Chilly

I'm thinking it was only ^04. Haven't used them for a while, but it was mainly for bits for christmas mobiles, so wouldn't have needed to be be fired any higher.

Can you change the title of this post please. It will help with searching in future.

Buy a bucket of the cheapest slip you can find. Dust out a mould, apply mould straps/bands. Fill to the brim with slip. Set a timer for 5 minutes. Top up with slip, reset timer for another 5 minutes. Repeat once more. Pour slip out of mould back into container, leave inverted for 1 hour. Stand right way up. Leave until you can see the clay start to shrink away from the edge, all the way round. Undo straps. Lay on side with mould seam horizontal. Gently prise top half away from bottom half of mould. Hold a piece of sponge/folded towel covered with thin cloth in one hand. Tip bottom half of mould onto hand and catch clay model. This next part is the most important. Examine the casting to see how good it is. Moulds have a life of only 20 to 30 casts, as some ingredient in the slip eats the plaster. If the cast is good, try with the rest of them, then find somewhere to fire/sell them. Welcome to the rabbit hole called pottery.

And fill the cavity with something, sponges, crumpled towels, anything to help support the clay, so it doesn't try to collapse. Unless it is very dry, of course.

I read somewhere that you can up to one third dried slip to new slip. I allow it to dry thoroughly, then bash it (dust mask) and add it to a very small amount of water, just enough to cover - tall narrow vessel rather than wide. Then add back to the slip. I often make slip by drying clay and adding water and a couple of stops of sodium dispex.

Low fired, unglazed clay will do this. I have (commercial) red terracotta plant pots, and they change colour quite quickly when they are damp. They leach out salts, from the clay, soil and water, they grow moulds. Can't imagine any way round this at low-fire temperatures.

Try getting hold of a Stoke-on -Trent telephone directory. Even then, I think lots our-source to China now.

Very probably impossible, yes. There will be someone on this planet who will say it works, but if you do any mould-making, you will know that you have to pour a piece in one go or it won't take properly. There are situations where you can add more, but you have to make a good and substantial "key" between the old and new - usually deep grooves on the old, before pouring the new. My plaster drying/wedging slabs all look like yours. I just ignore the black holes, leave them in the sunshine to dry out, they work fine.

I had this problem on the inside of some cups. When I broke one, the glaze was too thick. Next time, I applied a thinner coating and it was better.

I own two, a genuine corde Dremel, and a battery Lidl one. Both are great and do the job. One was expensive, the other cheap.

COE comes up regularly on here and other forums. Most of us have enough trouble getting clay and glaze to fit properly, even when purchased from the same supplier and supposedly compatible. I also work with fused glass and in that community there is a very clear message to NOT mix different COE glass, or to mix any unknown glass. I'm very firmly in the "don't do it" camp. You wouldn't put diesel in a petrol-engined car, just to see what happens. Don't mix glass and ceramics, they don't play well together.

You can use any material for your master. It's how you get from the master to the mould that changes depending on what you want to cast. If your finished item is to be slip-cast, then your final mould must be plaster.

If you search this forum, using the term moulds (use the English spelling), you will find lots of posts where I have answered questions, and put lots of tips for both mould-making, and slip-casting.

Depending how much clay and glaze you have in stock, it might be cheaper to sell/throw/give it away and buy new, than to fire your new kiln to cone 10. Take into any calculations the extra cost of firing, and the wear and tear on the kiln elements.

Could be the stones. Have you fired this clay any other way? Electric kiln?