Jump to content
Sign in to follow this  

Adding Water To Purchased Slip Yielded Greasy Surface

Recommended Posts



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?



Share this post

Link to post
Share on other sites


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.


Share this post

Link to post
Share on other sites



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


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



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).

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  


Important Information

By using this site, you agree to our Terms of Use.