organic burn out

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ulrike preuss
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Joined: Sat Aug 02, 2014 9:18 am

organic burn out

Post by ulrike preuss »

hi, what is the best way to prepare a mould for an organic burn out, in this case a large thistle, hair sprayed to toughen, seeing as it has to go through 2 firings, one for the burn- out and 1 for glass casting.
plaster and flint- hand built or poured?
and/or added material to strengthen mould? glass fibre bands?
or just plaster and flint with chicken wire around it?
thank you for all replies but only if you know, thanks.
Morganica
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Location: Portland, OR
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Re: organic burn out

Post by Morganica »

It depends on how much ash the plant part generates and what type of glass you are using. I do something similar, a pate de verre student project, where we use weeds, seed pods and other garden material to make bas relief panels, like these:
Image
These are open-faced molds, and we generally pull out as much loose material as we can, then do burnout and glass in the same firing. The material will burn out around 700F (or lower); the glass doesn't come down into the cavity until around 1150F or so. Typically, there's not much ash left, and what there is is pushed out of the way. There's a residue of ash on the glass surface, which rubs off.

If the object is larger and the mold is closed (uses a reservoir with a relatively small opening), as I suspect your project might be, I fire the mold empty to about 750f, with the lid cracked to 2-3 inches and all vents open (if you have a vent system, so much the better). If I'm using fine frit or powder, I'll add the glass at that point. If I'm using larger pieces of frit or billet, which might thermal-shock, I either heat them in a small, separate kiln or at the other end of the kiln, away from the mold.

Once the material is burnt out, I'll open the kiln and blow out residual ash from the mold (gently), then add my glass. If it's been heating in the same kiln, I'll wipe off the billet. I've never had much problem doing it that way. I know some plant materials contain silica, which could be an issue.

Personally, I prefer hand-built molds because they provide more uniform heating/cooling (important if your thistle has variable thicknesses) and I have more control over mold strength. I reinforce my molds with fiberglass fabric strips, soaked in refractory plaster and wrapped around the second and third mold layers. If I am dual-firing a mold I allow sufficient cure time (at least 48 hours), I'll make the strength layers a bit thicker, and I will keep the burnout cycle as low as possible.

I never use chicken wire for reinforcement; it tends to add flashing along the wirelines.
Cynthia Morgan
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http://www.morganica.com/bloggery
http://www.cynthiamorgan.com

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