It will probably work just fine with the HCl, depending on the quality of your powder. Ideally you would ball-mill it for a short time to help break up the powder and mix it well. Add the HCl to the water first, maybe pH = 4-5. Check the pH as you add powder as it will probably start to rise, you should continually add more HCl as needed to keep the pH near this level. If the pH goes basic the slurry with clump and will stay clumped, even if you lower the pH again - you'll have to start over. Your goal should be to mix >1.5g powder per g of water, and still have a fluid. 1g : 1g would be acceptable but not ideal. If you lower the pH to ~3 and still don't get good results (paste instead of liquid) I would stop because adding more acid probably won't help. At this point you would need to try a dispersant, such as ammonium poly(methacrylate).
Pour the slip into plaster molds, wait ~10-15 minutes, and pour the remainder back into the original container. Let the crucibles dry for a couple days before firing.
Keep in mind you will need quite a high temperature to get the MgO sintered. Think 1300-1600C. Temperatures at the low end of this range will give you porous crucibles, but this is not likely to be a problem for melting metals, and is obviously needed for cupellation.
Edit - as another note, pure MgO crucibles might not resist thermal shock well - i.e. don't know how they'll stand up to torch melting.
Steve
