3
Handworking
Handworking of Platinum
Specific Handworking
Processes
Platinum Working Equipment
Manual
Index
3.1
Handworking
Handworking of
Platinum
Hot-Working
At its simplest, an alloy is hot-worked above the
annealing temperature so that any work-
Mechanical working
of platinum jewellery
alloys, whether by
hand for individually
crafted pieces or
small batches by
specialist
manufacturers to
catalogue designs,
is similar to the
working of most
white golds. For
economic reasons,
most workers prefer
as smooth a surface
as possible rather
than remove metal.
It is very important
to maintain
cleanliness to avoid
contamination that
leads to
embrittlement,
particularly when
hot-working.
Otherwise hot and
cold working are
standard. Platinum
alloys are oxidation
resistant, formable
and easily annealed,
but die and tool
wear can be a
problem for the
large scale producer
if suitable
precautions are
not taken.
effect. In most cases, though not all, this is
undesirable because it complicates the finishing
and polishing of the piece.
hardening is relieved as quickly as it is generated.
Hot-working is best if economic batches have to
be subjected to considerable reductions or
repeated shaping blows. There are no significant
problems in hot-working normal platinum
jewellery alloys. Even casting alloys can be hotworked. A typical example of hot-working
platinum is the open die forging of 5kg cast
ingots of chain-making alloys at 1100 to 1200ºC.
Reductions of up to 70% may be made between
reheats to form slabs for thin strip rolling or bars
for wire drawing prior to automatic chain making.
Figure 3A. Rate of work hardening of several (initially annealed)
platinum alloys.
Cold-Working
When cold working platinum:
The general annealing temperature for platinum
• Keep all tooling clean
alloys is about 1000ºC and the jobbing jeweller
• Do not use excess lubrication
rarely needs to work them so extensively that
prolonged or repeated heating above 1000ºC is
justified. Cold-working and annealing is normally
• Keep roll surface and dies highly polished
• Clean drawplates of beeswax, other
preferred because it more accurately controls the
lubricants, and metal particles that could
properties of the piece prior to joining and
damage a high finish
polishing. Such cold-working operations include
rolling, section-drawing, hammering and pressing.
• There is a tendency to smear or drag wear
on working tools
A few platinum alloys work harden fairly rapidly
and the work necessary to continue deforming
• Keeping a high tool polish minimises wear.
those that start off relatively hard is significant;
cracking is less of a problem.
Figure 3A shows examples of some 5% and 10%
alloys of platinum compared with a typical
18 carat gold alloy. Alloys like platinum-palladium
have a lower rate of work-hardening. With most
platinum jewellery alloys it is possible to make
reductions in cross-section of up to 70% between
anneals if there is adequate power. For good
repeated practice it is better to choose
reduction/annealing sequences to suit each alloy
and so control grain size and mechanical
properties. Normally, reductions of less than
30% between anneals should be avoided
because a combination of light cold work and
frequent anneals can cause coarse grain
Annealing
Almost all platinum jewellery alloys resist
oxidation during annealing and either
conventional torch flames (air- or oxy-fuel gas) or
electric furnace annealing may be used. Furnace
annealing allows better control of the level and
evenness of temperature and, in turn, the
mechanical properties of the component. Most
cold worked alloys begin to stress-relieve at
600ºC (dull-red) and they soften rapidly at
1000ºC (bright orange) which may be regarded
as the general annealing temperature. Because
of the speed and ease of annealing and oxidation
resistance, many workers use a gas torch for
inter-stage anneals.
growth and corresponding orange peel surface
Index
3.2
Handworking
Annealing Temperature Guide
For most metals and alloys, the annealing temperature in °C is a little over half the melting point in ¡Kelvin.
(Add 273 to the normal melting point in ºC., to get ºK.)
Standard Silver
9ct Gold
14ct Gold
18ct Gold
Pt 1000 fine
Pt 585 fine
Pt-5%Cu
Pt-5%Ir
Pt-5%W
Pt-10%Pd
mptºC
890*
800-960*
850-920*
900-990*
1773
1580
1725
1780
1830
1740
mptºK
1163
1153
1158
1218
2046
1853
1998
2053
2103
2013
AnnealºC
581
577
579
609
1023
927
999
1027
1052
1007
}
about
600ºC
*mpt varies with ratio of the other metals alloyed with gold.
