Excuse me if I use the wrong terms. I am working my way through mistakes in learning casting aluminum and brass for various glass related purposes. (link below) Brass (usually smaller and invested) is coming out fine. Aluminum giving me more headaches. Two problems focused my attention today. 1. If investment casting a cup shape, should the spuing and venting be set up so the cup is upright when the metal is poured or bottom side up. IOW - should the metal fill the form from the bottom of the cup to the rim or from the rim to the bottom (with different venting of course) Or should it be cast on its side - I hope not considering investment sleeves I have. 2. I have been trying to sand cast a half bottle shape, to make a pair of molds to blow glass into. Since I don't care what the back looks like, I had expected to pour an open casting on the sand. So after I remove the positive, I have this rectangular opening with a mound of sand in the bottom that is like half a bottle. When I pour the aluminum, it goes great until it covers the top of the bottle shape, at which time, great bubbles form in the aluminum - 1 to 3 inches across. I guess I know what is happening - moisture is coming out of the sand at the highest point. But I have the sand just damp enough to hold together. Do I have to put a top on this thing - cope and drag - and sprue it? Haven't gone that far yet.
-- Mike Firth Hot Glass Bits Furnace Working Website