Platinum Do’s and Don’ts For Wax Model Preparation

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!

Platinum Durability and Scratching One of the most frequently asked questions about Platinum concerns scratching. It seems that the consumer equates durability with scratch resistance and is then disappointed when Platinum jewelry scratches and show signs of wear. Let me say that all metal scratches. From the finest steel to the finest Platinum, no metal is immune to scratching. When two materials come into contact, the harder one will always prevail. Whether you use a kitchen utensil or a wrench while wearing a platinum ring, a scratch will result if contact is made. The same is true for door hardware, garden tools and any other item that is harder than Platinum. Since these are the objects that we come into contact with each day and impart scratches, the Platinum will develop a patina-like appearance that will last. This is also the reason why many designers prefer satin finishes, brushed and sand-blasted finishes over the bright polished look. There are, however different alloys that have a different hardness and thus a greater scratch resistance. This does not mean that any of the alloys are completely impervious to scratching, it only means that some alloys have somewhat more resistance to scratching. Another point to keep in mind is that there is a difference between strength and durability. Strength is measured in pounds per square inch (PSI) and scratch resistance is determined by hardness. The following is a comparison of some Platinum alloys:

Finishing platinum is really no more difficult than finishing any other precious metal. It is, however, quite different. With a little practice and a proper understanding of the procedures and materials, a very fine finish and a histrous polish can be achieved. This will take time, effort, technique and the right polishing materials. When we talk about the jewelry finishing and polishing process, we are discussing the reduction of scratches and the removal of small pits and other blemishes on the surface of the metal. A finish is defined as the surface treatment that will alter the appearance of a cast product. A finish is defined as the surface treatment that will alter the appearance of a cast product, with techniques such as Florentine, matte, brushed or hammered or otherwise textured in some fashion. Polishing, on the other hand, creates the mirror-like luster on the metal’s surface. Until the decision is made to either apply a mechanical finish or a high luster, all the initial steps of preparation are the same. Platinum’s density and scratch resistance require a succession of finer and finer abrasives to reduce the scratches until they are microscopically small, thus presenting a high polish to the eye – a luster that will outlast any other shine. The surface should be mirror-like, without any blemishes or waves. There should be no visible porosity. After the removal of the sprue or after a repair is completed, use minimum filing to create a smooth surface. Just remove the high spots with a file. Be sure the file is cutting forward strokes and is not dragging across the platinum on the return stroke. This technique will prevent smearing as well as clogging the file. If porosity is encountered, a highly polished tungsten burnisher should be used to burnish the porosity away. A small amount of lubricating oil, such as oil of wintergreen may be used to facilitate the burnishing. The burnishing will work harden the surface of the casting, this will make it somewhat easier to hold a high polish. In some cases it may be necessary actually fill porosity by fusing small amounts of metal into the affected area(s). The metal used in this operation can come from the small sprue that is removed in the early finishing. Use progressively finer abrasives, such as silicon grade carbide or alumina papers on a sand paper stick. Grits 220, 280 and 320 are most commonly used. Unitized wheels in grits 400, 500 and 600 can be used to create even finer scratches. Sand papers are also available in those grits. As you switch from one grade to the next, sand at 90 degrees to the previous direction, removing all scratches from the previous grade. To prevent cross-contamination from gold or other metals, segregate all abrasives for platinum use only and keep them, as well as other tools in a separate location. There are some new tools on the market where ceramic particles are bonded and mixed with a rubber carrier material. The ceramic surface is quick to cut without clogging. The different grits are quickly identified by color coding. Red and blue are coarse, #80 and #120, green and purple are medium, #220 and #320, and orange is a fine grit at #500. They are mounted on 3/32 shanks for flex shaft motors and can be run up to 30,000 rpm. After all of the surfaces have been prepared to a uniform 600 finish, with crisp edges, the actual polishing process can begin. Using a hard felt wheel, charged with a polishing compound, a progressively finer scratch can be obtained. There are many compounds that will accomplish this job, however we recommend the compounds supplied by Gesswein. The gray 800 grit compound, followed by the white 1500 grit, then the white 8000 grit is a way to create a terrific polish on platinum. The final luster can be achieved using the carrot or orange rouge compound. It is important not to cross-contaminate individual wheels and to clean the jewelry piece prior to using the next finer compound. In other words, if a wheel is used for 8000 white compound, do not use it for any other compound or for anything other than platinum jewelry. It is helpful to polish individual components prior to assembly, while they are easily accessible to the polishing wheel. It is especially important to pre-polish the platinum if gold pieces are to be joined to the platinum. This is because it takes more steps to polish platinum and one would tend to over-polish the gold. Because platinum does not oxidize during the fusing/welding process, the finish will not be altered at assembly. A firecoat of boric acid and denatured alcohol is recommended to protect the gold components from oxidation. If you follow these instructions, polishing platinum will not be that complicated. All of these steps will become second nature and the results will be a luster that can only be achieved by platinum. 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.