Adding Water To Purchased Slip Yielded Greasy Surface

[eobet]

Hello!

 

I've been casting stoneware with factory made slip and it's been going well. When I pour it, it's grey and of course glossy when wet, and then dulls down as it dries and becomes white like plaster in a day or two. For one of my mould experiments, it was simply too viscous/sluggish to work well, so I added some water to the slip to make it flow easier.

 

However, the next day the surface of the result was dark and greasy/oily!

 

I have not tried to fire that result yet, as I want to glaze it and everyone says that glaze won't stick if the piece even has too much oil from your fingertips, so I come here to ask for tips on how to deal with this. Has anyone encountered anything like what I'm describing? If so, what did you do?

 

Thanks!

[TJR]

ecobet;

Casting slips are defloculated. This means that they are designed to have less water and more clay in the mix so they can cast faster and stonger. A typical defloculent woud be soda ash. There are a few more-can't think of the names. You have kind of messed up the proportions by adding water. As far as grease on the surface, this will burn out in the bisque. Not really my area of expertise-sorry.

TJR.

[PeterH]

eobet,

 

Slip can behave very strangely when it goes wrong. General advice is given in:

http://www.lagunaclay.com/support/art-of-slipmaking.php

... and there is a trouble-shooting guide on page 12 of:

http://www.wbbminerals.net/publications/pdf/casting_slip_booklet.pdf

 

I cannot see how your casting can really be oily, rather than slimy. But even if it was, a biscuit firing

would burn it out. I can imagine that it is a lot wetter than usual and in poorer physical shape (e.g.

flabby, brittle, ...) -- so drying it out and getting it into the kiln undamaged may be a problem.

 

Adding more water to a thick slip can sometimes help, but it has to be in really tiny amount. Adding

a deflocculant (e.g. Darvan) is more likely to help. [but see p12 of the above reference.]

 

As slip is thixotopic (thins when stirred) it's worth trying to thin the slip by thorough stirring (without

entrapping air bubbles); then cast ASAP. This certainly will help pouring into the mould, but if the

slip is too thixotropic it will thicken too much to pour out of the mould cleanly after casting-up.

 

Regards, Peter

 

As a point of reference my slip uses 2.1 litres of water and 2.5 Kg of porcelain powder (with factory-added

defloculant). Adding just 0.05 litres of water makes a very significant difference to its viscosity/thixotropy

(that's adding just 1% by weight of water to the mix).