Potters & Sculptors - Making Rock from Mud
I work with a tan cone 6 stoneware clay body,(Trinity cone 6), and I am wanting to layer on a white basic slip over my clay body in different areas so I will have different appearance in one glaze.Example; one glaze with two surfaces, tan and white, should give me some variation.
In order to get a white slip, I am drying out some frost porcelain to use as slip. I realize the COE of the porcelain slip will be different than the stoneware clay, therefor will likely craze on the surface of the slip.
So my question is: will this crazing on the surface from the slip effect the body of the stoneware enough to not use a glaze intended for stoneware? Or should I just make a glaze specifically designed for porcelain and use that on the piece? One stoneware glaze and one porcelain glaze or one stoneware glaze over both? Or scrap the idea of using white porcelain slip and just use a universal white slip/engobe ?? Any thoughts or suggestions?
You will probably find the difference in the shrinkage rate between your Stoneware and New Zealand Frost Porcelain to be of far greater importance than the relatively small COE difference. Check with Trinity, but the shrinkage rate of the New Zealand Frost is almost certainly far greater than your Stoneware. A thin layer of porcelain will take on the COE of the underlying stoneware clay, so long as the two types of clay remain attached.
People at our studio routinely apply a thin layer of Laguna CN-4110D Oriental Pearl casting slip over other clays to provide a whiter base without cracking, in spite of it's 14% shrinkage rate. It creates no problems in glazes. http://www.lagunaclay.com/clays/slips/cn4110d.php
This photo will give you a good idea of what you can expect from applying a thick layer of high shrinkage porcelain over lower shrinkage Paper Clay.
I applied a 1/4 inch to 1/2 inch layer of New Zealand Frost which shrinks 11% with a 6.99 COE - http://www.lagunaclay.com/clays/western/wc437.php
over Max's Paper Clay which shrinks 8% with a 6.05 COE - http://www.lagunaclay.com/clays/western/wc953.php
I made the cracks resulting from the 11%-8% clay-shrinkage mismatch a design feature using Cone 6 glazes, with a subsequent Cone 06 glaze application and firing.
This is the piece with the Frost Porcelain over the Paper Clay after a bisque firing in an old kiln sitter which liked to shut-down too soon - notice the "carbon lock" of the incomplete oxidation of carbonaceous material primarily from the paper clay under the porcelain.
Even after firing to a full bisque this piece looked the same, with very few cracks, except it was white.
Most of the cracking,caused by the shrinkage difference, occurred in the Cone 6 firing.
Thanks so much for the response, it makes a lot of sense. I'll give it a go.
Juli, when you get results will you post the information? I am trying the same with porcelin over a red clay. It will be several weeks though before I can fire it. I would like to know the name of the clays you are using. I have been trying out numerous clays this spring. Trying to find a good fit with clay and glazes, oooh, the struggle.
sure will! I have a few in the kiln right now. I will post info in a couple of days. The clay I use is from Trinity ceramics, and it is their cone 6 stoneware. Its buff colored. The porcelain is Frost, from a bag I had. The shrinkage on it is 11%, and the shrinkage on my Trinity cone 6 clay is around 11/12. The universal slip I use is: Bringle Universal slip. (without the borax)
Nepheline Syenite 25
Ball Clay 20
I mix up a couple of hundred grams at a time to yogurt consistency . I put it on at leather hard or bone dry. Cant remember what I added for grey and black. probably red iron ox, and manganese dioxide, and copper carb. tweeted it with the white to get grey.
For colors(in %)
white zircopax 10%
gold Rutile 20 to 40 %
Blue cobalt 3%
Brown Iron ox 20 -40 %
pea rly green cobalt 3%
I put the slip on crosses, bells, and pots. I'll post some pictures of some crosses since I have photos of those.
Wow, I really like your crosses. Thanks for posting the pics. I like the rugged look of them. I may have to try something similar. My sister would love one. I like the bells too. I will have to compare your slip reciepe to the one I have. The one I have goes on supper smooth. I first was using it for carving like on your bells. Have been working on glazes the past six months. About ready to give up.
I like the way you have your glaze buckets covered. I can never get the lids on, then back off. I just lay them on top now. It would be a good idea to use. I have acess to free buckets, but no lids.
Thanks for the kind words! Old picture of buckets. I now use dry cleaner bags, tied at the bottom, and they completely incase my buckets. I then use heavy duty twist ties to tie them up at the top.
Super easy for me, and as long as I see condensation on the bags I know all is well. My buckets are odd shaped, and lids don't fit .
As far as my slip, it goes on smooth also, I just at times like to put it over texture, so I like it dry then so I can kind of dry brush it on. I have tons of bells and crosses, I don't like to sell them, I just keep them around for my amusement. I am not real religious so to speak, I just like having them all over the walls. The bells on the other hand, take so much time to complete . Making the beads, and stringing them up. I try to use something I like to hang on the ends for weight. Favorite now are deer antlers.
I have a couple of test tiles with the porcelain , I hope I can see a big difference with the glazes.
Probably not a good idea to get my hopes up, if you know what I mean.