Annealing temperatures are not exact; they vary in practice with annealing time and with the amount of prior
cold work. Heavy work at low temperatures lowers the normal annealing temperature. Except for the extremes of
carat platinum alloys at the low end & Pt-Ir or Pt-W alloys at the high end, an annealing temperature of 1000ºC
is adequate for most purposes.
Where very thin, heavily cold-worked or highly
to be caused by arsenic, boron, magnesium, or
stressed sections are to be annealed it is
silicon. While it is not common, it is possible, in a
advisable to stress-relieve by heating for period
reducing flame during melting, joining or
of a few minutes at 500 to 600ºC before full
annealing, to liberate aluminium or silicon from
annealing or strongly heating for soldering or
refractory materials such as crucibles or support
welding. Time at the full annealing temperature
bricks, or carborundum (silicon carbide)
will depend on the thickness of the section, but
abrasives. This can only occur under reducing
1 minute per mm of section thickness should
conditions, when the platinum alloy is in close
be sufficient for thicknesses up to about 5mm.
contact with the support, well above red heat.
Too frequent annealing or annealing for too
Hydrocarbons such as oils or grease are typical
long can cause grain growth which may make
contaminants that can give rise to unexpected
subsequent working or polishing to a smooth
reducing conditions. It is important to avoid
surface more difficult. Except for platinum-
trapping potential contaminants in the metal
ruthenium, platinum alloys do not generally age
during working processes such as rolling, drawing,
harden and so can be either cooled in air or
hammering, cutting or abrading. Good
water quenched from the annealing temperature
housekeeping prevents these potential
without harm.
problems by avoiding contamination.
Contamination
Most metal contaminants of platinum surfaces
may be removed by pickling in hot, but not
Platinum combines with most molten metals.
While this is useful for making alloys with
controlled characteristics, it can also cause
boiling, 10% nitric acid, prior to annealing. Iron
contamination is best removed with hot
hydrochloric acid.
trouble. For instance, small amounts of
aluminium, lead or tin can cause low melting
If contamination has occurred after heating, it is
point compounds with platinum and this can lead
very difficult to correct other than by cutting it out
to intergranular cracking called hot-shortness
and repairing by joining in a new piece of clean
(silver, indiscriminately alloyed with platinum
alloy. Attempts to solder intercrystalline cracking
can cause intergranular cracking, but this is
or to dilute the effect by annealing usually results
very unlikely under ordinary annealing
in spreading the problem.
conditions). Intergranular cracking is more likely
Index
3.3
Handworking
Contamination
Some Possible Causes:
Some Preventatives:
• Lead-Tin solders on the same work bench
• Avoid reducing flames and furnace
• Charcoal soldering blocks
• Abrasives rolled into the surface
atmospheres
• Use oxidising conditions; platinum itself will
not oxidise
• Filings from other precious or non-precious
metals
• Avoid trapping contaminants in platinum
during working
• Filings or drillings from gemstones or
mineral crystals
• Degrease and positively clean surfaces
before heating
• Oils, grease or soaps used for drawing,
rolling, etc.
• Avoid contaminated supports during heating
and joining
• Machining suds (lubricants)
• Prefer refractory supports to charcoal
when joining
Specific
Handworking
Processes
Forging
The term "forging", while metallurgically correct,
sounds heavy for jewellery processes. It covers
squaring or rounding by planishing, spreading
widthwise, (Figure 3B) tapering/pointing (Figure
3C), or upsetting the cross-section of the
jewellery component, usually between a smooth
faced hammer and an anvil or stake or swaging
between open or closed dies. Much the same
principle applies in ring forming, forming tube
sections, and on a small scale, hammer
Figure 3B. Spreading platinum strip widthwise using a thinned
and polished cross-peen or small raising hammer to move
metal outwards at right angles to the long axis of the piece. The
back of the hammer head needs to be weighty to provide force.
texturing, embossing and chasing. Platinum is
very malleable and forges well, but it does require
deliberate and forceful blows especially when
dealing with large cross-sections. A good
indication of its malleability in the annealed state
is that strip sections up to 3 or 4mm thick can
be folded double and compressed by hammering
without cracking. In fact, this is a good way of
combining the excellent strength of cold-worked
platinum in thin sections, with bulkier sections
achieved by folding and hammering without
soldering or welding (Figure 3D).
Figure 3C. Tapering/pointing a section. Hammer moves in a
constant short arc, finishing normal to the taper angle
(otherwise, a square section will offset to one side to form a
rough rhomboid shape). Work may be rotated radially beneath
the hammer blows to achieve pointing (all round tapering) for
start end of section drawing.
