Advice on a generic recipe for mold mix, please

[Buttercup]

After several days reading everything I can find on mixes for reusable slumping molds, not necessarily deep casting, I am thoroughly confused.

I have narrowed the results down to the following excerpts. Please pardon the editing, but I excluded references to brand-name products as I'm looking for generic, readily-available products I can buy locally. I like the idea of the mold having a 'built-in' release, that is not needing KW, and wonder if I'm dreaming. I like the simplicity of Wallace Venables 2004 post and wonder if alumina hydrate could be substituted for the 5% KW but also wonder if the later posts are more reliable mixes?

(I also came across a much older post by 'mikefromitaly', on a different board, asking more or less the same question but not getting a clear answer.)

by Wallace Venable » Tue Apr 20, 2004 6:48 am
I use equal parts of:
Plaster of Paris (from Lowes)
Diatomateous Earth (swimming pool filter media from Walmart)
to which I add about 5% kiln wash (Hot Line or Bullseye)

It may not be "the best" mix, but the only exotic stuff is the shelf wash, which you should already have.

This mix can be reused several times if kept below about 1450° F.

I've run over ten slumpson a single mold when bending stained glass panels for "Tiffany" replacements.

by Morganica » Thu Jun 07, 2012 7:55 am
......I get the cleanest, clearest results with pieces made from a single billet, dripped into a pre-fired mold containing little or no silica flour.


Re: Kiln wash mixed into investment
by Morganica » Thu Jun 07, 2012 8:14 am
....I don't add kilnwash to all mold mixes--it doesn't buy you much once you get past the face coat so it's a waste of money to use it anywhere else. In face coats, it does indeed facilitate release of the glass, especially in pate de verre molds. BE kilnwash also tints the face coat pink, which makes it easier to ensure that you've fully coated the mold with the next layer....

......There are three components in a mold mix: Refractory filler, binder and additives.........
.....It's the ratio of all three components, not just the amount of plaster, that's important.

by Bert Weiss » Thu Jun 07, 2012 1:50 am
Standard kilnwash is a 50/50 mixture of alumina hydrate and kaolin. Both of these are reasonable substitutes for silica flour in an investment recipe......
I once did a lot of research in to investment recipes. I added about 10% kaolin to my mix. I did not add alumina, but it is on the list of workable materials. I know people who use it. The most important ratio in an investment mix is the percentage of plaster by weight. The rest of the stuff is filler, designed to be able to exit steam without cracking.

Bert

As noted above, I'm looking for generic components and sincerely appreciate any advice, especially relating to ratios and prep. Jen

[Bert Weiss]

Jen

The key word in your needs is reusable. The standard plaster/silica - with variations - mix is basically a one shot mold with a few exceptions where the glass is easy to remove and the mold is not moved. What you want is a refractory cement mix. I have never seen a recipe posted for how to make one of these. In the USA there are 2 commercially available mixes I know of, Castalot and Best Mix. They are pretty expensive, but they are durable. If somebody has a recipe to make one with basic ingredients that are available from a refractory supplier, I'd like to hear about it too, along with some of the issues about how the recipe is formulated.

[Morganica]

"Permanent refractory" and "needs no release" are pretty much mutually exclusive, which makes sense if you think about it. Castalot and Best Mix both require kilnwash or some kind of release.

Nothing should go into a mold mix without a reason, and the only time I use a "generic" mold mix is when I'm doing small, fast stuff that I don't want to think much about and doesn't need to be all that strong. In those cases, I'm using 40% hydrocal, 45% silica flour or alumina (whichever I have on hand), 10% kaolin (which improves detail), and 5% BE kilnwash.

[Buttercup]

Cynthia and Bert, thank you so much for your prompt replies.

Thanks Cynthia for the basic recipe and the advice. Is that pre-fired before using or just thoroughly dried then kiln washed?

Bert, you've given me something to think about regarding refractory supplies. I'll see what I can find out locally and report back if I get any info.

Applying KW isn't in itself an issue.It's getting hold of the commercial stuff.

Thanks again, it's much appreciated. I'll try your 50/50 Alumina hydrate and kaolin mix. I have both on hand.

Thanks again, your input is much appreciated.

Jen

[Bert Weiss]

Cynthia and Bert, thank you so much for your prompt replies.

Thanks Cynthia for the basic recipe and the advice. Is that pre-fired before using or just thoroughly dried then kiln washed?

Bert, you've given me something to think about regarding refractory supplies. I'll see what I can find out locally and report back if I get any info.

Applying KW isn't in itself an issue.It's getting hold of the commercial stuff.

