I pickle my casting tree (Silver or Gold) after quenching it in the
water, with 50 percent phosphoric acid ( 50% water plus 50%
phospharic acid) for 15 minutes, heat up to 45 degrees centigrade in
a Borocil beaker or a borocil bowl. all the oxid film is removed as
well as the investment soften downs which can be easily removed with
a tooth bursh or water jet pressure, efforlessely with no traces of
investment left. Always store the used phospharaic acid in a glass or
plastic container. Use it carefully even though it is not corrosive
as sulphuric acid to skin contact. Nuetralize the acid on the
castings with Baking soda solution prepared in clean water.
Use protective eye wear while using the acid, keep baking soda with
in reach to nuetralize the acid,in case of accidental spills. Put
acid in to water, while mixing it to make 50 percent strength.USE
THE ACID WITH CARE.
One can use the above acid at room temperature by soaking the
castings for an hour or can keep over night with out any damage to
the castings, whether silver or gold. Use copper tongs or plastic or
rubber coated tongs or brass tongs to place and remove the castings
from the acid.
I am using the above acid and method for softening the investment
and remove the oxide film from the casted tree, since last 15 years
sucessfully with out damage to the castings of gold and silver,
brass and copper castings can also be successfully cleaned with this
method and use of phosphoric acid as above.
Hope this helps.
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Strive to be Happy