Co-Casting
Co-casting, also known as bi-metal casting, is a terrific technique that can potentially yield quite amazing results for both you and your customer.
We are seeing more and more platinum 18 karat gold combination in both men’s and women’s jewelry. In most cases two or more pieces are cast separately and then soldered together to attain the bi-metal look. By necessity, these designs have to be relatively straightforward and they must lend themselves easily to all subsequent assembly operations.
By comparison Co-Casting can be a viable alternative that can be used to achieve much more complex, interesting and even hard-to-believe designs.
This technique requires that you prepare the wax for the platinum portion of the design, have it cast and then completely finish the platinum casting. Subsequently, the wax for the 18-karat portion is added directly to the finished platinum casting. You may need to add an anchor or two and the design will dictate just how you are going to secure the 18-karat to the platinum. In some cases, because of the complexity of the design, no anchors will be necessary at all. Also, you need to make the wax just a little bit higher than the platinum everywhere. This is to account for the small amount of shrinkage that will take place as well as the polishing that will have to be done to the gold.
When considering the overall design you can get as complicated as your imagination will carry you. Undercuts and interlocking designs are no longer a problem, as you will be working in wax and there will be no need for complex or impossible metal-to-metal assembly operations. The wax is completed just as you would any other model, except it is now an integral part of the finished platinum casting. We will add appropriate gates and cast the piece as usual. The casting of the 18-karat will actually form a metallurgical bond at the grain level and no further mechanical assembly will be required.
Platinum iridium alloys are recommended for this process as they do not oxidize. Only the 18-karat will require finishing subsequent to casting. The finishing is further enhanced in that the platinum is harder than the gold. Some care needs to be exercised, however, to prevent a “step” at the junction of the two metals because of aggressive polishing. This is precisely the reason for making the wax just a touch higher than the platinum. When finished with this technique, the two surfaces will be at the same height.
There are only a few guidelines that require your attention when considering this approach as follows:
- A non-oxidizing platinum alloy needs to be used because you want to avoid having to polish the platinum after the gold has been cast unto it.
- If the platinum piece is relatively smooth or has no irregular shape to anchor the gold, an anchor (hidden in the case of anchoring the gold to the platinum) may have to be added to the platinum to prevent the piece from moving during the burnout cycle as well as casting. If you give this point some thought it will be obvious as to what will be necessary. We can also assist you in this matter.
- Some preliminary consideration should be given to the overall design in terms of it’s ability to produce a successful casting. For instance, the gold should not take on the form of a fine wire or long thin piece. These forms should be fabricated. Another consideration is to avoid any space that remains between the platinum and the wax. Here the possibility exists that a thing “wall or sliver” of investment is formed. When the molten 18-karat is delivered into the mold, these very thin portions almost always will be broken off and will result in pits in the gold—remember, the mold is a closed system and the loose investment has nowhere to go. This situation is also present when an attempt is made to fit a platinum piece to some unsuitable injections where the fit is not complicated piece with hard to reach areas. These pieces will almost always present finishing difficulties either from loose investment or excessive gold being cast over the platinum’s surface.
With some careful attention to the details and a little experience, you can create some really amazing pieces!
Make sure to check out our Jewelry Casting section for more information about our casting services. If you have any questions concerning the above suggestions, or we can help you with any piece, please contact us. We will be happy to assist you.
