Knowing how to cast jewelry properly demonstrates another aspect of quality in your shop.
Porosity. It’s an ugly word for bench jewelers and can be expensive for store owners and customers. Just close your eyes and picture a block of baby Swiss cheese. The cheese contains millions of holes caused by air bubbles, and while their appearance in the cheese may be appealing and the taste unaffected, similar holes inside your jewelry are neither pleasing in appearance nor in effect.
That mental picture of Swiss cheese is a close approximation of how porosity appears in cast jewelry. But the real problem with porosity is not its appearance, rather the damage it causes.
Porosity in cast jewelry weakens the basic structure of the piece, causing ring shanks to break, prongs to break off and galleries to crack.
This article illustrates what porosity is, how you can detect it and how to advise your customers when you find it in their jewelry. Along the way, we’ll look at two major causes of porosity – shrinkage and gas – and some minor causes.
Casting Metal into a Mold
This cross section of a flask shows how metal enters a typical mold (#4) and begins to cool off, first at the farthest point in the ring cavity (#1) then progressively down the sprue (#2) and toward the reservoir of metal called the button (#3).
Typically, gas porosity results from overheating in the casting process. In its molten state, metal is very volatile and can absorb gas, usually hydrogen. When the metal enters the mold, the gas concentrates in small pockets and remains there after the metal solidifies, creating pits and holes (see dark outlines).
If the ring is thick at top, the metal can cool first at the bottom, trapping molten metal above it (shown in deepest red). As the trapped metal cools, it suffers shrinkage porosity, which normally would have occurred at the button.
Porosity can appear anywhere on jewelry. In this case, it’s on the side of a highly polished ring (shown in red circles). Be sure to inspect the entire ring because porosity can be easily “concealed” on the outside (see box at right).
Concealed Porosity: Nightmare Unto Itself
Porosity can be concentrated in certain areas or spread throughout a ring. When it’s prevalent in certain spots, sometimes finishers try to conceal the pits by “burnishing” the area, in essence, compressing the metal by rubbing it with a hard object as illustrated here.When the ring undergoes standard service and refinishing, however, the pits will reopen and become visible. Reburnishing porosity is laborious, and the ring gets thinner each time it’s done. Burnishing also can distort the shape of the ring and flatten fine details.
Shrinkage porosity generally occurs in the thicker portions of cast jewelry. When it’s visible on the surface, it often looks like starbursts or irregular cracks.
Gas porosity can be scattered throughout a piece of jewelry, making it brittle and prone to breaking. It often appears as blotches in highly polished areas. Magnification reveals these blotches as holes. Because the metal is porous and contains holes, it’s harder to polish and may appear darker than non-porous metal.
Gas porosity also can appear as cracks, as shown in the junctions of these prongs. Be careful when examining an item for porosity.
If you detect cracking, starbursts or tiny patterns of holes, proceed with caution. Call in a more experienced associate if you have any doubts. Porosity can be a very expensive problem; if you miss it during the take-in, your store may end up paying the bill. Another word of advice: be sure your store didn’t sell the item before you launch into a litany about porosity!
JA Professional’s Guide to Fine Jewelry Craftsmanship
Porosity and Other Casting Flaws
By Tom Weishaar and Mark B. Mann
Results of Improperly Cast Jewelry
- Gas porosity. This may be scattered throughout the piece of jewelry 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 jewelry, this appears as starbursts or irregular cracks in the metal.
- Cracks. Look for cracks anywhere in the cast jewelry, 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 jewelers 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.
Illustrations by Lainie Mann
© 1999 Jewelers of America
Knowledge of the some of the information in this feature is required for the third and fourth levels of written testing for the JA® Bench Jeweler Certification program.
Copyright © 1999 by Bond Communications.
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