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I have a large rectangular plaster tile with an interesting relief pattern of bumps, dips and waves, but it's broken and I need to make a copy of it. It's not a bad break, 3 clean snaps making 4 pieces that go back together easily

My plan is to:

  • Make a wooden frame around it, a few inches higher than the highest relief and lined with a strong plastic membrane
  • paint it with several layers of latex
  • talc coat and peel this latex off and then lay it back on so it's loosely on the original
  • mix up some normal drywall plastering plaster (I've got loads kicking around, too old for plastering the wall with) or sand/cement mortar mix
  • pour this into the mould on top of the latex and let set to form a supportive reverse mould
  • lift off the plaster/latex reverse mould and separate the latex/plaster from each other
  • flip the whole thing over and reassemble the reverse+latex+frame and set about filling it with plaster of Paris and maybe some gauze/mesh/glass fiber weave that I have to add a bit of reinforcement
  • separate the newly cast panel from the mould, peel the latex off it and repeat if I need more

Have I thought of everything? Will the latex separate easily from drywall plaster/plaster of Paris? Do I need a release agent? Will my chosen materials suffer any shrinkage or warping (they don't seem to when left in a bucket after a plastering job; it'd be a lot easier to clean if they did!)

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You're on the right path; the only things I would change in your plan would be not to remove the latex mould (often known as a print mould) before adding the supportive plaster mould (known as an overmould or mother mould). Shawn Thorsson's blog has a good tutorial on one piece moulds - he works in silicone rubber and plastic, but the principles are the same.

Plaster* and latex don't stick together much, so you shouldn't have any issues without applying talc or another release agent as long as you let the plaster dry for long enough, but there's no harm in applying one - the Hirst Arts website has a good guide to them.

Both latex and plaster do shrink a little, so consider applying multiple coats of latex to get the correct thickness when building the mould. Since you're doing a one piece mould, you can deal with the plaster shrinkage by over filling it and then scraping off the excess. Once again, Hirst Arts has a good tutorial on casting one piece plaster moulds.

*Admittedly my casting experience is solely with dental stone, which may be less 'sticky' than drywall plaster. Your mileage may vary.

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  • So you reckon I'll be able to separate the original tile from the latex, if I don't first peel the latex layer off it after it's set.. i.e. if I go straight to casting the supporting mould on top of the latex, I'll still be able to get them apart easily? There are no features of the pattern that would lock them together physically (no part of the relief has any undercuts etc), it's just quite a large tile (more than a metre by a metre) that i'd be separating from the latex. I figured i'd have to peel the latex off while had the flexibility of it being thin, but i'll give it a go without..
    – Caius Jard
    Commented Aug 21, 2017 at 20:14
  • It should be all right. The issue with trying to cast the mother mould without the master, is that the latex is very flexible and will almost certainly deform without the master to keep it in shape. If there's nothing locking the print and mother moulds together, you can take the mother mould off, then remove the master from the print mould. The large (and flat) size should make it easier as you won't need much deflection to pop it out.
    – walrus
    Commented Aug 22, 2017 at 8:27
  • Oh, I was thinking to peel the latex off the master to break any adhesion between them, and then immediately re-lay the latex back onto the master in the exact smae oritentation that it just came off with, so the master is supporting the latex but definitely not stuck to it, then pour the mother mould drywall plaster over the latex+master pair.. In the same way before I begin casting copies, I would delaminate the latex from the mother mould but then lay it back on - it should ensure I never end up with a situation where two lumps of plaster are adhered to a latex interim (however loosely) ?
    – Caius Jard
    Commented Aug 22, 2017 at 9:19
  • OK, I'm not sure if that would provide any real benefit but I can't see how it would cause any problems. Do comment back with your results, I'm interested to see if it works as well with drywall plaster as with dental stone.
    – walrus
    Commented Aug 22, 2017 at 9:39
  • Just waiting for my drum of latex to arrive, then I'll post up some results :)
    – Caius Jard
    Commented Aug 22, 2017 at 9:49

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