Monday, February 23, 2009

Using an Elmer's Glue Bottle for Slip Decoration

For years I have been using a fine tipped glaze bottle for my slip application and decorations. For just as long I have been immensely frustrated when the tip gets blocked and expels a giant blob of clay slurry all over my design in 30 second intervals. Of course, it's my own fault for putting up with it as long as I have as well as forgetting to purchase a slip bottle when I am at my ceramic supply store.

Just the other day the heaven's were singing when I spied a near empty Elmer's glue bottle in the closet. The light bulb above my head signaled that this container would make a good slip bottle. I rigorously cleaned the container with hot water, my small sponge on a stick tool and a hooked wire to clean the difficult top edging. I also removed the orange plastic top and cut off the inverted "Y" white plastic tip to permit more slip flow. Taking a small drill bit I enlarged the opening of the orange cap from the underside out. Mixed up a nice smooth red clay slurry, filled the Elmer's glue container by way of a funnel, closed it up and squeezed to my hearts delight. No more giant slurry blobs on my designs or toads. My toad house production is going to get a lot easier now.

The only question is how to keep the slurry in the container from drying out since I removed the inverted "Y"? I decided to pour in 1/2 inch of water to keep the slurry moist and found a thin piece of copper wire to set in the hole. The sitting water will not mix with the slurry so long as I don't vigorously shake the container and a fair amount of the water can be squeezed out before the next use.

Glaze Porosity, is it Food Safe?

This is a really good question and one that you will often hear from customers wanting to buy your functional pottery. Fortunately, there are a lot of tried and true glazes available out there but that doesn't necessarily mean that they will be safe with your clay. In order to have absolute certainty I recommend having your clay and glazes professionally tested which can be a costly affair but if you are selling functional dinnerware to the public, it's a safe investment.

Concerning our native clay, I have covered the shrinkage rates as well as the rate of absorption in my previous postings. Now we want to test for the absorption rate of your clay and glaze. In order to do this you need to completely glaze your test tiles and fire to maturing temperature in the kiln. In the realm of earthenware clay your bisque firing temperature is normally 1 to 2 cones hotter than the glaze firing temperature.

After completion of the glaze firing we will again measure our 100mm lines and weigh the tiles. The final lengths of the tiles will determine our overall shrinkage rate from dry to glaze for our native clay. Again, under 15% is good. Next you will need to boil the glazed test tiles in water for 1 hour. Safely remove the tiles from the hot water and allow them to cool to the touch before weighing them again. The difference and average of dry glazed test tiles to the saturated glazed tiles will allow us to determine the rate of absorption of our clay and glaze. If your percentage averages out below 3% you have a good clay and glaze fit. Chances are very good that this clay and glaze is food and water safe. Higher percentages will result in weeping water as well as wares that are unsanitary.

A final test involves acidity. Take one test tile and soak it in vinegar or lemon juice for 24 hours. If the glaze is discolored and showing craze lines your glaze is not safe for acidic foods.

If you happen to have a poor clay and glaze fit you can still create wares that are not crafted for food and drink as well as test with other glazes.

Continued Shrinkage Testing and Rate of Absorption

Upon completion of the bisque firing you will again weigh each tile as well as measure the lengths of the 100mm line. Using averages you deduce the shrinkage rate of the native clay from wet to bisque. Example, all tiles A- E began with a 100 mm. After the bisque firing to cone 05, Tile A measured 92mm, B- 92mm, C-95mm, D-95mm, E- 96mm. Differences being 8mm, 8mm, 5mm, 5mm and 4mm. Therefore, to reach the average will will add these numbers and divide by the total number of tiles. 8+8+5+5+4=30mm divided by 5=6% shrinkage rate from wet- bisque. You can also go further with the data you have collected and deduce the average shrinkage rates from wet to dry, wet to bisque, and/or dry to bisque. An average shrinkage rate under 15% will make a good clay body for continued testing.

The next test we will do is for lime blowing. In firing clay above 1632 degrees Fahrenheit calcium carbonate decomposes into lime. Clays with an excessive amount of lime or calcium carbonate will cause problems in the future. As the clay absorbs moisture from the air the lime inside will expand causing pieces of the clay to flake off. Lime is easily recognized by small pits and a white or yellowish film on the surface of the pots. There are remedies one can do to correct problematic lime blowing such as firing at lower temps or firing in a reduction atmosphere. For the lime blowing test, soak your bisque fired test tiles in distilled water for an hour and allow them to dry for a few days. If lime is present you will see a film form on the surface of the tiles.

Without certainty I believe that lime is causing a problem in this 180 year old house that I reside in. The red brick chimney we have is mortared in place and probably dates back to it's origins. Due to a slow leakage around the flashing of the chimney it might be possible that the brick has too much lime or perhaps the mortar has slowly leached into the brick causing large chunks and flakes to fall off.

The rate of absorption for the bisque clay will tell us how well the clay will absorb the wet glaze. For this test we rely on the weight measurements we have been taking. Boil your bisque test tiles in water for 2 hours. With tongs safely remove the tiles from the boiling water and allow them to cool to the touch on a dry towel. With your triple beam or digital scale take a weight measurement for each saturated test tile. The difference and average between the dry bisque and saturated bisque tiles will provide us with our rate of absorption. A percentage higher than 10% will provide a good absorption rate. Percentages under 10% will make it difficult for the bisque to absorb the glaze. If such is the case you can bisque at a lower temperature, cone 06 or cone 07, or heat the pots before glazing. Having a good rate of absorption with your bisque fired clay makes it easier to get a even coat of glaze on your vessels which will provide a uniform glaze coloring as well as a good clay and glaze interface.