jnapier Posted August 5, 2015 Report Share Posted August 5, 2015 Hi guys, I have some pieces I want to cast in plaster for a mould. The pieces will not be particularly flexible and I worry they wont release properly. Its a basic vase shape that narrows in at the top like a pair- this taper part is a 6cm diameter x 8cm long pipe that I feel the plaster will grip too tightly to release and slip out. I haven't had much luck with soft soap and I'm wondering could I rub a candle on my piece to give it a shiny and slightly slippery surface for the plaster to cast? I worry it will ruin the porosity of the mould though. Has anyone done such a thing? Other solutions are welcome but I'm mostly wondering if wax will ruin plaster or there are similar products that wont. Cheers! Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted August 5, 2015 Report Share Posted August 5, 2015 ?? You're not supposed to use soft soap on the mold for casting. It's for creating the plaster mould from the original. You are correct about the wax removing the porosity of the plaster. So will the soap. Your mold should have nothing at all on it. The casting slip should shrink away from the plaster as it sets up and dries. Just leave your piece in the mould a bit longer. Link to comment Share on other sites More sharing options...
oldlady Posted August 5, 2015 Report Share Posted August 5, 2015 maybe a dumb answer but in the very long history of casting molds in plaster has anyone ever suggested using wax this way? i am sure that if it had worked, there would be some reference to it somewhere.. not saying i know everything but i have never heard of this being done. wax on plaster would really ruin the plaster's ability to absorb water from whatever slip you would use. i think your mold is of something like a pear? a fruit? there is a potter who has very successfully made pears but i do not remember his name. they are usually combined on a long tray. there is a book, "250 tips techniques and trade secrets for potters" that shows a lot of plaster mold use. page 135 could be the answer you need. Link to comment Share on other sites More sharing options...
PeterH Posted August 5, 2015 Report Share Posted August 5, 2015 I'm assuming that you are trying to cast a mould from a 'found' object. - What is it made of? - How rough/uneven is its surface? - How many parts will the mould be? - Ideally, can we have a picture of the object? Regards, Peter Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted August 5, 2015 Report Share Posted August 5, 2015 Sounds like you are trying to make a one plaster mould for a vase. Are you imagining the mould to look something like this? Link to comment Share on other sites More sharing options...
jnapier Posted August 5, 2015 Author Report Share Posted August 5, 2015 Thanks guys, It was a difficult one to explain- to clarify I have a 3d printed object that looks like a basic vase like the eiffel tower shape (in sillouhette only) with a little bit of wider base. Like an 'A' with a curve. It's 3d printed in a slight rough surface because the polished version is too expensive. It's to make a mould off, NOT to cast off. The casting will be fine, but the moulding ive found is very tight around the shape and with the long 'spire' at the top I fear it will grip it too tight to release. @Joel this is what I'm after but a much longer top part, and a two part horizontal mould, not a vertical one like here. The split would be at the widest 'waist' part of the shape' so it would slip out the bottom, and other half out the top. I will try soft soap again, but I find it does nothing really for my past moulds. Thanks guys! Link to comment Share on other sites More sharing options...
BeckyH Posted August 6, 2015 Report Share Posted August 6, 2015 If you are worried about the mold catching, make it in more parts. Split the spire part from the body, and if you have to split that piece in two. But for the most part the clay shrinks a bit as it dries, releasing itself. If you want to make the surface smooth before you cast the object, try coating it with a thin layer of silicon casting material that you can get at the art supply store. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted August 6, 2015 Report Share Posted August 6, 2015 I guess the problem is because plaster does expand slightly and the long sort of nail like, I assume, top of the mould will not really want to come out. Link to comment Share on other sites More sharing options...
Marcia Selsor Posted August 6, 2015 Report Share Posted August 6, 2015 Maybe you should hand polish the form before casting. Then it will be smooth. Or you could fill any rough spot with plasticine oil base clay which is great for casting in plaster. I would coat the whole piece with WD 40 to assure release. Marcia Link to comment Share on other sites More sharing options...
Chilly Posted August 6, 2015 Report Share Posted August 6, 2015 If you are worried about the mold catching, make it in more parts. Split the spire part from the body, and if you have to split that piece in two. But for the most part the clay shrinks a bit as it dries, releasing itself. If you want to make the surface smooth before you cast the object, try coating it with a thin layer of silicon casting material that you can get at the art supply store. +1. Make the mould from 3 parts around the "spire". They'll pull away from the model more easily and when casting. Link to comment Share on other sites More sharing options...
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