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4.12 Using already cast material
Because of high costs casters do not like to use every time fresh alloy. For some alloys
customers mix “second hand metal” up to 50 %, mainly Au, Ag based alloys. Please pay
attention to following:
Never use finished jewellery pieces. The alloy composition is not known, solder in
casting material will give no good casting result.
Never mix two alloys, even if they have the same contents of gold.
All already cast metal should be very clean, sand blasting and pickling are ade-
quate methods. Caution: rest of glass beads, investment material or rests of acid
are unwanted. Acid residues can be cleaned in ultrasonic bath wih distilled water.
Alloys with zinc content: give a little bit more fresh metal to “old” metal.
Alloys with high carat (Au 21/22/24 kt) can be re-used to 100 %
Prevent oxidation by melting under vacuum and protective gas only.
Never use boric acid for melting!
4.13 Notes on the properties of some metal alloys
Brass, gold 8/9/10
kt, some alloys
gold 14 kt
These alloys have a high content of zinc. Some of the zinc evapo-
rates during the heating phase. If vacuum is used in the melting
chamber, much more zinc would evaporate. To prevent this problem
there is a small trick:
Put the metal in the cold crucible, set in flask at the same time.
Push button start (heating) and start (vacuum) again immediately.
Now a vacuum is built up, but the metal, still not liquid, cannot evapo-
rate zinc. After reaching full vacuum, push another time start (vacu-
um will be released by gas). After reaching casting temperature, cast
immediately.
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