Clay choice is everything in ceramics—from durability and firing considerations, to the aesthetics and longevity of your finished work. When making tile, clay is the foundation upon which everything is built, so treat it accordingly.
Virtually any clay can be used to make tile, but some clays work better than others. Historically and geographically, choices in tile making were dependent upon the local materials at hand and the technology available for firing (aesthetics followed thereafter). With advances in mining and globalized trade, we now have an endless variety of material resources and technology at our fingertips. With so many resources available for material and aesthetic inspiration, the biggest hurdle is narrowing down those choices. In terms of clay bodies, the most important technical considerations for tile making are warping, durability, and porosity. Warping is the bane of all tile makers and, to complicate things, it occurs during both drying and firing for many different reasons. Roughly half of the shrinkage that occurs in clay happens during the drying process, and the warping that occurs at this point is primarily the result of moisture leaving the clay unevenly.
There are five relatively simple and surefire methods that can reduce wet-to-dry warping, and I highly recommend that you develop an awareness of these to aid you in assessing, solving, and avoiding future problems at this stage in the process.
- Use a clay body high in non-plastic materials or fillers such as grog, molochite, and sand. These materials minimize the need for excess water, which will cut down on warping by allowing your tile to dry more evenly. These fillers create micro channels that allow water to escape evenly throughout the tile as it dries.
- Work as dry as your process will allow. If you are rolling slabs by hand, the clay will have to be wet enough to be pliable, but see how stiff you can handle it before the clay is unworkable—less water, less warping. The same goes for slab rolling, extruding, hand pressing, etc.
- Move your tile as little as possible throughout the making process. Lifting, flipping, bending, and otherwise disturbing the tile during the making or drying process creates memory within the clay that will come back to haunt you.
- When the tiles are dry enough to move without bending, move them to an open-air wire rack. This will help equalize drying with air circulating from above and below.
- Dry your tile in an area free from breezes, uneven heat, and direct sunlight. New tile makers often forget or omit this consideration, leading to much hand wringing and head scratching when trying to assess warping issues.
- Durability can refer to both green (unfired) and fired clay. We will focus on fired strength and porosity because green strength is not nearly as important with tile as it is with wheel-thrown or handbuilt pieces.
1, 2 Avoid excessive handling when forming and cutting clay tiles to minimize warping. Tiles can be cut with a tile cutter or using a wooden template and a pizza cutter. Photos: Colleen Eversman.
When considering durability and porosity, become familiar with the term vitrification. Vitrification is the transformation of any material into glass during the firing process. Clay and glass are the most prominent examples of this. The ring test is a simple (albeit not exactly scientific) way of checking various levels of vitrification. Imagine holding a pot and striking it with your finger. The higher pitched the ringing sound, the glassier and tighter the bond in the material. In other words, it has a higher level of vitrification. Vitrification in clay means it is stronger and less absorbent, therefore in most applications it is less water permeable. Fully vitrified tile is so dense that it may not require glaze to prevent absorption. Tile that is unvitrified absorbs water and can eventually grow bacteria that may break down and cause damage. Unvitrified tiles are far more vulnerable to freeze-and-thaw damage in cold climates. As water penetrates the tile, it becomes deeply embedded in the clay’s matrix. Once freezing occurs, the water expands and breaks the clay apart in a condition called shaling. Throughout history most tile has been unvitrified earthenware—what the industry lumps into the terra-cotta category. The solution to making unvitrified tile more durable is to glaze and seal it. Glazing earthenware tile goes a long way to prevent moisture penetration because you are adding a glass boundary to the exposed face of the tile. Even with glaze, unvitrified tile is still absorbent to a point and certainly can’t handle moisture and freezing to the degree that vitrified tile can.
Note: A kiln load can have varying heat zones, and therefore your tile can be more or less vitrified (porous) throughout a firing. I always err on the side of caution and encourage people to seal my tile as a precaution (and because my tile is not glazed).
Special Clay Considerations For Tile
Your method of making tile will factor into what type of clay you use. Bagged clay is roughly 25–30% water. It is soft and perfect for rolling slabs by hand because a rolling pin can only apply so much pressure and your body has a limit to what it can do. A softer clay is necessary for this method. A slab roller is a simple machine that is able to handle stiffer clay than a rolling pin can handle. More complex tile-making equipment such as tile extruding pug-mills can handle clay that is even stiffer, followed by hydraulic presses with plaster molds that can handle much stiffer clay, and finally hydraulic presses with metal molds, which are able to manage clay that is too stiff to work by hand.
I make my tile with a mix of commercial bagged clay and powdered dry mix of the same clay body as an additive. A little known fact is that most manufacturers will sell their commercial clays in powdered form. Mixing bagged clay with powdered clay can be done by hand or in a clay mixer. This creates a consistency between that of bagged clay and leather-hard clay. Another option for removing water content from your clay is to leave bagged clay out in thick coiled arches or small blocks long enough to achieve your desired firmness. Pitfalls to this method are that it sometimes takes hours, the clay can dry unevenly, and when you go back to check on it, it may be too dry and unworkable.
Excerpted from Handmade Tile: Design, Create, and Install Custom Tiles by Forrest Lesch-Middelton, published by Quarry Books, an imprint of The Quarto Group. To learn more, visit www.quartoknows.com/books/9780760364307/Handmade-Tile.html, www.amazon.com, www.barnesandnoble.com, and https://ceramicartsnetwork.org/shop/handmade-tile.