serger formula i.e. potassium, silica and alumina.
Due to absence of colourants, feldspar was expected to result in white shade of
glaze.
The feldspar glaze was white and glossy in appearance.
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6.0 CONCLUSION
The experimental results obtained showed clearly that waste wood available in
Kenya can form glazes with small additions of feldspar. The glazes obtained had
different colors depending on the amounts of different oxides found in the
respective sample glazes.
During the entire project there were some hindrances encountered, correction of
which could have resulted in a more accurate and conclusive analysis of the glaze
resultant characteristics. These included; Use of oxyacetylene flame instead of use
of kiln which could have affected the glaze colors especially for the iron containing
samples which is sensitive to firing temperatures. The available thermocouple had
a range of 0 -1400oC while the flame had a top temperature of 3500oC, hence
inaccurate temperature measurement with some elements being burnt off by the
high temperatures of the flame. Some samples were also blown off by the
relatively high flame pressures. The need for an alternate firing method was
evident given the above setbacks of oxyacetylene flame as the firing method.
Chemical analysis of the different samples was performed using AAS and XRF
methods and there were some variations between the two methods with results of
some samples having extra elements in a particular method, while varying in
percentage amounts in others. This was due to the technicalities involved in the
given tests, where syenite was used as the base of analysis for the AAS therefore
giving the percentage of elements in the samples as present in the syenite base.
That is why there were elements like Phosphorous oxide, Cobalt oxide and Zinc
oxide detected in XRF were missing in AAS. Also some were present in traces
hence their presence could be neglected. After maturing the glazes, some samples
were observed to have the color distribution in form of patches, which would have
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been as a result of uneven mixing of the feldspar and the ash samples. The areas
that had a high amount of feldspar would have more of white while those areas
with high concentration of ash had different colourations depending on the oxides
present.
The varieties of colors obtained for a glaze were dependent on the oxides present
in that sample. Coloring elements could be classified into; those that always give
colour, those that give colour under special conditions and those that will not give
colour in any case. The most pronounced oxides in colour determination of the
resultant glaze were Iron, Cobalt and Manganese oxides. While Calcium and
Magnesium oxides influenced mainly the crystal formation of the matured glaze
and thus influence texture and light transmission or reflection properties of the
glaze.
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REFERENCES
1. Wikipedia, (2009) Wood ash glazes: www.wikipedia.com
2. The Fundamentals of the Glaze Preparation, Nagoya International Training
Center, Japan International Cooperation Agency.
3. PKF Consulting Ltd and International Research Network, (2005), Wood and
wood products Kenya, Nairobi, Kenya.
4. Properties of Wood for Combustion Analysis, K.W. Ragland, D.J. Aerts:
Department of Mechanical Engineering, University of Wisconsin –
Madison, Madison, Wisconsin 53706, USA.
5. Digitalfire Ceramics Technical Articles: www.digitalfire.com
6. Ash glazes by Phil Rogers
7. Ceramics arts daily: www.ceramicartsdaily.org
8. Temperature ranges for firing glazes: Pottery.about.com
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