ACID DEVESTING SOLUTION
Acid devesting solution is the major cause of cracking in karat
gold castings. Hydrochloric/muriatic acid is one of the main
chemicals in many investment removers. Hydrochloric acid attacks the
grain boundaries, dissolving part of the alloy mixture. What can be
observed in these cracks is a black shiny compound of silver
chloride. We previously believed this compound was oxides of
silicon. For many years we have attributed most cracking failures to
silicon oxide formation. This has been erroneous in direct terms.
The severity of the cracking will depend on many contributing
factors, such as alloy silicon content, gate/sprue relationship and
temperatures involved.
Hydrochloric acid is the most effective treatment for removal of
investment powder. The key to trouble free use is careful control of
the time in solution, and the strength of the solution. Freshly
mixed solutions are more powerful and therefore require less time to
work. The problems are greater in I Ok gold, because of the higher
alloy content. Controls should be in place to monitor the
replenishing and changing of the solution. Better yet, use a
solution without hydrochloric acid.
SUBSTITUTES FOR HYDROCHLORIC/MURIATIC ACID
- Phosphoric Acid 50% & 50% water
- Ammonium Biflouride
- Sulfuric Acid
- Hydrofluoric Acid (use with extreme caution)
- Ammonium Carbonate (Food Grade)
(Must be used at room temperature. DO NOT HEAT.)
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