May 1999

For Your Staff:Selling Quality

Knowing how cast jewelry is made demonstrates another aspect of quality in your shop

by Mark B. Mann
Director of Professional Certification
Jewelers of America

Lost-wax investment casting was formally established in the early 1900s in the dental industry and was embraced enthusiastically because it ensured great accuracy in creating reproductions. It soon evolved into a process for jewelry, where it has grown to be the largest manufacturing category.

The casting process that has resulted from nearly 100 years of developing technology is not so scientific that it's fail-safe. In the hands of those unprepared to meet its technical challenges, casting has the potential for defective results.

To make sure you have the know-how to avoid these pitfalls, we'll focus the next two installments of the Quality Assurance Guide on these important aspects of casting:

1. Methods and observations for identifying cast articles of jewelry (in this issue).
2. Quality features of cast articles of jewelry (in this issue).
3. Identification and analysis of common defects resulting from improper casting (written by Tom Weishaar and appearing in the June issue).

Overview of the Casting Process
The goal of lost-wax investment casting is to create wax copies that allow mass production of articles basically identical to a handmade original. For anyone unfamiliar with the process, here's a much-simplified explanation:

• Begin with a piece of metal jewelry called the model.
• Make a rubber mold by covering the metal model with a solid mass of layered, unvulcanized rubber and subject it to heat and pressure to vulcanize or "cure" the rubber.
• Using a surgeon's knife, dissect the rubber mass and remove the original model. Unless the rubber mold is removed carefully, the lines where it's cut – called parting lines – may show on the resulting wax model reproductions.
• Inject molten wax into the rubber mold; the resulting wax reproduction is used as the master to produce mass numbers of the article.
• Place the wax model in a flask, surround with investment (a plaster-like compound that withstands high heat).
• After the investment has set, the assembly is placed in an oven to create the mold for the jewelry.

Identifying Marks of Cast Jewelry

Irregular, Free-Flowing Design
Both these rings are cast. The free-flowing design of the ring at left makes its cast origin fairly obvious. That origin isn't as easy to detect in the other ring, which has a symmetrical design that easily could have been hand-fabricated.

Lack of Precision at Wire Junctions or Other Junctions

The illustration at the top shows wires fabricated together. The detail is not "joined," rather it's crisp and precise. The illustration at the bottom shows the identical junction of wires, but it is rounded and "joined" – typical of cast reproductions.

Unfinished Areas Are Unpolished and Have Slightly Granular Surfaces Cast jewelry often has hard-to-reach areas that – because of time and labor – are impossible or impractical to polish and finish properly.	Parting Lines Unless the rubber mold isdissected very carefully when the wax model is made, the result may be a seam or parting line on the wax that is never removed by the wax finishing department. This defect is transferred to the final piece and sometimes not removed by the metal finishing department.
Rounded Characteristics Cast items are only nearly as good as the original model. Right angles or square corners often become rounded because the model's fine detail is typically lost in the casting process: first during the intricate steps of precasting and then in actual casting and finishing.	Irregular Shape: Wax Was Pulled from the Mold Too Early Often, the factory technician removes the wax from the rubber mold before it's fully cooled. The result? The item becomes deformed when it's handled, and the deformation remains in the piece through the casting process.
Errors in the Casting Process Unfortunately, common defects can also serve as identifying characteristics. Here are some of the more common defects or characteristics that make a cast article easy to recognize. Casting "Skin" The surface of a cast article (sometimes referred to as a "skin") is slightly granular and lacks luster.	Assemblies of Two-Color Castings Don't Match It's difficult to cast one piece of jewelry requiring two alloys of different colors (such as white and yellow gold). Instead, these articles are typically cast separately then soldered together after prefinishing. Too often, the joining is not done with precision.   Filled Pits Pits are the result of errors in workmanship and poor or inadequate casting procedures.

Next month: Tom Weishaar, JA's first Certified Master Bench Jeweler, will discuss issues surrounding porosity.

Illustrations by Lainie Mann

JA Quality Assurance Guide
Integrity of Cast Jewelry

Properly Cast Jewelry

Here are the signs of a properly cast article of jewelry.

1. The appropriate alloy is used.
2. The article's detail and pattern are maintained.
3. The design and alignment of components are consistent.
4. The piece is free of porosity (no small-to-large pits or cavities on the surface).

 

 

 

Potential Problems

Ring Too Thin and Lightweight This may be a second-, third- or fourth-generation reproduction. Molds for this ring were made from reproductions, resulting in rings far too thin and lightweight for normal wear.

Porosity This ring has visible pits on the surface of a highly polished area; pits result from errors during the casting process. There is no remedy – the ring must be redone.

Cracks As a result of errors in the casting process, the ring has cracks in the prongs (a critical point) and another very visible one on the shank. There is no remedy – the ring must be redone.	Small Nodules As a result of errors in workmanship when finishing a cast ring, small nodules can appear inside the shoulder of a shank.
Parting Lines Mold lines that occurred when wax was injected into the rubber mold before casting still show on the side of this ring. It would be impossible to remove them without destroying the ring's design.	© Jewelers of America This information is required for the second through fourth levels of certification (written tests) for the JA® Bench Jeweler Certification program and the first and second levels of the JA® Sales Professional Certification program.   Illustrations by Lainie Mann

 

 

Copyright © 1999 by Bond Communications.

 

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