"Pond scum" mummy wrap

[John Kurman]

Reading up (or catching up?) here on what seems to be the bane of the glass casters existence: mold cracking.

Lots of good stuff here about it. Lots of important points that make a difference, re: thorough mixing of dry materials, long slaking times, and getting the water out of the mold. From all the reading I’ve done of the available literature, it seems that, when it comes to plaster/silica molds, physical structure is everything. When you take plaster/silica up to casting temperatures, say goodbye to plaster bonding. It (the plaster) breaks down, and the only thing holding the mold together is pretty much a “magicalâ€

[charlie holden]

Thinking out loud here. (Even though you can't hear me.)

I don't actually know the physics involved but I think the point of putting silica in the mix is that it forms some kind of bond at higher temperatures. So you try to offset the increasing weakness of the plaster with increasing strength of silica. After all, rigidizer is just colloidal silica. So if it binds to and rigidizes ceramic fiber after firing above 1300 F, it must be sticking to the plaster, even after the palster has been calcined.

This idea is why I think it is so important to completely mix the dry ingrediants first. When plaster hardens it grows long, spiky crystals that get tangled up with each other and therefore can't move. You want every grain of plaster to be next to a grain of silica, and vice versa. That way the plaster cystals grow around individual grains of silica, rather than being blocked by clumps of silica.

But it seems that the plaster must be turning back into powder as the water is burned back out again. And the silica is rigidizing powder rather than fiber. (I would be interested in knowing what actually happens to the crystals as they heat up.) The addition of fiber glass fibers means that there is something for the plaster and silica both to bind to that covers some distance.

So that leads me to wonder whether you couldn't just use rigidizer on your fiberglass matt? Maybe you could just layup dry, or slightly damp mat, then spray rigidizer on and cover with another layer of plaster/silica. You've already got the shape in place with the existing plaster. The rigidized fiberglass would act like a very thin shell.

[John Kurman]

I've considered using colloidal silica as a binder, but plaster/silica flour is almost free in cost comparison. I'm basically a cheapskate and am unwilling to use my colloidal on what I still consider to be an unproven technique (it has worked a dozen times with only 1 failure, but this could be dumb luck). Plus, I've found that I have a tendency to reinvent the wheel and figure the good ideas are generally already been done. And, I know others have done ceramic shell casting with glass and what a pain in the butt it is to remove the shell from the glass.

And this could all be just another case of unnecessary over-engineering. I tend to do that a lot. But, what the hey. Next time I cast something, I'll try the spray binder technique and let you know.

[rskrishnan]

I've considered using colloidal silica as a binder, but plaster/silica flour is almost free in cost comparison. I'm basically a cheapskate and am unwilling to use my colloidal on what I still consider to be an unproven technique (it has worked a dozen times with only 1 failure, but this could be dumb luck). Plus, I've found that I have a tendency to reinvent the wheel and figure the good ideas are generally already been done. And, I know others have done ceramic shell casting with glass and what a pain in the butt it is to remove the shell from the glass.

And this could all be just another case of unnecessary over-engineering. I tend to do that a lot. But, what the hey. Next time I cast something, I'll try the spray binder technique and let you know.

Hello John & All,
I love the idea of a "thinner" plaster mould. I have made plaster + silica moulds which are about say 11" x 11" x 11" (not quite a cubic foot). And man is it a PITA (a very big one) to dry the mould, dewax it and then the damn thing cracks during firing. I make the mould walls about 1.5 - 2" thick depending on projections/undercuts etc.
I have also found that after the initial splash coat of about 0.25" thickness - I spray a bit of sodium silicate then add a new layer of plaster + silica - on and on. Seems to give less cracking and the mould is not as "brittle" after the firing.
I'd love to get more details on the fibreglass mechanism if John would care to write it up - seems worthwhile since it reducres mould thickness - hence heating time/costs!!

Thanks,
Krishnan

[John Kurman]

Hi Krishnan,

Serendipity and all that. I've been "out of the loop' here for a month or so (casting bronze, busting through walls and I-beams laying black pipe for an air hose, building a sandblasting booth, - my main distractions), and just happened to catch your dewaxing thread. And now this inquiry into "pond scum".

First of all, I'd really really like to get my hands on some of that refractory mold mix from Japan that all the folks at Corning are crowing about (just had a friend return from a class there who sang its praises). This stuff, whatever it is, rumored to be used or developed by the Higuchis, is probably what you are looking for. Word is that you can make very thin molds with it. It's expensive, but you don't use that much per mold, so the cost evens out.

Second of all, I'm trying to track down another rumor about a technique to "grow" glass using TEOS (tetraethyl something or other) that is being used by the tech boys out in Silicon Valley. Supposedly it is kitchen table stuff, but I'm having a devil of a time getting info on it. Probably a wild goose chase.

Sorry to say I haven't done any glass casting for the past couple of months. I've been meaning to put the "pond scum" to an acid test, and do comparisons between thin "mummy wrap" molds and regular plaster-silica molds with equal amounts and shapes of glass - but those plans are on hold. I'm sorry but there's not much more I can report.

The technique I described is pretty well summarized above. I haven't done any "numbers" work on the stuff, if that is what you are hoping for. I'm using an old roll of fiberglass I found lying around, and more than likely, given anecdotal reports from other sources, it is a borosilicate fiberglass, but maybe not. This (boro) would explain why the dran molds hold up so well at 1650F, but then I don't know. I sure ain't going to send a piece to the lab for testing. But it's an old roll, probably from the late 70s, so who could guess.

Sorry to be of no help here, but I'll keep you posted on more rigorous tests I do in the future.

[rskrishnan]

Hello John,
Thanks for your reply. I've heard of the Japanese material - but I dismissed it as "marketing blab" but looks like there some truth to it. No ide about the TEOS - if you get me more info I can search since I'm pretty much in Silicon Valley (==> SillyValley).
Well, I plan to try the "pond scum" methodology on my next mould - will post results once I'm done with the casting.

Thanks,
Krishnan

[Suzan]

When you refer to fiberglass as a strenghthener, are you talking about pink fiberglass insulation, or something else, and if so, where is it obtained?

Cheers,
Suzan

[Ron Coleman]

When you refer to fiberglass as a strenghthener, are you talking about pink fiberglass insulation, or something else, and if so, where is it obtained?

Cheers,
Suzan

Chopped fiberglass strands are avaliable through places that specialize in making molded fiberglass products. Some boat and auto supply stores may have it too.

One place that has it is;

http://fiberglast.com/products.php?sess ... 9b34c55c16

Ron

[rskrishnan]

When you refer to fiberglass as a strenghthener, are you talking about pink fiberglass insulation, or something else, and if so, where is it obtained?

Cheers,
Suzan

Well, I've used something slightly diff from what John uses. I use the fibre glass "mat"or "mesh cloth" type of thing. It's basically a cloth with a mesh size of about 1 sq. mm (or perhaps a bit less). This form of fibreglass is a bit safer - no shards, no poky stuff, but provides similar strengthening qualities. I make small strips of this fibreglass cloth and soak in plaster/silica mix and pretty much "bandage" the wet mould - after you've added the initial splash coat.

If you feel like you need more support you could get small mesh chicken wire and sort of make a "fence" around the wax positive and then poue the plaster/silica mix. Works quite well for me

Krishnan