Tile spread. Tiles made from 75% glass and 25% clay with body stains added to vary the color.

Tile spread. Tiles made from 75% glass and 25% clay with body stains added to vary the color.

Goal

The approach described here focuses on how including high percentages of recycled glass into clay tiles can save energy by lowering firing temperatures and create unique effects, as well as reusing a material instead of sending it to a landfill. The process focuses on making tiles comprised of 75% recycled glass and 25% clay that can be fired and cooled quickly (the entire process takes only
8 hours).

 


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Recycled Glass and Clay Tiles


Loose wooden mold frame, Hydrocal bottom, and fine screen release layer. The mold frame is made from four pieces of 1x2 stock cut into 4-3/4 inch lengths to create a 4x4 tile. A single screw at each corner is tightened to set the mold and loosened to release the tile. These molds are simple, so make several.

Fig.1 Loose wooden mold frame, Hydrocal bottom, and fine screen release layer. The mold frame is made from four pieces of 1×2 stock cut into 4-3/4 inch lengths to create a 4×4 tile. A single screw at each corner is tightened to set the mold and loosened to release the tile. These molds are simple, so make several.

Researching Glass Sources

The first thing you need is a consistent source for crushed recycled glass—both plate and container glass work.

Some communities that are too far from container manufacturing plants have installed glass crushing systems and make crushed recycled glass available to the community, usually for things like construction projects and garden mulch. Do some checking around for such a system near you.

You could also make a glass crusher. Search the Ceramic Arts Daily website to find information from potter Matt Jones on a simple crusher design that is hydro-powered.

The glass doesn’t need to be perfectly clean, but it is critical that the particle size ranges down to dust size, since these very small particles supply the ultimate strength. The source of glass must also be consistent because part of this experience is developing heat and time profiles that work for your glass, clay and kilns. The optimal profile changes if any of these things change.

 


Supplies

  • Crushed plate and container glass ¹?8 in. and finer (also called U.S. Standard 8 mesh)
  • Plaster or Hydrocal
  • Wooden mold forms
  • fine mosquito netting or very fine screen material
  • Gram scale
  • Electric kiln with digital controller
  • Dry clay
  • Sodium carbonate (soda ash)- used as an additive to make the tiles self-glazing.
  • A temperature-adjustable warming table
  • 3 in. putty knife (or other troweling tool)
  • Whiting (calcium carbonate) for mold release
  • Shaker table (not necessary but very helpful)

Mold filled with clay/glass mixture.

Fig.2 Mold filled with clay/glass mixture.

Making the Tile Mold

The molds used to form each tile are made from 1×2 inch boards, a removable Hydrocal bottom and a piece of screen material that helps the tile release from the mold (figure 1).

1. Make the sides of the mold. The length of the boards you use for the mold depends on the size of the tile you want. Lap the edges and number the sides so the molds disassemble and reassemble easily. A 2° angle (draft) built into the mold makes it easier to remove the tile.

2. After assembling the molds, create the removable bottoms. Use Hydrocal as it is harder and more resistant to both chemical degradation and heat than #1 Pottery Plaster. Each 4×4 inch mold is made from 200 grams of Hydrocal and 75 grams of water. After measuring, sift the Hydrocal into the water, let it slake for a few minutes, then mix until there are no lumps and pour.

3. Prepare the mold for use. Make a mold release solution of 10 grams of whiting in 20 grams of water and brush a light coat on the wooden sides of the mold, but not on the Hydrocal.

4. Place a release layer in the bottom of the mold. This facilitates mold removal and extends the life of the mold bottom. Use fine mosquito netting cut into 4×4 pieces, or any material that allows water through while also trapping particles will work. Whatever you choose, be sure it has enough rigidity so that the release layer lays flat as the material is poured onto it.

Place the mold on a small block and slide it down to reveal the green tile on the hydrocal base.

Fig.3 Place the mold on a small block and slide it down to reveal the green tile on the hydrocal base.

Preparing a Clay and Glass Mix

You’ll need a water/soda ash solution to promote self glazing, so dissolve 10 grams soda ash in 100 grams of water. Set aside so it’s completely dissolved when you need it. Baking soda can be used but it seemed to yield less reliable results. If baking soda is all you have, try that.

Put 150 grams of crushed glass into a mixing bowl. Add 50 grams of dry clay. Add enough of the water/soda ash solution (about 45 grams per 200 gram batch of dry materials) to make a mixture that stands up when still but begins to level out when shaken. For these tests, Redart and Plastic Vitrox (PV) Clay were used. They both have a moderate plasticity that works well in this technique.

Spoon or pour the glass/clay mixture into the mold, pat it a little to get the screen to lock in and shake it into the corners. A shaker table works best, but manual shaking works. If necessary, trowel the surface smooth with a wide putty knife or other tool (figure 2).

After any water standing on the top of the tile has disappeared, set the warming table to 200°F and leave it for an hour.

After an hour, the sides of the tile should have pulled in a little. Place the tile, while still in the mold, onto a piece of wood that is cut slightly smaller than 4×4 inches. Loosen the corner screws and slip the mold down over the tile to reveal the greenware tile sitting on the Hydrocal mold bottom. Doing this rather than loosening the corners and pulling the mold off results in nicer sides (figure 4).

Place the green tile, still on the Hydrocal base, back onto the warming plate and turn the heat up to 250°F.

These tiles dry so fast because they are 25% clay and 75% glass. Since the glass doesn’t absorb water, it exits the tile very quickly through the release layer into the Hydrocal mold bottom. This makes the drying process more efficient than with pure clay tiles. The movement of water from the green tile into the Hydrocal bottom also carries the dissolved sodium to the surface of the tile, making the tiles self-glazing.

