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What is Preferential contamination of direct filling gold is done by: and explain in detail?
After pellets of gold are rolled, they may be conveniently stored in a gold foil box, which is divided into labeled sections for various sizes of pellets. 
Cylinders of foil and selected sizes of other types of gold also may be stored in the box. Preferential contamination is suggested by placing a damp cotton pellet dipped into 18% ammonia into each section of the box
This serves to prevent deleterious oxides from forming on the gold until it is used.

Key Concept
Contamination of direct ﬁlling gold is done by 18% ammonia.
 
 
Sturdevant's art and science of operative dentistry. 7th edition page e69