After thirty years I again tried Lost Wax casting. I still had the cristabolite enabled gypsum as investment and the flasks. I used candle wax, put them in a rubber bowl, painted them with dishwashing detergent and denatured spirit to prevent sticking of the investment to the wax castings. Then I filled the flasks with investment prepared with water which had about the viscosity of yoghurt. I put the filled flasks under a transparent plastic bowl which I evacuated using a vacuum pump which drains to less tha 10mbar (0.15psi). But then it started foaming like milk which over boiled. After allowing air again the foam collapsed. Later on, I realized that water boils below room temperature at that pressure so the investment-water mix really overboiled and steam got trapped inside the investment. But I did burn them out (1.5 hour to 200 C to vaporize water) and then to 750 C after which I poured the metal in it, one with silver and the other with copper. As expected, the castings were still full of a kind of bubbles. Should I keep the pressure not too low when evacuating ? Any ideas on this ? The failed castings: lots of metal leaked out in steam cavities.
i dont do investment casting but i think you would want the investment to stick to the wax to get into all the detailing. Putting your investent under a vacuum is standard practice from what ive learned so I dont think it wuld be causing problems like boiling. I think that frothing is just the air coming out of the investment. I know some people just wrap some thick plastic around the flask high enough to accomodate the expnding investment. Others just degas the investment in a large container before pouring, then pour gently into the flask
The use of a de-bubblizer is to release the air bubbles, not to keep the invest from sticking.. Dropping the flask in water after the metal sets will release most of the investment.. A brush will tale care of the rest.. A hot pickle with sulfuric acid and water will clean up any discoloration on silver castings.. Liver of sulfur will antique your silver castings.. Sodium bisulfate can be used instead of sulfuric acid.. A pickle made with either will still eat holes in your clothes. They wont show up until you wash them.. The solution doesn't need to be really strong. The investment needs to boil to remove the air.. If you vacuum the flask with the waxes, you don't need to pre-vacuum the investment.. Rubber Flask Extenders (riogrande.com) Tape will work for a one time use.