Terrazzo style tile made from 50 grams PV Clay, 50 grams coarser glass wetted with soda ash mixture, and 100 grams finer glass.

Terrazzo style tile made from 50 grams PV Clay, 50 grams coarser glass wetted with soda ash mixture, and 100 grams finer glass.

Finishing

Scrape and brush any clay and mold release from the boards and sponge off the mold bottom. Every two cycles, soak the mold bottoms for a couple of hours in clear water, then wipe them off and dry them out a little. Otherwise the sodium solution will clog and degrade the mold. During the tests shown here, after ten cycles of making tile, there still was not any severe degradation.

After about 30 minutes on the warming plate, the tile should seem pretty dry. Take it off of the Hydrocal base and turn it over on the warming plate. Pull off the release screen from the tile and rinse it off for another use. If you like the texture left by the screen surface, designate it as the top of the tile. Make a solution of dry clay and water and brush the back of the tile. This prevents it from sticking to the kiln shelf. Return to the heating table for 15 minutes to dry out the wet clay.

Brush the top with some of the 10% soda ash solution. The sodium in the tile and the soda you brush on the surface form a continuous surface glaze during firing.

Firing

Place the tile on a kiln shelf. You need to fire these tiles on shelves because they get too soft during firing to be self-supporting. Use the thinnest kiln shelf you can find, to save energy and because thick shelves cause a big lag between the kiln temperature and the actual temperature experienced by the piece. Fire as follows:

  • Up quickly to 250°F and hold for 15 minutes
  • 30 minutes up to 1000°F and hold for 15 minutes
  • 30 minutes up to 1760°F and hold for 15 minutes
  • Cool normally

This is a total controlled kiln time of just 1¾ hours. If you have a slower kiln, try using the same temperature set points. If your kiln is too slow, the self-glazing may not work as well because the glass may de-vitrify.

It may be counter intuitive, but having an overpowered kiln saves energy. It’s the long kiln cycles that use lots of energy, and so reduces the thermal mass and speeds up the cycle.

Tile made with pieces of art glass placed in the mold first and the glass/clay mixture poured on top, locking them in place but keeping the color on the surface.

Tile made with pieces of art glass placed in the mold first and the glass/clay mixture poured on top, locking them in place but keeping the color on the surface.

Assessing the Fired Tiles

Judging vitrification: I judge the vitrification of a tile by doing a water absorption test. Weigh the tile, soak it in water overnight, then weigh it again. Divide the dry weight by the wet weight to find the percentage of absorption. If it absorbs less than 8 percent water, it’s fully fired.

Efflorescence: There may be some efflorescence or movement of solubles to the surface of the tile. I assume this is excess sodium that does not flux into a glaze on the surface, or from glass within the tile structure. I have found that sealing the tile with a potassium silicate product like Safecoat Grout Sealer (www.afmsafecoat.com) stops the efflorescence.

Glaze durability: I have not had tests done on the durability of the glaze. To see whether the glaze endured acidity, I have soaked the tiles overnight in pure vinegar with no discernible effect.

How the Process Works

Soda-lime glass holds it together. Redart clay by itself has 10 percent absorption when fired to cone 01 (about 2100°F). But when used as the binder in this process, the 25% Redart/75% glass tiles have 5% absorption. What gives? The fired strength is coming from the fusing of the glass. Soda lime glass softens and begins fusing at about 1250°F, then gets softer and softer until it goes into a full melt around 1850°F. At 1760°F, the glass is soft enough to form a full glassy matrix, but not so soft that it distorts or melts the tile.

Using different clays: Use any clay that has some plasticity. Kaolin by itself is hard to use because it lacks plasticity. For non-plastic clays, or clays that lack adequate green strength, adding one or two percent bentonite helps. You can also change the color of the clay body by adding coloring oxides or mason stains in different percentages.

Glass/Clay Compatibility: Why aren’t there compatibility issues between the glass and the clay? As long as there is a mixture of coarse and fine particles, the glass and clay work out the compatibility issues. Tiles made from all coarse particles are too brittle and don’t hold together. Using all fine glass requires too much water and causes bloating in the finished tile. Mixing in larger pieces of glass works, as long as you wet the larger pieces first with the salt mixture. Wetting with the soda ash helps the finer pieces of glass fuse fully with the larger pieces. Without it, trials using larger pieces of glass tended to be weak and break along the large/small particle interfaces.

Glass Quality: The cleanliness of the glass definitely affects the appearance of the final product. Small pieces of label residue turn into ash that generally can’t be seen. Larger pieces can cause spots on the surface or get between pieces of glass and prevent fusing, causing the glass particles to pop out later. If all you can locate is larger pieces of crushed glass with label paper in it, a shallow kitchen screen with about eight openings per inch can be very useful. If you have no other way to separate the paper from the glass, pre-heat it to 1100°F to burn it off. Afterwards, mix it up to integrate the glass with the ash. Don’t sift out the fine particles.

Tile made from Redart clay and glass.

Tile made from Redart clay and glass.

Contribute your Research

This process is very new, and is not in any sense fully developed. I believe that in the future, recycled glass will be a common raw material in ceramics simply because of the way it lowers firing temperatures. It’s not often that you have the chance to contribute to a new and exciting field. I hope you’ll help.

Robert Kirby, a consulting engineer living in Seattle, writes for Ceramic Industry, and his latest research on recycled glass products can be seen at www.bottlestone.com. The research for this article was partially supported by a Waste Reduction & Recycling Demonstration Grant from the Wisconsin Dept. of Natural Resources. For questions, send an e-mail to recycledglassceramics@gmail.com.

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