Press-Molded Tiles

I believe that using a one-piece plaster press mold is the most effective way to create handmade tiles. Tiles made this way are less likely to warp than tiles rolled out by hand or with a slab roller. I have found that press-molded tiles don’t require a special clay body, nor do they need to be flipped while drying (most of my tiles have relief designs, and flipping would ruin the surface detail).

Calculating Shrinkage

I usually prefer to make tile in the same dimensions as commercial tile so that it can be installed as accent tile. In order to create precisely sized tile, it’s important to first calculate the shrinkage of your clay. Roll out a small slab, then measure and mark a 10-centimeter line (1). Fire the tile to the clay body’s recommended temperature, then re-measure the line. The fired difference of the tile equates to the shrinkage of the clay. For example, if the 10 cm line shrunk to 8.5 cm, then the clay shrank 15% (1.5 cm ÷ 10 cm = 0.15 or 15%).

1 Calculate the rate of shrinkage for each clay body you use.

2 Measure the fired tile. My clay shrinks 12% when fired to cone 10.

Next, draw an inch ruler on a piece of paper (2). Use a copy machine to enlarge by the percentage of your clay body’s shrinkage. Now you have an accurate template for making tiles. Next, take this ruler and re-mark the inches on a carpenter’s square with a permanent marker (3). Using this square, you can cut out perfectly square tiles that will shrink to the desired measurement.

Create the Tile Mold

To make many of my designs, I press clay into bisque molds that I’ve created using plants and wildflowers, in this case fern leaves (4). To make a tile mold of the fern design, place a wet slab (with the fern texture facing upward) on a work board; I prefer to use thick Plexiglas or sections of laminate countertop because they are very smooth and make flat-lying tile. Use the carpenter’s square to measure and cut a perfectly square tile with the desired composition (5), then remove the excess clay.

If the clay is wet and compressed to the board’s surface, natural suction will adhere it. If the clay is leather hard to dry, or if another material is being used to create the original tile (foam board, cardboard, etc.), apply a thick layer of petroleum jelly on the back to adhere the tile to the work board.

3 Enlarge a drawn ruler by 12% on a copy machine, then copy it onto a metal ruler.

4 Press clay into bisque molds pre-made from plants and wildflowers.

5 Place the slab facing up on a piece of Plexiglas. Measure and cut a square tile.

6 Secure cottle boards around the tile, with at least 2 inches around the edges.

Position cottle boards around the tile, with at least 2 inches between the tile edge and the inside of the cottle board (6). Secure the cottle boards with clamps, and seal the outer seams where the boards rest on the work surface and against each other with a coils of wet clay.

Mix the plaster, and then slowly pour it into the side of the tile mold box, so that the plaster flows naturally over the surface of the tile (7), reducing the probability of air bubbles. Pour the plaster at least 3 inches thick (8). Remember that the mold is a tool, and it will suffer some abuse—it’s better if the mold is thick and sturdy. Before the plaster sets, tap the table (not the mold or work board) with a rubber mallet to help move air bubbles away from the tile surface.

7 Mix plaster, then pour to the side of the tile, so it flows over the tile.

8 Pour the plaster at least 3 inches thick so the finished tile mold is sturdy.

Once the plaster has completely set (after it has become warm, and then has started to cool), remove the cottle boards (9), trim off the sharp edges of the mold, and sand the outside surface smooth (this helps keep plaster bits out of your clay) (10). Remove and discard the original tile (11), then clean the mold with a sponge and water. Inspect the mold for undercuts and trim these with an X-Acto blade. Bevel the edge of the mold, which will help the tile release easier (12). Thoroughly clean and dry the mold.

9 Once the plaster has set and cooled, remove the cottle boards.

10 Clean and smooth the edges with a fettling knife and drywall sanding pads.

11 Remove the tile from the cast mold and discard the clay.

12 Bevel the edge of the mold, which will help the tile release easier.

Pressing the Tile

To press a tile, use a much larger piece of soft clay than will fit into the mold. I press this clay into the mold by hand (13), cover the clay with a piece of canvas, and gently tamp the clay into the mold with a rubber mallet (14). Remove the excess clay with a wire tool (15), and then use a wooden ruler to scrape from the center to the outer edge in all directions to remove the last bits of excess clay and make sure the tile is flat (16). This is one reason it’s imperative that the mold is made with a work board that has a smooth, flat surface—if the surface of the mold isn’t flat, the tile will be difficult to make and it may not sit flat once finished.

After the clay is pressed into the mold, it may take some time before the tile is dry enough to release from the mold and drop out. For this reason, I make dozens of tile molds, and on the days I make tile, I press several dozen molds before I try to remove the tiles. By the time I have pressed clay into the last mold, usually the first tile is dry enough to release safely from the mold. Be careful not to bend the tile when removing it from the mold—if the tile bends, it will most likely warp in the final firing.

13 Press a large piece of soft clay into the mold by hand.

14 Cover the clay with canvas, then tamp it into the mold with a rubber mallet.

15 Remove the excess clay by dragging a wire against the surface of the mold.

16 Scrape the surface with a ruler from the center to the edge in all directions.

I dry my tiles on drywall ware boards. If it’s a really dry day, place a sheet of newspaper over the top of the tiles to help them dry more evenly.

David Scott Smith is an assistant professor of ceramics at Salisbury University, in Salisbury, Maryland. He is the co-owner of Little Lane Pottery with his wife, Paula, who is also a potter. To see more of his work, visit https://davidscottsmithceramics.com or on Facebook @littlelanepottery.

Comments

Leave a Reply

You must be logged in to post a comment.

Enter Your Log In Credentials
This setting should only be used on your home or work computer.

Larger version of the image

Send this to a friend