I keep having small chips appear in plaster cast components. Aside from the obvious stop being so careless, I have tried preparing with thinned-down blacks of various flavours but it does not seem to penetrate the surface sufficiently. Has anyone experimented with ink or anything else to see if it penetrates better?
Guy
Thought: Mix some colouring with the plaster when casting. Then it appears "grey" or "pink" when it chips.
- Nigel
Guy,
Maybe too late for plaster already laid down, but I have a distant memory of a friend adding some potassium permanganate into his plaster mix to turn it a brown colour and prevent the white chip problem. That's from forty years or so ago and I'm not sure how easy it is to get small amounts of potassium permanganate these days, or whether Health and Safety would even allow it.
Just found this
Jim.
On Mon, 6 Apr 2009 09:31:43 +0100, "Nigel Cliffe" said in :
Yes, I thought about that, but I don't know what colouring will avoid weakening the cast, and if you use acrylics the colours are too washed out. I wonder if you can use clothing dyes in the mix?
Guy
Try googling on "plaster coloring materials" and "... colouring...". I found a good hit first try, generally you should use earth colours (water soluble formulations, I gather), as these are "lime proof." These also happen to be the colours we need most for rock and masonry castings, and such -- bonus!
You're right, additives of any kind affect the strength of the plaster, but they also affect the setting time. Eg, from [from
Interesting question, and not one often asked! The hydration reaction that results in the hardening of Plaster of Paris proceeds very rapidly as your question infers. This reaction can be slowed somewhat by the addition of several common organic acids. Tartaric and citric acids (tartaric acid in particular) have been used for this purpose, and are known as "retarders".
Regards,
Bob Wilson."
Clothing dyes are generally organic salts, so they would slow the setting time - don't know, never tried it.
IMO, the best bet to prevent chipping is to paint the casting with acrylic paint, a thin wash to penetrate, followed by a creamy mix to form a (pretty tough) skin on the plaster. That's what's worked for me, anyhow. OTOH, I don't subject plaster bits to much handling. Water colours and inks will also soak in, so you could first colour the plaster with these, then add the acrylic paint.
Another consideration is the toughness of the plaster itself. Ordinary plaster of Paris is pretty skimpy stuff. It crumbles easily, often because it had insufficient water in the mix. PoP should be quite shloopy, about the consistency of thick cream. I think people are often afraid of making the mix too wet, under the impression that PoP dries - it doesn't, it sets, and it needs plenty of water to do so. Dental plasters, while pricey, are better materials for making plaster castings. Over here, Hydrocal (tm), originally made for the plaster molding trade, is a harder and stronger version of plaster, that modellers use extensively for those reasons. I'm pretty sure that an analogous product is available in the UK. Consult as plasterer.
HTH
wolf k.
On Mon, 06 Apr 2009 11:55:19 -0400, Wolf K said in :
Thanks for this and the other info, Wolf, very comprehensive and all useful. I do use plenty of water, I learned this doing house plastering...
Guy
Cheers,
wolf k.
Just try not to get yourself plastered. ;-)
I add dry powdered poster paint to my plaster mix. Here in New Zealand the paint is sold by educational supply shops for use in primary schools= . It's very cheap. Only problem is it takes a lot of mixing to get it righ= t through.It takes only a very small amount. Haven't noticed any fading.
Greg.P. NZ
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