Casting

[Laurence Finston]

The depressions turned out alright, but with the domes, the papier-mâché stuck to the bottom of the depressions in the plaster molds. It's possible to touch them up after removal and filing and sanding is probably necessary anyway. Making round things is usually a challenge with most materials, at least the ones I've tried, and even if the very tip sticks to the mold, what remains is still a good basis. However, this isn't a very satisfactory result so I will try to improve it.

I think the paper may have stuck because the molds weren't completely dry. I have now let them dry for almost a week and they seem to be fully dried out now. I've filled the depressions with papier-mâché and will let it dry for a good long time. I've found that it dries more slowly than I expected, especially the paper within the depression.

I have a couple more ideas if this doesn't work:

1. I could try making a two-part mold, with the parts side-by-side. Then it might be possible to work the cast out without it tearing.

2. I could try using a release agent, such as wax, green soap or vaseline. I'm reluctant to do this because it seems to me that sealing the plaster would inhibit its ability to absorb water, thereby losing one of the main advantages of using it in the first place.

3. I'm not obsessed with making everything out of paper, so I could use something else. Making plaster casts from either the negative or positive mold should be no problem, so I could just make domes and depressions out of plaster. Once they were cast, I could hollow them out with gouges and/or files. Another possibility would be thick foil or thin sheet metal (which are the same thing), which could be painted. The dapping tools could be used for this directly.

[Laurence Finston]

Actually, I don't think ideas 1 and 2 are very good. I think in a two-part mold, the paper would stick just as much as it does in a one-part one and I don't think the idea with a release agent is worth trying. So, I'll let the current casts dry completely and try to get them out and see if it worked. If it doesn't, my conclusion would be that paper is not the right material for this purpose. However, it will have been worth a try and I can use the molds for plaster, concrete and possibly acrylic.

[WVA]

I would suggest you try waxing the mold (after it is completely dry). If you will use a hard wax such as Johnson floor wax, it will dry to a hard finish, and not transfer to your part. I have used this before, and it works well. One thing I would warn you about is shrinkage! After a week or so, your part will be nowhere near the original size. Jim

[Laurence Finston]

Originally Posted by WVA

I would suggest you try waxing the mold (after it is completely dry). If you will use a hard wax such as Johnson floor wax, it will dry to a hard finish, and not transfer to your part.

Thank you, that's a good suggestion. I have some carnauba wax, which is the hardest known natural wax and is definitely harder than the other kinds of wax I have. It also has a higher melting point and doesn't melt well in the hot water bath: the wax at the top of the container starts to harden while still in the boiling water.

I am worried about the cast drying even more slowly if I seal the plaster, though. However, I do plan on trying this.

I have used this before, and it works well. One thing I would warn you about is shrinkage! After a week or so, your part will be nowhere near the original size. Jim

I hadn't thought of that. I knew about papier-mâché shrinking but failed to make the connection. That's another argument for using a different material, because for this application, I really want a result that's as accurate as possible. I suppose it would be possible to wet the dome, apply wet papier-mâché to it and press it into the mold again and repeat the process as often as necessary until the result stabilizes at the right size. I'm not sure this would be worth it, though.

I had another idea: Since the "caps" or "craters" or "depressions" turned out well and can be separated easily from the mold, I thought I could make one of them, adding some color to the papier-mâché in the form of tempera paint or powder. Then I could build it up so that it was more stable or add a plaster "jacket" and use it as a mold. I could add white papier-mâché and when it was dry, I could remove as much of the mold as possible by cutting and then use files and sandpaper to get down to the cast. That still leaves the problem of shrinkage, of course.

[Laurence Finston]

The dome from the mold made out of alabaster plaster popped right out, after I'd waited for it to dry completely and loosened it with a plaster knife. There is a small hole in it, which, however, can be easily patched. So, the proof of concept has worked, I won't need to use a release agent or any of the other possibilities and I am extremely pleased.

On the other hand, the domes in the molds made of ordinary modelling plaster are stuck tight and I'll have to chisel out the paper. My guess is that the pores of the modelling plaster are larger than those of the alabaster plaster.

However, this isn't a problem, since I can just use the original positive plaster molds to make negative molds with the better plaster.

[Laurence Finston]

I'm not sure whether there would be any potential interest in this and maybe I'm getting a little ahead of myself here, but I've rented some webspace for an online shop and plan to start trying to sell some artwork there, primarily prints. However, I also plan to offer cast objects as well as plaster casts themselves for sale.

While these are still in the prototype stage, I would sell them for cost plus a few dollars or euros to compensate me for my time. "Cost" would mainly be the packing material and shipping, since the materials themselves don't cost much and I won't charge for stuff that I've already had around. If I do manage to produce a really saleable product, this will change, since it's work that requires a certain amount of skill and time.

This is all at a very early stage, so if anyone is interested (you never know), please either reply on this thread or send me an email. I've put my email address here: Laurence Finston's Online Shop (it's down at the bottom).

I've turned off private messages on this forum because I'd rather just get all my messages at one place.