WORKING WITH 18KT ROSE GOLD
Rose golds can cause problems when allowed to cool slowly, as do most castings before being quenched. The problem is caused by the ability of a gold/copper alloy to from an alternate crystal structure, which differs from usual structure of the metals. This structure forms generally at temperatures below about 800F, and is highly brittle and unworkable. This problem is especially noticeable with 18K rose gold containing only copper and gold, which can be made so brittle by this problem as actually shatter if dropped on the floor.
There are two general cures. One is to use an alloy which still contains enough silver, in addition to the gold and copper, to avoid most of this formation.
But the other is simply properly annealing the metal after casting. Assume that the raw casting is hard and brittle. Before any working is done, anneal by heating to a dark red (around 900F). Allow the red glow to just barely disappear and then quench. And here’s the trick. If you quench in water, you run the risk of sometimes cracking metal. So instead, quench in alcohol. Be careful not to set the alcohol aflame with annealing torch, and plunge the hot gold into the alcohol quickly, so it’s immediately totally immersed. Done that way, the alcohol will not ignite (be sure your alcohol is in metal or glass container, not plastic, so hot metal won’t damage the container, be sure it’s on a firm safe surface so it won’t knocked over, and have a lid handy. Should it catch fire, don’t panic. Just drop the lid on the jar). It cools the good more gently than does water, so no cracking, yet fast enough to deny the gold a chance to recrystalize in that brittle structure. If, during later working of the casting, such as soldering operations, you are again heating the rose gold, you should then again quench. Don’t ever allow it to slowly air cool, if you can help it, and your brittleness and cracking problems should go away. Be aware too, that danger is hat if you work the original casting without annealing, even if it doesn’t appear to fail, you might from micro cracks in small hardened areas of what may otherwise appear to be a reasonably soft casting. Those cracks don’t then heal if later annealed, but then open up at a later time. So simply be sure that you never take chance of forming such micro cracks, by being sure that any time you work the metal, it’s properly annealed and quenched. If you’re working on a piece that already has stones set or otherwise be quenched, you may be able to avoid most of the problem by chilling the ring quickly with compressed air, instead of quenching, but this is likely less reliable.
As to your casting defects, examine the usual suspects of spruing and burnout. Be sure your melting flame is only slightly reducing, not extremely so, (molten copper can dissolve some gasses, notably hydrogen, which can lead to annoying porosity). Use enough flux melting to keep the metal clean, and be sure not to overheat the metal when melting. Some rose gold casting alloys have less it the wax of deoxidizers than normal yellow golds, as the deoxidizers can tend to reduce the red color, so you have to be more carful in how you handle it. And you may notice that some rose golds tend to melt suddenly, with less of a “slushy” stage. These alloys, when cast, also solidify more suddenly, which makes proper sprueing all the more important. If you feel this may be part of the problem, one thing you may try is to increase the flask temp a hundred degrees or so, then when getting ready to cast, pull the flask out of the oven a couple minutes earlier, setting it sprue hole on an insulating board. That allows the back end and outer surface of the flask to start chilling, but the center core of the flask will not loose much heat. When you then cast it, you’ve set up a temperature gradient between the center of the flask, where the sprue and button are and the outer portion of the flask, into which the models extend. That will help to increase the degree to which metal solidification occurs progressively from the end of the model towards the sprue and button, and this may help reduce shrinkage porosity and sprue related problems.