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tghattaq tghattaq is offline
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Location: Malta
Posts: 10
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Quote:
Originally Posted by Richard J Kinch
tghattaq writes:

Can anyone advise me how to avoid this occasional surface cracking on
the profile?


Cracking comes from stress. Is this induced in the casting itself, such as
from shrinkage during curing, or is it from being bonded to another
structure which moves?

Do you use a polymer admix?

I'm curious how you make these molds. What patterns from what source, what
materials, what process.
Thanks for your reply Richard.

To give details of the form work in chapter and verse. I build up the form in good, clean timber like first grade red deal. It has to be pretty good to successfully router to shape. As I stated, it is a good mental exercise to devise how this will demount leaving support underneath whilst freeing the face to dry after a couple of days. To the mould I have grain filler applied and sanded, then several coats of the sort of commercial epoxy that hobbyists use on furniture. When this is fully dried and sanded I have several coats of classic beeswax polished applied and polished with plenty of time in-between each coat. The release is as near perfect as I could wish for.

My primary source for design is Isaac Ware's 'Vitruvius Britannicus" (know I should be using Piranesi) but I never copy directly as I wish to add a slightly more Mediterranean feeling. An exaggeration. Guess I generally close my eyes and think of the Amalfi coast rather than Colchester, drink a large glass of Grappa and get stuck in. I have just cast a moulding 43cm x 380cm which came out perfectly without blemish. Cors its sod's law that this is one that I want to paint over anyway.

The surfaces that I am casting onto are totally stable. There is no movement or shrinkage what-so-ever.

I think that the answer may be to add short strands of glass fiber to the mix. We leave it a half hour running in the mixer for the moisture to be absorbed. I have found that any wetter mixture leaves nasty bubbles in the upper surfaces where excess water has accumulataed and then dried out. I do add a polymer in the form of the old-fashioned compound Pudlo which greatly increases the slump of the concrete and makes it settle very well into the complex mouldings.

Sometimes it works and at othertimes ......

I more than welcome you comments. Giles