Thanks again, it's much appreciated. I'll try your 50/50 Alumina hydrate and kaolin mix. I have both on hand.

Thanks again, your input is much appreciated.

Jen

Jen

When I first got the kilnwash recipe, I started making my own. I soon figured out that the grade of alumina hydrate I got from my local pottery supply was a bit coarser grind than what is used in the commercial Bullseye mixture. So, I set about to find a finer grade. I was successful finding this. I have no idea what is available down under. I ended up with a finer grade than the BE. I like it. Somebody got an extremely fine grade and reported problems, so I guess there are limits. The stuff I use is rated to some percentage sitting on a 400 mesh screen.

[Ralph]

ButterC - about local refractory sources...
Walker Ceramics has calcined alumina in 100# and 300#. Qld Potters Supplies should also have both. They'll also have alumina hydrate but you're buying quite a bit of water. Zirconium silicate (zircosil 5), a very fine ceramic opacifier is also available. It's refractory, but the particles are so fine that they get picked up at the glass/mold face regardless. You should be able to get what you need with calcined alumina.

Particle size is very important in casting refractories. It's a trade-off situation...fine particles help with detail but often marr surface finish of the casting.

I've used mixes around 50 parts of 300# calcined alumina with 50 parts of potters plaster by weight extensively in open-face molds. Excellent detail -- shows fingerprints, but single use only.

If you want re-usable, one approach is to mix clay (kaolin and/or ball clay) with calcined alox. Increasing additions of clay will progressively strengthen your molds. Try Eckalite kaolin first - even up to 50% by weight. Form your molds from a plastic mix or make casting slip. If using plastic mix, ball clay will improve the "formability".

You'll need to carefully dry, then pre-fire the molds -- essentially a bisque firing -- with the usual precautions. Don't make thick cross sections. Ask a potter if you're not sure.

Depending on the clay-refractory proportions you may need to spray a very fine layer of 300# alox mixed in CMC as a separator on the finished molds. This needn't be thick enough to obscure detail. Higher proportions of clay in your mix will strengthen the molds, but you'll be increasing the chance of glass sticking and the molds will shrink more when drying. Higher-clay molds should last a long time -- possibly indefinitely.

Depending on the glass you're casting (both in composition and whether frit or sheet) it should be possible to find a mix that does what you want, possibly without a separator or just a few grains thick. I've found BE glass works well, with less sticking in small crevices. 96 seems to pick up more refractory. Frit applied wet also picks up more refractory than frit applied dry. Trade-off again -- wet frit provides better detail but the surface is not as good as dry. Sheet glass is hard to get deep down in narrow places because of surface tension but gives the best surface, at least at my temperatures.

Of course you could simply use clay bisque if your designs allow - but then you'll certainly have to spray on separator.

[Bert Weiss]

We have discussed this before. In the USA, calcined alumina is very expensive and not necessary. In Australia, the cost is far more reasonable.

Thanks Ralph for the idea of adding clay to a plaster mixture. I wonder about using clay and refractory cement?

[Buttercup]

Thanks Ralph and Bert for the additional input. I probably should not have posted my query in the kiln casting forum but that was where I was reading about molds. As mentioned in my original post:

...''mixes for reusable slumping molds, not necessarily deep casting"....

I don't have a pottery kiln so can't high-fire molds before using them for slumping. With all the good information that's been provided, along with an informative PM, I think I have a clearer picture of the possibilities and limitations. Much as I'd like to make an indestructible mold I don't plan production runs at this time, just to be able to make 5 or 6 pieces using the same shape.

Does anyone (Cynthia?) know if substituting Bert's 50/50 KW formula for the 5% BE KW in Cynthia's formula would work, not for strength, but for separation?

The alumina hydrate, kaolin and plaster of Paris have all been stored in less than ideal conditions. Does putting them in the kiln at a low temp, with the lid cracked, dry them out sufficiently to make them useable, or are they doomed once they have been exposed to damp? I know they're inexpensive to replace but I'll feel environmentally irresponsible if I toss them.

I'll certainly keep all this good information as there could be a pottery kiln in the future. I keep looking. i appreciate the local supplier info, too, Ralph.

Thanks so much again,Jen

[Peter Angel]

An alternative is to make the slump mold out of clay, and then fire the clay a little bit hotter than the temperature you will fire the glass.

You will need to use kiln wash.

Peter

[Buttercup]

Thanks Peter. I don't want to fire clay in my glass kiln as I've heard mixed reviews on doing that. I believe a bisque firing has to go to 1730F. If and when I do get the pottery kiln that would definitely be an option or I might consider getting it fired elsewhere meanwhile. Jen