One quotient, no other terms
Investment casting works because the cavity left behind has exactly the volume of the wax that was burned out of it. Volume is preserved; only density changes. So:
metal weight = wax weight x (specific gravity of metal / specific gravity of wax)
There is nothing else in it. No allowance for the process, no fudge factor, no dependence on the shape. Whatever the wax weighed, multiply by the ratio and that is the metal in the finished piece, before any sprue.
The ratios are larger than people expect because wax is close to the density of water and metals are not. Against a wax at 0.95, sterling multiplies by about 10.9 and fine gold by about 20.3. A carving that weighs three grams in wax is 32.7 grams of sterling or 61 grams of gold, and that is the moment a project changes character.
| Metal | Specific gravity | Multiplier on wax at 0.95 | From a 3 g wax |
|---|---|---|---|
| Aluminium | 2.70 | 2.84 | 8.5 g |
| Yellow brass | 8.47 | 8.92 | 26.7 g |
| Sterling silver | 10.36 | 10.91 | 32.7 g |
| 14k yellow gold | 13.07 | 13.76 | 41.3 g |
| Fine gold | 19.32 | 20.34 | 61.0 g |
| Platinum | 21.45 | 22.58 | 67.7 g |
Why sterling is 10.36 and not 10.49
Silver is 10.49 and copper is 8.96, and sterling is 92.5 percent silver by weight. Densities do not average by weight, though, they average by volume. A hundred grams of sterling contains 92.5 grams of silver occupying 8.818 cubic centimetres and 7.5 grams of copper occupying 0.837, for a total of 9.655 cubic centimetres. A hundred grams in 9.655 cubic centimetres is 10.36.
The same arithmetic explains why karat golds vary between suppliers. Fourteen karat is a fixed 58.3 percent gold by weight, and the remaining 41.7 percent is silver, copper, zinc and sometimes nickel or palladium in proportions the refiner chooses. Yellow, white and rose at the same karat sit at genuinely different densities, and two yellows from different refiners can differ enough to matter on a large piece. Ask for the figure.
The button, and what it is for
Metal shrinks as it solidifies. A casting that has no reservoir to draw from pulls that shrinkage out of itself, and the result is porosity somewhere inside the piece, usually in the thickest section where it solidifies last. The button is a mass of metal above the tree that stays liquid longest and feeds the shrinkage downward.
How big it needs to be depends on the tree, the sprue diameters and the sections in the pieces, which is why this page takes a percentage rather than supplying one. Ask whoever is casting. If that is you, it is a number you develop from your own failures rather than from a table.
The button is not consumed. It comes off the tree with the sprues and goes back for reuse or refining, so the metal cost that matters over a year is the metal that leaves as finished work plus whatever is genuinely lost. The cost line on this page separates the two for that reason.
Weighing the wax
Weigh the model without its sprue, because the sprue is accounted for separately as a percentage, and counting it twice inflates the charge. If you are working from CAD, the volume comes straight out of the file and is more reliable than a scale reading on a fractional gram.
The specific gravity of the wax itself is worth checking if you use several. Carving waxes, injection waxes and castable print resins are not the same material and do not have the same density, and since the ratio divides by it, a ten percent error there is a ten percent error in the metal.
For the kiln time that burnout takes, the kiln firing cost calculator adds it up segment by segment. If the master is being printed rather than carved, the resin print cost calculator covers what the pattern costs before it reaches the flask, and the silicone mold calculator handles the rubber if you are moulding a master to inject waxes from.
Questions people ask
How do I work out metal weight from a wax model?
Multiply the wax weight by the specific gravity of the metal divided by the specific gravity of the wax. The cavity has the volume the wax had, so only density changes, and no other term enters. A 3 gram wax at specific gravity 0.95 becomes 32.7 grams in sterling at 10.36, since 10.36 divided by 0.95 is 10.91. Weigh the model without its sprue and add the sprue and button separately.
Why does the calculator ask for the wax specific gravity?
Because it is in the denominator, so an error in it goes straight through to the answer. Carving wax, injection wax and castable print resin are different materials with different densities, and machinable waxes differ again. If you use one wax for everything, measure it once by weighing a known volume and you will have a figure good for every casting after that. If you switch, the number switches with it.
How much extra metal for the sprue and button?
That is a decision for whoever is casting, which is why the field takes your figure. It depends on how the tree is sprued, the diameter of the sprues, and how much reservoir the heaviest sections need to feed their shrinkage. A casting house will tell you what they want. If you are casting yourself, it is a number you arrive at from your own porosity failures rather than from anyone else table.
Do I lose the metal in the button?
Not in the sense that it is gone. Sprue and button come off the tree as clean scrap and go back into the melt or to a refiner, so over time the metal that actually leaves is the finished work plus genuine losses from fire scale, filing and polishing. What the button does is tie up capital in the flask, sometimes a lot of it, which is why the cost breakdown here separates model metal from button metal instead of quoting one number.
Are these specific gravity figures exact?
They are working figures, and the karat golds are the ones to be careful with. Fine gold, fine silver, platinum and aluminium are elements and their densities are settled. Sterling is a fixed recipe and 10.36 follows from it. Karat golds and bronzes are families rather than single alloys, and the density moves with the exact mix, which the refiner chooses. For anything where the metal cost is significant, use the number your supplier publishes for the alloy you are actually buying.