Index
3.4
Handworking
To maintain good shape control, use square or
corners and re-entrant sections. While hand-
radial blows as appropriate; otherwise the
made draw plates may be satisfactory (with acid
section tends to offset to one side or the other.
cleaning) for an occasional piece of craft jewellery
Once the near final dimensions are achieved, the
in platinum, economic batches demand best
section can be trued by planishing, die drawing or
quality tungsten carbide drawing dies kept in a
even rolling. It can of course be finished by filing
high state of polish, particularly in the work zone;
but it is always preferable to use compression
otherwise platinum will further roughen the
processes to conserve metal weight and minimise
surface and lead to platinum build up and loss of
the need for extensive surface finishing.
cross-sectional accuracy.
Anneal after about each 30-35% reduction in
cross-sectional area even though platinum will
usually withstand greater reductions without
cracking; its workhardening ability is such that it
requires considerable increases in force to
continue working. Heavily work-hardened
platinum alloys can dent stake, anvil and hammer
surfaces, making it progressively more difficult to
maintain a good surface finish on the work.
Figure 3E. Starting a thin walled tube from strip with a taper cut
smoothly at the lead end, hammered into a channel to start
curving the tube wall and completed by die drawing (with seam
welding if necessary).
Reducing tubular cross-sections with the wall
finding its own thickness is known as sinking
(Figure 3F). This is a simple variant on wire
drawing and wire drawing dies may be used.
Where the wall has to be controlled for short
platinum hollow sections for jewellery it is
convenient to use a mandrel. This is a polished
steel rod or hard-drawn wire (piano wire as
slender as 0.5 mm diameter may be used)
Figure 3D. Platinum strip/tape can be repeatedly doubled back
on itself and, in effect, cold forged to produce a thick section
(which may then be locally annealed, if required) attached to a
filigree section (which may be left cold-worked).
whose external shape matches that of the
required internal cross-section of the platinum
component. The mandrel is placed inside the
Section and tubular
section drawing
starter tube and the two are co-drawn through
the die sequence. In the first few passes the
mandrel is loose inside the platinum, but after
With draw plates, channelled and taper-ended
being drawn down onto the mandrel the platinum
platinum strip can be formed into more precise
wall is controlled between the mandrel and the
channel sections, hollow ring stock and seamed
die. The total reduction in the wall cross-sectional
tube (Figure 3E). Thin rod and wire can be
area between anneals should be limited to about
modified to many different cross-sections.
35%, particularly after the platinum is in full
Prepolished dies are available commercially, but
contact with the mandrel, so an inter-stage
some craft jewellers prefer to draw simple
anneal before the platinum grips the mandrel is
sections through a hand-made draw plate. This is
advisable. The mandrel must start with a length
still possible with platinum, but time consuming if
greater than the finishing length of the platinum
complex sections are attempted. The limiting
tube or it will be enclosed. The mandrel also
factor is the smearing ability of platinum on
introduces another dragging friction interface
insufficiently polished die surfaces or where the
that, with platinum, requires careful lubrication
interface pressure is very high such as at tight
and techniques to remove the mandrel.
Index
3.5
Handworking
than the outer diameter of the platinum tube to
be removed. These are “played like castanets”
along and around the mandrel/tube combination
to achieve many small pinching blows to loosen
the tube (Figure 3G). The loose tube, now off the
mandrel, may be given a finishing pass through
the last die hole again to clean up the surface
and shape.
Figure 3F. Tube sinking with relative wall thickening compared to
mandrel drawing for short lengths. the mandrel controls the wall
thickness and also takes part of the load at the short end.
Spinning and Doming
The ductility of platinum makes it quite amenable
to spinning. The jobbing jeweller is unlikely to
Lubrication and mandrel
separation
need to spin large pieces into deep shapes but, if
A hard soap such as “Pale curd industrial” in bar
the need for too many annealing stages. Virtually
or powder form is used on the die and the
all alloys can be spun to produce shallow dished
mandrel for lubrication and to ease the removal
shapes for jewellery pieces in a single stage. The
of the platinum tube after drawing is completed.
metal is worked against a former at a shallow
Bar soap can be applied to a stout mandrel by
angle and the strain is not severe. Relatively deep
rubbing; soap powder can be tapped into the
pieces may need two stages with an interstage
tube bore prior to mounting on the mandrel. To
anneal, mainly to reduce the off-centre pressure
get a better finish, particularly on the outside of
to be exerted by the burnisher (Figure 3H).
the finished section, it helps to use a soft soap
Making small round shapes up to hemispherical
lubricant for the last two or three passes.
depth is best accomplished by doming. Platinum
so, the most ductile alloy is preferable to avoid
jewellery alloys respond well to this technique.
