
Ever since I spent a year in Ireland, I have been enthralled by Celtic knot designs. There is something exceedingly captivating about the intricacy of the weaving lines creating beauty and motion with a sense of whimsy. I began to create my own designs, and a number of years ago I decided to design a Celtic knot alphabet and number set.
I began by finding a simple, bold font to use as a base. The font I chose happened to have an outline version, and this was perfect for my needs since I would be drawing the knot designs into each letter and number. Using a photo editing program, I made revision to the letters or numbers as needed. I threw a border around the each letter, and printed out templates. Then it was then just a matter of sitting down and designing the knots. In perfect procrastinator style, I was not very diligent with my designs, and the completion of the set took my quite a number of months (read: over a year).
From Paper to Clay
Most of the designs had been completed before I began my foray into ceramics, and it was a logical step to turn them into tiles. The numbers came first since I was able to complete the set in a shorter amount of time than the letters. The Celtic knots are designed with many fine lines. So, instead of carving a master tile from clay and creating a mold from that, I thought I would be able to have a bit more control over the line uniformity and thickness of my knot numbers if I used a plaster carving technique. Using this technique, I create a blank plaster mold first, and then carve my design directly into the plaster, creating my master tile mold. This allows for very thin and detailed lines in the final piece, and I have used it to create some of my other tiles.
Project:
Carving Tiles in Plaster

- Clay (enough for the size of the tile to be made)
- Slab roller -OR- rolling pin
- Cookie cutters -OR- template/ruler and clay knife
- Materials for mixing plaster (see mold making post)
- Design template
- Graphite or carbon paper
- Pencil or ball tip tool
- Carving Tools
Step 1: Making a Tile Blank
A tile blank is just a slab of clay in the size and shape of the tile I wish to make. I use a slab roller to roll out a slab of my desired thickness. In the case of my number tiles, I used my standard thickness of 3/8”. A slab of clay can also be rolled out using a rolling pin and dowels.
I then cut a tile blank from the slab of clay in the size and shape desired. I have made a few clay cutters (basically cookie cutters) in standard shapes that allow for quick cutting, but I have also used a ruler or template and an hobby knife to cut out my tiles. Make sure the surface of the clay is smooth. I use a piece of canvas with my slab roller, which leaves a texture on the face of the clay. I smooth this out using a rubber rib (actually a re-purposed squeegee blade). At this point you can also round the corners and edges to give the tile a finished look.
Where the shrinkage rate is the percent of shrinkage in decimal format (i.e. 10% = 0.1)
Example: x = 4″ / (1 – 0.1) = 4.44…”
Where the shrinkage rate is the percent of shrinkage in decimal format (i.e. 10% = 0.1)
Example: x = 4″ + (4″ * 0.1) = 4.4″
Step 2: Casting the Tile Blank
The next step is to make a plaster mold from the tile blank. I explain my basic process for mold making in my post Meet My Good Friend, Plaster: Making a Plaster Press Mold for a Tile.
Step 3: Carving the Mold
The clay blank has left a cavity in the plaster mold, and the design will be carved into the face of the cavity. The option of carving your design freehand is always available. You can also use a soft pencil or marker to draw your design directly on the plaster.
I like to carve the mold while the plaster is still a bit damp since it is softer at this point and easier to carve. It also leads to less dust. Still, there are drawbacks to this method. When transferring your design, it is very easy to mar the surface of the plaster with your pencil or scribing tool. The ink from markers will bleed a bit, and pencils sometimes do not draw very well on wet plaster. Also, if you use a paper template, the paper will become damp and warp a bit as you trace your design. If you carve your mold when the plaster is completely dry, take precautions to minimize your exposure to the dust.
Be sure not to use your mold for pressing tiles until it is completely dry. New plaster takes a while to cure before it reaches its full strength. Damp molds will also be slower to release their tiles.
Step 4: Making a Working Press Mold
I usually use my original carved mold as my master mold and then create a press mold from the master mold for general use. This allows me to keep my original mold in good condition for a long time. If the press mold breaks or if the details dull due to prolonged use, I can always make another one from the master mold. Simply press one tile from the master mold. Use this tile to create a second mold. The new mold becomes the general purpose press mold, and your master mold is stored until it is needed to create a new press mold.
And the Finished Product
Using the above technique, I made 9 carved plaster master molds (0-8 with the 6 doubling as the 9), and press molds from each. And I was ready to press my Celtic knot number tiles!

Health and Safety Notes:
1.) Always wear a dust mask while mixing plaster or generating plaster dust. Plaster contains silica, and breathing the dust can lead to respiratory problems over time.
2.) When mixing plaster by hand, wear rubber gloves. Plaster can dry out the skin and cause irritation.
3.) Do not pour any plaster down the drain or clean out plaster equipment in the sink. The plaster will harden in the drain and ruin the pipes.
4.) Keep plaster away from the good clay in your studio. Even the tiniest piece of plaster can ruin a finished ceramic piece.