Potters & Sculptors - Making Rock from Mud
Ease of installation is a marketable quality in tile. Tile with similar dimensions as commercial tile simplify installations. To control finial size, allowance for shrinkage needs to be calculated. My clay shrinks about 12.5%. (I have rounded it down to 12% to simplify calculations) How to calculate shrinkage; I made a 6 X 1 X 3/8 inch clay block. When it had dried to a hard, leather hard stage, I layed a ruler against it and copied the markings. Bisqued it and fired it to mature temp. Then I compared my ceramic ruler to the actual ruler. My 1 inch marks where now 7/8 of an inch apart. Math to find shrinkage; (Dry measurement - fired measurement) divided by Dry measurement X 100 = % shrinkage. EX; 1 (dry) - .875(fired) divided by 1(dry) X 100 = 12.5% I converted the fraction to decimals (7 divided by 8 = .875) to make it easier. You can also measure the distance across a thrown cylinder when it is dry. Fire it to matureing temp and compare measurements. That way if you want to make a 6 inch tall mug you know it needs to be 6 3/4 inchs tall when you throw it, at least at 12%. Why does this matter? Maybe not so much in coffee cups but, at 12% shrinkage If I make a 4 foot backsplash out of wet clay it will be 5 3/4 inches to short when it comes out of the kiln ready to be installed. That's not good. At 12%, if I need a 6 inch tile I need to make it 6 3/4 wet. If someone has a fireplace front that they want covered It payes to have one that fits it when you empty the kiln. To me all this math is a pain so an easy way to get around it is to use a shrink ruler. You can buy one or you can simply put a ruler in a copy machine and enter 12% inlarge and press copy. Cut out and glue your inlarged paper ruler onto a paint stirring stick and you are good to go. Use your paper ruler to do all you wet clay work and when it comes out of the kiln it should be the right size. Also another note on measuring and making sure your tile is the correct size. If a tile is an 1/8 on an inch to big. No big deal right? That mistake will be an inch to big by the eighth tile. If you have a simpler or better way of figuring shrinkage please feel free to share it with us other clay mongers.
I pretty much figure it the same way but I have a shrinkage ruler that I find handy when I have to make a replace tile. I bought mine at NECECA , I think you can buy them at any ceramic supply shop now. Denice