additions to slip

[Gonen]

I want to cover a vase with combination of different clay slips (terracotta , black clay, porcelain , RIO clay slip , and cupper oxide clay slip  ) on different spaces over the clay body - cone 6 ox 
I want a shiny met look like the one of terra sigillata  

what do you think I should add into the slips ? and in what proportions ? silica ? fieldfare or maybe a frit ? 

thanks 

Edited December 25, 2022 by Gonen

[Hulk]

Hi Goren,

I don't have a depth of slip knowledge and experience to draw from*, just wanted to suggest that you run trials on test tiles or test pieces - same clay body, of course - before committing to your vase.

*I do use a red slip, which is made from a red stoneware that I use, with the sand sieved out. I apply it to just trimmed ware, usually with a brush, with the ware on the wheel.

[Callie Beller Diesel]

If you want a finish similar to terra sig, you’ll need to use a slip that has a finer particle size, so sieving as Hulk suggests is a good place to start. After that, you’ll probably have to add some kind of flux to the clay you’re using as slip. You really will have to do some methodical testing on those clays to see what works best, because how it turns out is going to depend on what you use. Your best bet is to set up some line blends, adding frit or feldspar to your clay in maybe 2% increments to as much as an additional 20%. That 20% is probably going to be overkill, but it’ll give you information about what happens.

[Min]

Have a look at vitreous slip or vitreous engobe recipes. When you put the slip on the pot will make a big difference in the composition of the slip/englobe. The drier the unfired clay is the less plastic materials you want in it to prevent it cracking or shelling off the pot while firing. If you are applying the slip to bisque ware then you need very little raw clay in the slip, more along the lines of an underglaze than a vitreous slip.

When are you applying the slip? 

 

 

[Gonen]

id apply it to leather hard clay body - i think to spray the place with water before the application , so i think the slip can be plastic and shrink ok with the clay body 
so i think ill add some flux ( feldspar or earthenware clay) to make a better connection to the clay body and cover the differences in shrinkage and a little silica or frit to form a little glasslike looks  i ask myself if i should add some sodium silicate to difluc the all thing ? 

what do you think ? 

[Min]

Have a look at the cone 6 engobe recipes here. To make them more glossy increase the flux, in this case the recipe uses nepheline syenite plus frit.  An earthenware clay should work too if you are okay with the slip not being white, try a blend of earthenware, kaolin or ball clay and nepheline syenite, silica and frit. If you don't have access to a frit try 5% borax (dissolve it in hot water first), this will precipate out of the slip over time so don't make a big batch of it.

It's going to take some testing to get a good fit between your claybody and the slip with the amount of gloss you are looking for.

Yes, I would probably add some deflocculant, if sodium silicate is what you have then try a tiny bit of that. Less water in the slip results in  less shrinkage thereby decreasing the chances of it cracking while drying. 

[Callie Beller Diesel]

If you’re adjusting slip/pot fit with differences in water content or deflocculant, a polymer based flow agent is going to give you more room for error than sodium silicate will. The North American go to is Darvan, and on the other side of the Atlantic, a similar product is called Dispex. If you’re using sodium silicate on small batches especially, it’s very easy to cross the line between deflocculated nicely and a substance that resembles pudding. 

[Gonen]

all about slips as glazes on digital fire 

 https://digitalfire.com/article/creating+a+non-glaze+ceramic+slip+or+engobe