Porosity in jewellery
So what is it?
Easiest explanation, its when gold is cast improperly and you end up with a "swiss cheese" look to the metal rather than a solid structure.
That mental picture of Swiss cheese is a close approximation of how porosity appears in cast jewellery. But the real problem with porosity is not its appearance, rather the damage it causes.
Porosity in cast jewellery weakens the basic structure of the piece, causing ring shanks to break, prongs to break off and galleries to crack.
It is found largely is mass produced styles rather than one off items, although it can occur during any casting process.
Results of Improperly Cast Jewellery
- Gas porosity. This may be scattered throughout the piece of jewellery and also may concentrate in corners, such as at the junctions of prongs or in galleries. It shows itself as tiny holes or blotches on highly polished surfaces.
- Shrinkage porosity. Generally located in thicker areas of a piece of jewellery, this appears as starbursts or irregular cracks in the metal.
- Cracks. Look for cracks anywhere in the cast jewellery, the result of overheating (or in some cases underheating) the metal.
- Investment inclusions. “Chunks” of the white investment powder used in the casting process may sometimes appear on an item. Usually removed through standard cleaning after casting, it leaves behind a square-cornered depression.
- Pits other than porosity. These include depressions from air holes the wax worker didn’t detect in the wax. If the wax has minute holes when it’s cast, it will absorb investment, ultimately leaving a void in the cast item.
Ring Has Cracks
As a result of errors during the casting process, this ring has one crack in a prong (a critical point) and another very visible one on the shank. There is no remedy for this error; the ring must be redone.
Flat Pendant Has Depressions
Powder was used as a release agent for the injection wax in the rubber mold. In this case, too much powder was used, leaving visible depressions on the surface. Trying to eliminate the depressions during the finishing process could remove too much metal.
Ring Is Deformed
In the wax stage, this ring was removed from the rubber mold before it was completely cool. As a result, the finished ring is deformed.
Areas of Discoloration
Some bench jewellers try to fill porosity with solder, a practice that results in speckled discolored areas (as shown). There is no remedy for porosity; in most cases, the mounting should be remade.
Most of our work is handmade but sometimes a design requires casting and each and every piece is checked fully for the above issues before it is presented as a completed item.