I tried making some figurines from tin by casting them in hand-carved forms in plaster. The tin was bubbling and the surface came out extremely uneven, with bubbles covering it.
Someone off-handedly commented I failed to bake my casts, but I never got any info how to do it. What temperature, what time, what other preparations? I heard plaster becomes very brittle after thorough drying too; how to prevent that?
Plaster comes in quality ranges. The plaster sold for slip casting is a high quality and, no doubt the site or store will have an answer for the beginner.
That said, all moisture must be out or near the outer surface of the mold before pouring your metal. Part of you problem may be that you have a one piece mold. A two piece mold allows the surface area you wish to reproduce to release moisture and dry when opened to the air, whereas a one piece contains an atmosphere that is usually more populated with water molecules than the outer surface, so the path of least resistance will be the outer surface and water will try to find it and remain in the plaster.
Your oven should be adequate to the task. Heat it to 350 or whatever the site or store recommends and expect to leave it there for hours, depending on the density and size. Also, be sure the mold is stabilized with the opening facing up to permit quick venting.
When pouring your metal into your very dry mold, venting will again have a roll. Picture water shooting from a hose and then picture that hose shooting water into a glass pitcher. Your molten metal will act like the water when it hits the bottom of the pitcher. You want as little of that as possible!
To help prevent the turbulence vents are added. The vents are created by drilling a few small diameter holes directly through the image surface and into the region that will be filled with tin. Gases, including water vapor, which also cause turbulence, have an escape path and will leave just before the hot metal does. The metal will find them and the escape of some metal will reduce the problem. These vents need to be chased off, after the mold is removed. Soft metals are easy to deal with and tin falls into that category. Hack saw, files, or sand paper should help hide the blemishes created by the removal.
You need to minimise bubbles in the plaster of Paris mould. I do this by mixing the plaster of Paris in a rather runny state and after pouring into the former for the plaster tapping it to release any bubbles that have formed.
Plaster sets relatively quickly but even after setting is still wet and in thick sections takes a long time to dry adequately, which is why baking might be a good idea if you can't leave the mould to dry thoroughly.
The mould will need a riser channel, in addition to the channel you are going to pour the metal into, to let the metal flow properly through the mould (and so you can see that it is properly filled). It will also need breather channels to let the air out of awkward corners and re-entrants.
Finally after the metal is poured into the mould, if it can be done safely, the mould should be tapped to help air escape.
Something else to consider is keying the parts of the mould together so that they properly align. Normally I do this by making circular/conical depressions in the first part of the mould to be formed, so the second poured onto it forms keys with these features.
