The Mysterious Black Clay

Laguna Clay company makes a black clay, WC-391 to be specific,  that I have been playing with for several months now and have been having some issues with. Not wanting to give up on it I thought I would do a post about it in hopes that if any other potters, who are also using this clay, read this post, they might offer some ideas.

My creative thought process was that if I could put the colorful water treatment of the frog bowls on the interior of the black body, sealing it in with a clear glaze and leaving the exterior of the bowl the raw black matte clay body that it would be a really dramatic look. So that is what I set out to achieve. I bought the clay, threw 6 large bowls with it and decorated them with white slip on the interior of the bowl along with the colorants that I have been using. Adding my signature frog sculpture to the rim in all his personality, I loaded the first batch into the kiln and fired it just like I have been doing with the brown and the white stonewares that I am using. Allowing the kiln to cool for at least 12-16 hours, I opened the kiln to find wonderful bowls that were warm to the touch but at all hot. I set the bowl out on the shelf to continue unloading and within minutes the bowl that I had just unloaded makes a loud bang noise and upon when I go to the shelf the bowl has broken in such a way that it has experienced thermal shock. The interior of the bowl looks like it has been twisted and separates. The bowl continues to make noise for several hours following, telling me that as it continues to cool it is really going through some additional changes. If this were the brown or the white stoneware that I use this wouldn’t happen.

Frog Bowl

Frog Bowl, Black clay

The first time it happened, I thought it was a throwing technique problem, that I wasn’t compressing the clay enough to compress the molecules within the body on the wheel. So I tried again. Going back to the basics of throwing and following my hands through the process making sure I compress the clay with a rib before lifting the walls of the bowl. Fired the bowls, same problem. This makes me realize that the kiln needs to cool longer. So I let the next kiln set for an additional day before lifting the lid. This seems to work.

Frog on Black Clay mug

However, an additional problem that the clay has that I don’t know that I can fix without just lots of testing, is the lack of ability for the clay to adhere to itself when I score and slip of two pieces. Spending plenty of time on the construction to make sure that the seams are sealed properly and securely doesn’t seem to help this problem either. After I had spent several days on a sculptural rectangular vase vessel, making sure that seams were sealed and smooth, the bisque fire opened up the seams leaving large gaps in the joints. I had noticed this also on the small fairy garden furniture that I had been making when I would make a butterfly bench and attach the legs to it they didn’t want to hold at the joint.

I have been known to push the limits on projects and materials and I believe that I was pushing the limits of this clay body as well. Laguna lists the shrink rate of this #391clay at 10% with a 2+% tolerance. The white body that I use, #609, on the interior of the black clay has a shrink rate of 13%, I’m sure, is putting stress on the interior of the black clay.

Without doing a lot more testing, I am afraid to sell the items that worked. I don’t know if the item will break after a person puts the mug into a microwave or puts the bowl into an oven to warm or cook food in it and I don’t want to put someone at risk. I have the adorable mugs with the frog on the exterior that I am afraid will pop off due to the two clay bodies not getting along enough to hold each other’s joint.

When I went to the Kiln Doctor on Saturday to buy more clay of all the colors that I use, I did buy more of the black clay and spoke with Mike, the owner, about the problems that I had been having. It just so happened that he was going to Seattle to a conference and Laguna Clay would be there. He took the information from me and planned to talk to them about it and find out if the chemistry of the clay is the problem and if they knew of anyone else that is having similar issues.

Black Bunny Mug

In the meantime, I will hold onto the items that I’ve made so far and not sell them and only make small fairy garden items out of the black clay for now. Hopefully, Mike will bring some answers back from the Pacific Northwest this week and I will keep testing the wares before I let them out of my studio. Just to be safe!