Kiln wash mixed into investment

[Jenna]

Does mixing kilnwash into investment degrade the strength of the plaster? I seem to remember Cynthia suggesting adding up to 20% by weight. My molds are all small, open faced, with very fine detail so brushing kilnwash on hasn't really been working for me. Ive been using MR-97 which works really well but is very expensive and I would love to skip this step altogether.
Does 20% provide a decent seperation? No investment stuck to glass?

[Bert Weiss]

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. You will probably find these ingredients cost less when purchased separately at the pottery supply. The alumina they sell will be ground coarser than that used in the kilnwash. For investment purpose this is not at all a problem.

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.

This means I would be wary of adding any material to a commercially devised investment. Somebody already mixed in a precise amount of stuff.

[Jenna]

Thanks for the reply Bert. I have to admit that since using the R and R 101 investment, the one test I haven't done is to fire a mold without a separator. I have no idea what they use in their mix and have a feeling they aren't telling. My previous experiments (Amaco casting mix) without a separator came out with quite a bit of plaster stuck to the glass and ever since I have convinced myself that some kind of a kiln wash was required. Self inflicted paradigms can be quite hard to overcome.
I am about to load the kiln and will throw in some molds without any separator as well as some where I mixed in some Primo primer and some with MR-97.
Let's see what happens.
Cheers, Jen

[Bert Weiss]

Thanks for the reply Bert. I have to admit that since using the R and R 101 investment, the one test I haven't done is to fire a mold without a separator. I have no idea what they use in their mix and have a feeling they aren't telling. My previous experiments (Amaco casting mix) without a separator came out with quite a bit of plaster stuck to the glass and ever since I have convinced myself that some kind of a kiln wash was required. Self inflicted paradigms can be quite hard to overcome.
I am about to load the kiln and will throw in some molds without any separator as well as some where I mixed in some Primo primer and some with MR-97.
Let's see what happens.
Cheers, Jen

Primo primer is different. I believe it is simply alumina hydrate with an organic binder.

[Jenna]

I saw a web page mentions that skutt kins had sent out a warning that alumina hydrate radically shortens the lifespan of your electric kiln elements. Anyone heard that before?
http://www.potters.org/subject00470.htm

[Morganica]

Yes, you can add kilnwash to R&R910. Works fine, but not 20%, more like 3-5%. What works better, though, is to kilnwash an R&R910 mold after it sets. I use a thinnish coat of Bullseye kilnwash. If the mold shape is appropriate, I'll dump in the whole batch, swirl it around and pour it out about 30 seconds later. If not, I'll brush it on very lightly. A single coat is usually sufficient, and doesn't hurt the detail.

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. The binder (in this case plaster) holds the mold together, the filler bulks out the mix to help reduce the tendency of the plaster to shrink and crack, and the additives change the mold characteristics in lots of different ways: Reduce cracking or shrinking, strengthen or weaken the mold, harden the mold for better detail, increase elasticity or permeability of the mold, etc. Additives can range from cements to alumina to talc to kaolin to bran cereal.

It's the ratio of all three components, not just the amount of plaster, that's important.