Design and The Intelligent Compromise
Often times, when creating new jewelry masters, questions come up around design ideas and where one might look for inspiration. Ideas come from everywhere—nature, architecture, music—the list is endless. But, probably a more pertinent question to ask is: “How close is the master you made to the design you had in mind?” We know: A direct hit to the panic button.
Making a master model look exactly like the intended design is one of the most difficult challenges faced by the model maker. Some challenges you face include:
- Making the master larger than the intended finished piece.
- Formulating the design for the end piece for the end piece instead of the master.
- Questioning delicate designs on paper and if they'll become nearly indistinct surface markings on a casting when accurately reproduced-things look quite different on paper than they do in metal.
This is why model making could be viewed as an exercise in intelligent compromise. It requires considerable interpretation of what the designer is intending to see produced, and what we think the design is showing. Sometimes, it just comes down to working with the designer to adjust the design for the optimal end result.
Shrinkage: Don’t Be Surprised—Plan For It
We know that most models are made in one of three ways:
- Direct fabrication
- Through casting
- Or, a blending of both
We also need to keep in mind that the model will be reproduced and, during the reproduction process, shrinkage will occur. The amount of shrinkage will usually range between 6 - 12% or more depending upon a variety of factors and methods of master replication. Some of these include:
- Whether the mold is metal of rubber
- What type of materials are used for producing the pattern
- The types of metals used in the casting process
- Temperature of the mold and metal
- Method of casting-vacuum or centrifugal
When working from a scaled design to the final reproduction, it’s best to compensate for shrinkage when carving the original wax model, or when the master is being made through a blend of cast and fabricated parts. Why? Because once the wax (or blended) master has been cast into a metal master, the amount of shrinkage will be the same as the fabricated master in the replication process.
The consequences of shrinkage on the design can vary dramatically. If the design is a freeform mounting without seats for stones, shrinkage may not be an issue. On the other hand if there are extremely tight tolerances for set stones, shrinkage becomes a most important factor. With this in mind it may be advisable to allow for slightly more shrinkage where stones are concerned.
This can also be applied to stone size as well, as explained through these examples:
- Example 1: If we allow 8% shrinkage for a 3mm stone, we would add .24mm for a total size of 3.24mm.
- Example 2: If the total shrinkage was 12% we would have needed 3.36mm for a difference of .12mm. The setting process might be able to easily accommodate this difference.
- Example 3: If the stone was 10mm with everything being the same as above, we would have planned for .8mm and needed 1.2mm. The result is we are off by .4mm. That much of a difference could be a problem.
The size and mass of the master is also something to think about when looking at shrinkage. Although the rate of shrinkage will be comparable throughout the piece, the consequences of the shrinkage vary. Let’s say, for example, the piece shrinks eight percent. One part is ten millimeters and another is two millimeters, it will appear that the ten millimeter part had considerable shrinkage whereas the two millimeter part had considerable shrinkage whereas the two millimeter part had little to none. In mathematical terms the ten-millimeter part shrank .8mm and the two-millimeter part .016mm.
HELPFUL HINT: The amount of shrinkage “you” get is dependent upon “your” processes; therefore the same set value is not possible for everyone.
You will have to determine the best value for you through some trial and error. Casting some wax wire of a known size can do this fairly quickly and accurately. Here’s how:
• Use the same type of wax used to make the pattern
• Keep in mind, round wire will produce different values than square wire due to the ways these shapes solidify
• Measure both the diameter and length
• Record the size before and after casting
The difference will be your shrinkage factor. If you take a mold from the casting, produce a pattern and cast that, your entire shrinkage factor is accounted for-more or less. Slight variations in processes from one piece to another will yield differing results but you should be in the same ballpark.
Another point to keep in mind is that, In addition to shrinkage, polishing also reduces the outside dimensions. Unlike shrinkage, this is seldom uniform. The amount of reduction is greater on the extremities since these are in greater contact throughout the polishing process. Interior corners are other areas of higher rates of metal removal due to the focusing affect of brushes and buffs.[Adapted from Model Making-Creating Master Models]