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
The blank zero template, and the sketch of my Celtic knot design for zero.
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.
Snowflake knot tile created using the plaster carving technique.
Project:
Carving Tiles in Plaster
Materials: 3/8″ slab of clay, rectangular clay cutter, hobby knife and rubber rib.
Materials:
- 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.
The tile blank: the slab of clay has been cut and the face, edges and corners smoothed.
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Remember that clay shrinks a certain amount when it is fired, and this shrinkage should be taken into account when cutting the tile blank. The clay manufacturer or distributor should note the clay’s shrinkage rate. For example, the red stoneware clay I use will shrink 10% when fired to my normal temperature of cone 6. If I want my final tile to be 3″ x 4″, I have to make my tile blank a bit larger to account for the shrinkage. Doing the math (I just used simple ratios), I find my tile blank should be 3.33…” x 4.44…” (or 3 1/3″ x 4 4/9″). Since I don’t mind if my measurements are a fraction of an inch off, I save myself some trouble by just adding 10% to the finished tile size of my smaller tiles, which is a calculation I can do in my head. So, for my number tiles, I cut the blanks at 3.3″ x 4.4″. The extra 0.033…” x 0.044…” is really not going to matter in the long run. For larger tiles (>10″) I would probably stick with the exact size calculations.
Exact Size Calculation: x = (finished size) / ( 1 – shrinkage rate)
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…”
Shortcut Size Calculation (for smaller tiles): x = (finished size) + (finished size * shrinkage rate)
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.
Tile blank in the mold form ready to be poured with plaster. My cottles have seen a bit of use!
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.
Shading the back of the template with graphite so the design can be transferred to the plaster.
Since my Celtic knot designs are sometimes quite complex, I tend to use a template for transferring my pattern. Commercial transfer paper is available, but in the interest of saving money, I like to make my own. I print out my design on a piece of scrap paper, and shade the back of the template with a graphite stick or a pencil. I then place the template on the mold and trace over it with a ball tool (or any other object with a rounded end). Don’t press too hard while transferring the design, since this could leave an unwanted impression in the plaster. Sometimes the transferred design is a bit faint so I go over it with a pencil.
Tip: Remember that the design you carve into the plaster should be the mirror image of what you would like the finished tile to look like. I have carved my original design into the mold only to realize when I pressed my first tile that the design backwards!
Tracing the design template onto the plaster mold.
I then carve my design into the plaster, keeping in mind that anything carved into the plaster will end up as a raised area on the final tile. I purchased a set of inexpensive stainless steel dental tools from Amazon, which work well for carving plaster (I also use them for clay and wax carving). The sets come with 10-20 various tool heads, which allow for different types of lines, but I tend to use only a few of them almost exclusively. I have also experimented with ball and sgriffito tools. Be aware that the tools will dull with use, but since the tool sets are not too expensive, they are easily replaced.
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.
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.
Carving the design into the plaster mold.
Once your design is carved into the mold, make sure all the plaster dust has been cleaned out of the cavity. Wearing a dust mask, I usually take the mold outside and brush out the inside of the mold with a paintbrush. There will still be a bit of plaster residue left, so I press my first tile with some scrap clay. The clay will pick up any leftover plaster, and your subsequent tile will be clean. Toss this clay, or keep it exclusively for working with plaster. Plaster bits in the clay will ruin a final ceramics piece!
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.
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.
Finished mold with carved design. Yes, the mold is a bit messy, but it will press a beautiful tile!
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!
Finished Celtic knot number tile glazed in translucent blue.
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.
Close-up of the Knot Number 6 tile.
