Clay -> Fiberglass -> Concrete

[Bluubekt]

This is my first post here, found this place through Google and figured someone here might be able to help with my process. I have read a lot of stuff on the web about fiberglassing, about modeling in clay and about casting concrete but nowhere have I been able to find specific information about combining these three materials/techniques the way I would like to.

I want to cast an architectural/ornamental item (1' long, 8" diam.) in concrete using a ridgid mold that can be re-used at least 24 times. The original for the object is made from ordinary sculpting clay (not a polymer or plasticene clay) and is shaped so that a mold can be pulled off (gets narrower towards one end and has no protruding details).

AFAIK molds used to cast concrete garden ornaments are often made from fiberglass so I was planning to use this on top of my (still wet) clay original. Should I seal this first before applying the fiberglass? Does a sealant like PVA work on clay that is still moist? Will I need any kind of mold release? The only information I've been able to find about clay originals -> fiberglass molds has been related to polymer clays and is probably not relevant in my case?

Any help/tips/comments much appreciated!

[fritchie]

Bluubekt - I have no direct experience in using fiberglass the way you were advised, but one caution comes to mind. It sounds as if you plan a one-part mold, with no separating joints. If that’s the case, be sure tiny irregularities on the sloping edges aren’t large enough to prevent separating original or casts from the mold.

The typical use of fiberglass in sculpture would be as a rigid mother mold over a flexible inner mold, with the inner mold commonly made of a rubbery polyurethane.

Hopefully you will get a reply from someone more familiar with your particular need. Participants here do represent quite a variety of experience.

[oddist]

Best advice is to go to a source.

Check out http://www.smooth-on.com/howto.htm.

They have instructions on mold making and casting in concrete.

oddist (http://www.torchandpalette.com)

[Bluubekt]

Thanks for your input fritchie and oddist. To clarify slightly the object (clay model) is "bullet shaped" or like the nose cone of a rocket, i.e. it narrows quite dramatically towards one end and I hope to acheive a very smooth surface. In the past I've many times had concrete set inside buckets (sometimes by mistake) and have had little problem getting it to dislodge, I'm hoping my mold will behave similarily.

The reason I don't want to use rubber is twofold:

Firstly I like the simplicity of using a single mold that can be slid off even before the concrete is completely set (casting will be done from the narrow "top" and the base will be left open, sealed against a piece of marine plywood). This way I'm hoping all 24 pieces can be cast and inspected for flaws in a quick succession. A bit like a kid making sandcakes!

Secondly, I'm concerned that a rubber mold will not be as ridgid/durable/exact as a fiberglass one. It is important that all the pieces have the exact same size - won't the weight of the concrete make a rubber mold change shape slightly?

Once again, thanks for your replies!

P.S. Yes, I have the Smooth-On site in my links list. I had already read their concrete guide but couldn't find specific answers to these questions.

[notso]

when using rubber moulds like this its very easy to make a box to fit the mould in (while the original is still in place) or then just build up a jacket of plaster etc around the mould, personally, i would still do the jacket in 2 piece though
then just coax your master out (just don't use this for a final, but as a back up incase of disasters)

for the sort of cast your talking about and the numbers of reproductions, you'll have no probs with a rubber mould lasting right to the end

your mould could also be made from plaster and then sealed with soap or shellac (if your sure about it slipping out easily) -this if done gently should get you through fine (esp' if its not used as a piece mould)

{i used a 3 piece plaster mould to slosh cast 50 beer bottles and they got me to the end just fine, with no blemishes}