MAKING CERAMIC TILES
At one point or another, most ceramists decide to make their own tiles. Often they are disappointed with their first attempts, most commonly because of warping. The following tips should help you make tiles that do not warp.
The first thing is to start with a good clay body for tiles. Generally this means tiles that have a high amount of grog. Clays designated for sculpture and Raku are usually good clays for making tiles. Some people swear by clay bodies that have a mixture of large and small sized grog.
For durability and water-proof-ness, you want a clay body that matures at the temperature you will glaze fire at. This is especially critical for tiles which are outside or in bathrooms. The absorption rate after firing should be 3% or less, otherwise water will seep in and cause the tiles to crack. If water-proof-ness is not a requirement, you can sometimes achieve flatness easier by under-firing the clay. This is something to experiment with, as other times a tile that is warped after bisque firing will become flat after glaze firing if the clay is allowed to mature.
Low fire, smooth, earthenware clay is generally the worst to use for tiles. Not only does it not fully vitrify, it's lack of grog can be very challenging. An example of earthenware tiles are Saltillo tiles from Mexico. Since these are low fired (unvitrified) and unglazed they become easily soiled and must be sealed several times a year.
WORKING WITH THE CLAY
Once you have chosen a clay, it is best to work with it rather dry. This reduces warping problems.
Make sure your tiles are thick enough. If tiles are too thin, they will tend to warp. An 8" square tile should be at least one half inch thick. This is thicker than commercial tiles you buy, but those are made with a special dry pressing process that allows them to be thinner.
It is important not to bend your slabs while they are wet. If you do, the clay will have a tendency to warp once it is dry. So find a way to handle the slab without bending them. How? For starters, if you have rolled the slab onto fabric, do not peel the slab off the fabric. Instead, put a hard board (plywood, drywall, etc.) on top of the slab, flip the whole thing, then peel the fabric off the clay. Or simply slide the slab onto a drying board without bending it.
Alternatively, you can roll the slabs directly onto a board and keep them there, flat, until they can be slid off. This works with slab rollers like the Brent where the clay stays stationary and the roller moves across it.
To prevent warping, some people roll out the slab, flip it over, and roll it again. Especially if using a slab roller that has a single roller. Since the North Star type has two rollers that the slab runs through, it is thought to compress the clay equally on both sides, so this step isn't needed.
You can use various materials over your canvas to eliminate the canvas texture on the tile, as well as keep your canvas from becoming wet and the clay sticking. Heavy Pellon is sometimes used. It is an interfacing material found in fabric stores that doesn't stretch or bunch up. Vinyl flooring, and old lithographic printer's mats are also used for rolling slabs onto.
DIRECTION OF ROLLING AND LOCATION OF TILE AFFECTS THE FINAL SHAPE
If you roll out a slab of clay in one direction, then cut square tiles, they will shrink to a rectangle, not a square. This is because of the way the clay particles align and stress is created in the clay. Rolling one direction, turning the slab 90 degrees, then rolling again is helpful to reduce this problem.
If you roll out a slab of clay (even if you roll in two directions as above), then cut it into squares, each square is likely to be a different size when fired. The reason is because different stresses have been created at the edges of the clay than in the center of the clay. Avoiding the use of the outer inches of your slab is helpful to reduce this problem.
Both problems above will be more pronounced if the tile is cut when the clay is wet than if the tile is cut when leather hard.
Each tile is going to be in a different location in your kiln, and that will also cause tiles to fire to slightly different sizes and shapes.
Some beginners become frustrated because their tiles are not perfect, but in reality no handmade tile are really perfect. Commercial, mass produced tiles are very close to perfect because they are made in an entirely different way, (for example, by steam pressing powdered clay.)
ALTERNATIVES TO ROLLING
Some people wheel throw their tiles. Just figure the diameter you need to reach corner to corner on your finished tile including shrinkage, mark that size with a magic marker on your bat, and throw to the mark. Weigh out the clay balls so the tiles are always the same thickness when thrown to the mark.
Some people press their tiles. You can make a bottomless frame out of wood, pound the clay into this form, use your cut off wire to cut off excess clay, then push the clay out the bottom. To avoid bending, use a piece of wood the size of the clay to push it through. You will have to let the clay dry to the point where it doesn't stick to the wood, or use something like WD40 or cooking oil to coat the wood.
