I hope someone here can help me. I have been doing pottery for about a year. I tried to attach mold forms to the side of a vase. (Sprigging, I think it is called?) I scored them and the vase to attach and they seemed to be on tight. After the bisque firing they had cracked away from the pot and some had peeled back quite a bit. What did I do wrong?
Yes, adding molded shapes is called sprigging. Basically it is the opposite of stamping which creates an indented impression in the clay.
I love sprigging, but it is deceptively time consuming. I plan on spending 15-20 minutes on each sprig. For my pots completely covered in such decoration, this means a good 10-15 hours of work. With that kind of time investment, it is devastating when a sprig cracks off.
Here is a list of steps that will increase the likelihood of success, but as with everything else in the world of pottery, there are no guarantees.
1. Match Dryness. Very wet sprigs on leather hard clay (or vs vsa) are more likely to crack away. Keep your piece quite damp and add sprigs in a similar state of wet/dry.
2. Score, Score, Score, Score, Score. A few lines drawn with a pin tool is not enough. Rough up that clay!
3. Use Slip. Some types of clay connect better with slip. This is especially true of porcelain and the smooth stonewares like B-Mix. I have read many Tom Coleman articles in which he swears off any scoring and only uses slip on his porcelain pieces. (UPDATE: At the Tom Coleman seminar in 2006 he said he was wrong about slip only and now scores all of his attachments.)
4. Press and Smooth. After pressing the sprig onto the piece, attach the edges firmly and smooth them down into the clay. Don’t leave an indent around the edge of the sprig that will encourage a crack to form.
5. Dry Slooooooowly. The slower the better. I usually allow two full weeks tightly wrapped before opening the plastic a bit to allow the full drying.
6. Dry Completely. Sprigs create a thick area of clay where they attach. Allow extra drying time for the piece to become fully dry before the bisque fire. In addition, preheating the bisque kiln slowly (60 degrees/hr until you reach 200 degrees) will catch any left over moisture in the thick area and prevent mini-explosions of steam trapped behind the sprigs.
I hope this helps. Let us know if it works.
Thank you! I did not lose any attached pieces on my last batch of work. You are right about the time though. I was spending about 1 minute attaching pieces, and having most break off! They really take a lot of time to do right.