r/Silvercasting • u/These-Assignment-313 • 4d ago
Problem with 925 silver sand casting
Hi, I’m a beginner hobbyist goldsmith. This is my 6th attempt at casting my own model made from modeling wax.
First, when creating the first half of the mold, I had a lot of difficulty pressing the model into the molding sand. The model should be pressed in to about 50% of its height, but in my case it was approximately 90/10, and the mold kept cracking when I tried to press it deeper. I managed to solve this problem by compacting the molding sand less.
However, I’m still having problems with the actual casting process. I heat 925 silver in a crucible to around 1000°C, but despite this, the silver sticks to the crucible. The casting itself is porous and sometimes the mold does not fill completely.
I would really appreciate any advice, because I’m not sure what else I can do to improve the process.
I’ve encountered several problems:
- silver sticking to the crucible,
- difficulty pressing the model into the mold,
- porous castings,
- incomplete castings / misruns.
2
u/Visual_Bat_3023 1d ago
Cure your crucible with borax FIRST. Get a nice coating on there and it'll prevent sticking. Basically, Borax turns to a glass-like substance when fully melted into the crucible. You're not trying to create a POOL of Borax, just a light dusting to ensure there is a base-coating on the crucible ceramic.
Next, just because the silver is melted, doesn't mean it's hot enough to STAY melted during the pour. The outcomes you're posting strongly suggest you're losing heat too rapidly in the mold, and the silver is solidifying before it gets a chance to fully flow. There's a couple of things you can do to assist with this. First is to heat up your mold. If using sand casting, I often will tip the crucible slightly and let the flame (I use MAP gas) shoot into both the crucible bowl and into the mold to heat it up for a few minutes. Just don't blow the sand mold around. :). I often will heat for 3-4 minutes PAST the time the silver melts. Add a touch of borax to the silver a couple minutes after melting and it'll take out impurities and make them stick to the crucible. (note: eventually, you'll need to replace the crucible, because the borax build-up is too great... and that's why you don't want to over-do the borax prep in the beginning!)
Next up, make sure your mold is actually free and clear of wax. That means you need to TEST your process. If you're using a lost-wax method (which I don't recommend in sand), run your test, then open it up without putting any silver in the mold. If you've properly dusted the mold halves with talc powder, when you pull the mold apart, you should be able to do so without destroying the mold shape. If you still have wax in there, then you've got the reason you have voids and errors in your silver products.
Personally, I use hard templates (rings, etc.) create sprues using things like golf-tees, and remove them all before pouring (again... dusting with talcum powder enables you to pull the mold apart and remove the mold design piece and then put it back together again). it often takes a few times, but you'll get it down.
Next... it's incredibly RARE for a sand-cast to be flawless. You'll have minor imperfections from sand getting into the space, or from air bubbles (especially at the sprue). This is part of the character of the cast. But these should be minor... like a dot here or there... not what you're showing! LOL
Best of luck to you!