Sunday, September 14, 2008

Testing the wet to bisque shrinkage rate of native clay

To quote Michael Cardew from his fantastic book, Pioneer Pottery,"An unknown clay should be presumed guilty until proven innocent." Commercially available clays often have the shrinkage, porosity and absorption rates listed in there catalogs, the work has already been done. But what about a unknown native clay?

In the near future my husband and I will be moving to our property in western New York state. Our parcel is filled with native terracotta clay and shale. As a potter this is a potential gold mine for me but I don't know much regarding this particular clay body. Is this a clay body I can use for throwing on the wheel, what temperature does it fire to? One thing I am certain of is that there were local potteries in the old days so chances are pretty good that I can utilize this clay.

After finding a nice pure vein of clay I was able to do a simple plasticity test by rolling out a wet coil and checking for cracks, it passed. Refer to my previous post- "Native clay processing on a large scale". My next step was to make a couple of small balls and bisque fire them in my kiln to cone 05. I fired these small balls of native clay inside a commercial clay bisque fired bowl as a precaution. I did not want the clay to melt and ruin my shelving, I could sacrifice the bowl. The clay survived and fired to the same rich red coloring as my commercial terracotta clay body. Now it's time for some more testing.

To test the shrinkage rate of a native clay works on averages and does not involve complex formulas, thankfully. To take from Tom Buck, Leon Nigrosh and Michael Cardew begin by rolling out a slab of native clay that is 1 cm thick. Allow the slab to set up for a few hours before cutting out your test tiles. Take a piece of box board and cut a rectangle 13 cm long and 4 cm wide. This box board will be your template for the tiles. Using a fettling knife cut out as many tiles as you can, preferably 10. Next draw a line 10 cm long down the middle of each test tile. Set them aside to stiffen up to the point of handling.

Thinking ahead to porosity and absorption testing were going to kill 2 birds with one stone. Label each test tile A, B, C, D and so on. Using a triple beam or digital scale weigh each tile in grams in it's wet state and write it down, always keep notes. Completely dry the tiles flipping as needed and/ or weighting them to prevent warpage. After the tiles have completely dried, measure the 10 cm lines (100 mm) and weigh them again. Example- Tile A is 96 mm (-4 mm), Tile B is 97 mm (-3 mm), Tile C is 95 mm (-5 mm). The average difference of A+B+C+D+E+F+G+H+I+J =X divided by the number of tiles equals your wet to dry shrinkage in percentage. Example- in mm- 4+3+2+3+4+3+2+3+4+3= 31 divided by 10 = 3.1% wet to dry shrinkage.

Next we are going to fire the test tiles in the kiln to bisque temperature, cone 05 or cone 04. After the kiln has cooled and you have removed your tiles measure each 10 cm line again and weigh the tiles again. Tile A is 89 mm (-11 mm), Tile B is 90 mm (-10 mm). Again, take those averages and divide by the number of tiles. This percentage will give you the average wet to bisque shrinkage rate. Somewhere in the range of 10- 15% is good and worth investing the extra time for glaze firing and absorption and porosity testing which will be part of my next post.

Saturday, September 13, 2008

Sponge on a stick, Throwing Tool

One of my favorite pots to throw on the potters wheel are narrow neck vases. Any one who makes wheel thrown pottery knows that all the free standing water within the vessel needs to be removed before cutting the pot free from the wheel head. Failure to do so will result in a cracked bottom as the clay absorbs all of that free standing water.

Commercially available sponge on a stick tools (does this have some other name?) are too wide for removing the water from the base of a narrow neck vase and they have the tendency to roll around while the wheel is spinning. I devised a very simple, inexpensive and quick method of making my own sponge on a stick tool that is narrow enough not to damage the neck of my vase when inserting it.

Three simple materials are all that is needed- 1" thick foam found at fabric stores and often sold in 2' X 2' pieces. You will need a strip approximately 2" X 3" X 1" for the making of the tool. Don't despair, the remainder of the foam can be used for throwing sponges, cleaning and as padding for pots. These sponges last for years before giving way. You will also need a 1/4" thick wooden dowel approximately 1 foot in length. And lastly, a thick elastic band like the ones that come with bunches of broccoli at the grocery store.

Simply roll your strip of foam around the dowel (2" thickness, 3" length) leaving 1/2" to 3/4" of the sponge to overhang the end of the dowel so that it will not puncture the base of your pot. Secure in place by wrapping the elastic around the top half of the sponge.

This simple tool only takes minutes to create and pennies to make. Make them as thick or as long as you need them. In the image my larger sponge tool is about 5 years old and still working just fine. The best part is that I can insert it into my narrow neck vases without damaging the neck and it won't roll around inside the pot while the wheel is spinning. Happy tool making.