Indeed, doming of a blank, and perhaps
annealing, may be useful as a start to spinning.
Some of the larger scale (deep drawing and deep
spinning) or more continuous working processes
(stamping), and processes like ring sizing more
akin to finishing operations, will be dealt with
later in the Manual.
Figure 3G. Loosening the mandrel by tapping with parallel bars
or “castanets”.
Attempting to drag the mandrel out of the tube is
likely to tear or buckle the platinum. To free the
tube from the mandrel after completing the final
pass, it must be burnished off with one or two
passes through rolls that just pinch across a
series of outer diameters and impart a slight
degree of twist. In effect, the tube wall is slightly
expanded away from the mandrel. For occasional
short lengths, it is possible to achieve the
roller/twist effect with a pair of parallel tool steel
Figure 3H. Outline sketch of small scale spinning arrangement
for making a collet for a stone setting from a sheet-circle using
a burnisher. The bottom section may be parted off and may be
useful as a bezel ring.
rods (or tungsten carbide for very long life)
gripped in a handle with a gap just a little larger
Index
3.6
Handworking
Cutting
One of the most frequently used processes in
handworking a piece of jewellery is cutting to
shape or length, by piercing and sawing, slitting,
drilling, filing or grinding. On a small scale, there
is little need to differentiate between tools,
grades and techniques for white gold and for
angle (about 15 degrees or less) to the cut
surface to avoid excessive snagging at the leading
edge. Filing should also be done at a similarly flat
angle with frequent changes of the file surface
and cleaning. The lubricant "oil of wintergreen"
has been proven to be effective in relieving wear
on files.
platinum. It is always worthwhile minimising the
“Sanding” papers should be chosen for their free-
amount of metal removed or scrapped, by
cutting ability, in which case the abrasive particles
carefully sizing the piece to minimise or eliminate
will remain upright on a stiff bonded backing
the need for cutting. Often, platinum wires, tubes
paper. 240 and 400 grades are usually adequate
or strips will be thin, taking advantage of high
in two stages. It is often better to part off
strength. As with any hard jewellery alloy, it is
platinum sections with thin alumina or silicon
unwise to cut thin sections with coarse settings.
carbide disks running at 1300 to 3800 M/min.
Jewellers' (tool steel) saw blades are rated from
peripheral speed rather than use a lathe tool
8/0 for the very finest pierced work, through 0,1
which tends to leave a coarse burr and
and 2 which are better for silver but too coarse
undergoes rapid tool wear.
for most platinum jewellery work, and on up to
size 8.
Platinum alloys appear to wear out saw blades
and similar hand tools more quickly than do gold
or silver. This is because the alloys cold weld very
well. Metal builds up on cutting edges, followed
by stick/slip binding/removal of the debris, and
this causes extra wear. Figure 3I sketches the
different effects between aluminium, gold/silver,
and platinum alloys on a coarse saw blade.
Compromise between too smooth and too
coarse. Use around 3/0 grade which is a good
general purpose blade for white gold and
Figure 3I. Sketch of different modes of debris build
up/clearance on a jewellers’ sawblade.
platinum. The blade should travel at a fairly flat
Platinum
Working
Equipment
Ideally, rolls, dieplates, dies, saw blades, chasing
If this is not economical, then the best
tools, files, grinding wheels, and similar tools
alternative is to apply good housekeeping and
where there is a degree of sliding movement
segregate tooling used for platinum; clean it
during working, should be kept specifically for
and the working areas before and after
platinum alloys only. This is because platinum is
platinum working. This ensures maximum
more vulnerable to contamination from other
recovery of platinum scrap and sweepings and
metals picked up under working conditions.
minimises cross contamination of platinum. For
Polished hammer and stake faces cause fewer
instance, when the same rolls are used for
problems because the impact is normally square
platinum and other metals, platinum should be
rather than sliding. Large platinum jewellery
placed in 10% nitric acid at 70ºC to remove any
producers, in Japan for instance, dedicate separate
residues of other metals before any annealing or
parts of the same factory, let alone separate
joining process.
tooling, to platinum or gold jewellery production.
Index