Some people cut their tiles directly off the block of pugged clay. You can buy devices that act like cheese slicers to cut the clay off at a certain thickness. Or, to make your own, take two wood dowels (or wood strips) and make notches at even intervals (one half inch apart for half inch thick tiles.) Wrap a piece of wire between and around the wood dowels, starting at the top notch. Grab onto one piece of wood with each hand, pull the wire tight, and sliding the wood pieces toward, pull the wire evenly through the clay. Make sure you hold the wood pieces vertical as you pull. Then move the wire to the next lower notch and repeat.
Finally, another way to make tiles is to extrude them. This tends to align the clay particles nicely and reduce warping.
CUTTING THE TILES
Wait until the clay is leather hard before cutting the actual tiles out. Also, if you are impressing designs make sure you do this before cutting the tiles out, or the pressing action will deform the shape.
Make a template from metal or wood to make it easy to cut out same size tiles.
Drying is very critical. You want to make sure the tiles dry evenly on both sides, or they will warp. Keep them away from drafts, and dry them slowly by covering with plastic. There are two main techniques used to dry tiles.
Place each tile between two pieces of drywall (also called sheetrock.) This is the stuff used to build walls. You can sandwich many tiles between layers of sheetrock and stack them up. The moisture is pulled evenly out of both sides. A variation on this is to dry between layers of three quarter inch thick plywood, or layers of calcium silicate board (often used as a non-asbestos fire proofing board).
Place the tiles on a wire rack to dry. This allows good air circulation. A variation on this is drying on plastic grids that are often used in fluorescent light fixtures.
Other tips are:
Use drywall method for the first day or two, then transfer to wire racks.
Wax the edges before they dry. This prevents them from drying much faster than the center. Or wrap dry-cleaning plastic over the edges.
Weight the corners with kiln posts overnight -- each stilt laid across the adjacent corners of two tiles.
For functional tiles, people often cut grooves in the back of the tiles. This allows more surface area for adhesion; they dig into the mortar bed better. An interesting way to get the grooves is to roll the tiles out onto corrugated, grooved cardboard. Grooves do not seem to affect warping.
For bisque firing, you can stack tiles on top of each other, or put them into a tile setter.
For glaze firing at low temperatures, tiles can be put into many types of tile setters. But if you are using high fire clay and firing it to maturity, the tiles will slump at high temperatures. In these cases you have to fire the tiles on a flat surface.
Fire tiles on a kiln shelf dusted with silica sand. This allows the tiles to move easily on the shelf, as they expand and shrink from the heat.
Make a moat around your tiles by putting bars of clay around their edges. This helps them heat from above and below, rather than the edges heating faster than the middle.
Remember that clay will shrink, and you have to adjust for this when making your tiles. To determine shrinkage, take a slab of clay and incise a line 100 millimeters long. Re-measure this line at the bisque and high fire state. I f the line ends up 90 mm's long then your shrinkage rate is 10%. If your ending number is 87mm's then your shrinkage rate is 13%, etc. Remember to make the first line at the same dryness stage as you will be cutting your tiles, as there will be shrinkage between wet and leather hard as well.
Quick tip for mosaics: To make a large number of small square tiles, go to a hardware store and get an acrylic grid used to diffuse fluorescent lights. Roll out your slab, spray cooking oil on the grid, and press the grid into the clay, cutting all the way through. Let the clay dry and remove the grid.
INTERNATIONAL POTTERS' PATH
Finally I would like to tell you about a very interesting tile project called The International Potters' Path. Located at the Chapel of Art, in the shadow of a 13th century Castle in the UK, this pathway will eventually be 50 square meters completely paved with 5000 tiles. What is unusual about these tiles is that they are all being made and donated by ceramic artists, potters and tile makers from around the world. You can donate a tile yourself! Details can be found here including specifications and some great examples of tiles already received. Tiles should reflect you, your environment, your country or culture, and include the name of your town/province or country on it.
(May 2012: Note that the link on the tiles.org page for the Chapel of Art has been taken over by a casino, so some of the information mentioned here is no longer available)