Casting Slip

[Momo]

Hello,

I've been making casting slip using only sodium silicate. I would like to make some more today but am almost out of sodium silicate - I do have a lot of soda ash though. Is it possible to make a good casting slip using only soda ash?

 

Thank you in advance.

[Min]

I wouldn't do this, you'll likely land up with a bucket of very sticky slip. I'ld wait until you get some more sodium silicate or dispersal / darvan and mix it properly.

[Mark C.]

wait and get the right stuff

[Momo]

OK. Thanks. Will do. But what exactly is soda ash for?

[Callie Beller Diesel]

It’s used more as a glaze ingredient, or in soda firing. When it’s in water it’s very soluble, so it definitely has an effect on rheology, but it behaves differently than sodium silicate. Like Min said, it’s better to use the ideal tool for the job.

 

[Jeff Longtin]

For those with a similar question I'll share this: soda ash is a very strong deflocculant.  It is rarely used on its own and commonly paired with sodium silicate. ("N brand" as I know it.)

From what I've read the sodium silicate works on the easy to defloccuate particles while the soda ash works on the harder to deflocculate particles. (As there aren't as many of those, in a casting body, you don't need as much.)

Darvan 7 and 811 really came on the scene, for potters anyway, in the 80's after the publication of the book, "Moldmaking for Ceramics", by Donald Frith.  (A wonderful book for those just starting to make molds.) Until the book came out most casters used the sodium silicate/soda ash combo to deflocculate clay.  (Also adding barium carbonate to iron containing casting bodies, to reduce scumming.)

The other reason soda ash is only used in small proportions is that it is very caustic. Caustic to humans and caustic to plaster molds.

 

 

Edited January 17, 2022 by Jeff Longtin

[Alan WS]

Sodium silicate is strongly alkaline, and is considerably more expensive than sodium carbonate (soda ash).  The clay particles have positive and negatively charged regions that stick together: binding of sodium silicate (or sodium polyacrylate for Darvan) to the positive regions makes the clay particles overall negative so they repel and remain dispersed.  My assumption has been that the reason to add soda ash is to neutralize any acids with a less expensive base (alkali) than the silicate, which needs to remain alkaline to be negatively charged.

Darvan, being less alkaline, does not need sodium carbonate, is safer and less harsh on the plaster molds so they can last longer.  It is even more expensive to use than sodium silicate, but has a more gradual deflocculation behavior so it's easier to hit the spot you want.