If a tile or any flat form is cooled too fast, it can crack as shown in the example above
Making Ceramic Tiles: An Overview
This tutorial covers the complete process of making ceramic tiles and flat slab forms, from selecting the clay and preparing even slabs to cutting, drying, and firing. It also explores alternative forming methods, surface details, larger tiles, and the special requirements of outdoor installations. Each stage affects the finished tile's shape, strength, and durability.
Making ceramic tiles requires attention to the clay body, the way slabs are formed and handled, and how the tiles are dried and fired. The following lesson explains practical methods for producing consistent tiles, with additional guidance for large formats and outdoor use.
Clay Bodies
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 waterproofness, 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 waterproofness 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.
See APPENDIX A for making weatherproof tiles for outdoors use
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. 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 warpage 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 warpage, 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.
Alternatives to Rolling Slab
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.
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
Drying flat forms 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.
1. 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).
2. 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 slab and tile making tips
- 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.
Grooves
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 warpage.
Firing Tiles
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.
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.
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. If 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.
Troubleshooting Ceramic Tile Problems
Problem
Possible cause
What to try
Warping while drying
Uneven drying, bending wet slabs, or uneven thickness
Keep slabs supported and dry both faces slowly and evenly
Cracks before firing
Uneven shrinkage or stressed edges
Maintain even thickness, round edges, and reduce drafts
Warping during firing
Overfiring or inadequate support
Check clay maturity, kiln schedule, and flat shelf support
Cracks during cooling
Thermal stress or uneven cooling
Review clay fit, kiln loading, and controlled cooling
Tiles finish undersized
Shrinkage underestimated
Measure a fired test tile and calculate cut dimensions
Tip: Change one variable at a time and record the results so you can identify which adjustment helped.
Considerations For making Large Tile (over 12" x 12")
Warping and cracking are a more likely to occur when making larger tiles. Below are some thoughts that will make you more successful.
Choose a grogged clay body. Consider a clay with higher grog content (10% or more), after testing its suitability for your project.
Increase tile thickness. Larger tiles generally need more thickness and even support.
Round and smooth the edges. Avoid sharp corners and weak edges that can encourage cracks.
Consider the firing range. A higher-cone clay fired below maturity may reduce some firing deformation, but it will also be more porous. Test carefully; underfired tiles may not be suitable for functional or outdoor use.
Plan for even heating and cooling. Large, flat tiles are especially vulnerable to thermal stresses. A heavy kiln shelf can cool at a different rate than the tile's upper surface. The underside of a commercial tile often has raised ribs or supports. If using similar supports, distribute them carefully to avoid sagging at firing temperature.
Dry slowly. Keep tiles away from fans, drafts, and heavy foot traffic that can cause uneven moisture loss.
Dry both faces uniformly. Use a suitable flat support or drying rack as the tile becomes firm enough to handle.
Control the kiln schedule. A programmable electric kiln can help manage heating and cooling more consistently.
Allow sufficient preheat and cooling time. For thick or large work, a slow bisque program with an appropriate preheat can help moisture escape. The original studio example uses approximately 180°F for 7–12 hours; adapt this to the clay, tile thickness, kiln and manufacturer's instructions. Where available, consider controlled cooling for bisque and glaze firings. See Skutt's KilnMaster operating manual for kiln-specific guidance.
Recognizing a cooling crack: A sharp, unglazed fracture edge can suggest that the crack formed after the glaze solidified, but its appearance alone does not establish the cause. Review the clay body, firing schedule, support and cooling conditions.
APPENDIX A - Making Tiles For Outdoors Use
Clay absorption rate of 3% or lower is required for tile to withstand frost. The absorption test typically used is established by the American Society for Testing and Materials. First, a fired tile is weighed dry (Wd), then soaked for 24 hours in cold water. Immediately after being dried with a towel, the tile is weighed again (Ws). The absorption rate is then calculated from the two measurements using the formula below:
Water Absorption Calculation
Water absorption (%) =Ws − WdWd× 100
Ws = weight after soaking and gently removing surface water Wd = weight of the completely dry, fired tile
Example: A dry tile weighs 200 grams. After soaking, it weighs 206 grams.
206 − 200200 × 100 = 3%
The tile has a 3% water absorption rate.
To test the fit between the glaze and the clay body. Put glazed tiles through a hot dishwasher cycle, then into the freezer for a day or two, then back into the dishwasher, and again returning it to the freezer. Repeat this process several times (7-10), then check for crazing.
See example specification of a good low fire clay for outdoors use:
Standard 417 Red Earthenware
Firing range: Cone 06 to Cone 02
Low-fire clay body with fire clay and a small amount of fine grog. Suitable for Majolica techniques and described as a superior throwing body.
Firing cone
Shrinkage
Water absorption
Cone 06
6%
5.3%
Cone 02
11%
1%
Making Ceramic Tiles
Baily's Slab Roller - we have two of them for years being used almost every day flawlessly
The spatula is a very useful tool for smoothing
stainless cutters come in 2x2", 4x4", 6x6" and 8x8". Each cutter has a spring loaded plunger/release button which pushes a release plate to eject the finished clay tile. The size takes in account shrinkage
Adjustable clay divider with 6 adjustable cutter - Bailey's
Roll out slab by hand using a rolling pin and two wooden slates
