If you have any experience with glass and clay you might be wondering what's the big deal? Some have used glass to decorate  pottery for ages. Well, this is a totally different way of using glass.

Dr. Robert Kirby

One of the presenters at NCECA 2009 was Dr. Robert Kirby who shared his research on using up to and over 60% recycled glass with clay to form the actual clay body, not just decorate it. 

Why mix so much recycled glass into the clay? One reason is that it significantly lowers the firing and maturation temperature of ceramics, which will reduce fuel consumption. The pieces get fired at a low temperature yet have the strength of higher fired clay. Using recycled glass in tiles, bricks and sculptures could be a good solution to the problem of what to do with all the glass communities gather. 

The glass cannot be home made. You just cannot crush glass yourself and have smooth edges so as not to rip your hands as you work. The product I used was made by a special crushing process that leaves the edges very smooth. It is so soft, some of the finer grades can be used as sandbox sand. It does have the added advantage of being silica free. 


I did not try different percentages of glass at that time.

The clay I used was White Earthenware from Highwater Clays in Asheville, NC.

In my most recent experiments, I tried to mix as much glass into the clay as I could. These high percentage mixtures definitely need to rest a day or two before you can use them with ease. I made one tile and one bowl from the resulting mixes.

recycled glass and clay

The tiles were divided into six sections to show plain clay, velvet underglaze, LUG underglaze, majolica, iron oxide and Cone 06 black glaze. The bowls were divided into plain, clear glaze, white glaze and black glaze.


pl we firwe fired


pl we w paper firedpaper clay test

** Note ... this is the first time I have had this clay body crack.


C 50 50 1recycled glass and clay


D 60 40 1D 60 40 2


K 50 -50Tile ex fired


M 62 38recycled glass and clay test

The mix of glass was the highest percentage of clear glass I could get. I could have used a brown mix or a green mix. The white still has a bit of both colors in it as you can see.

I also mixed some to use in making Garden Sculptures since I thought outdoor use would be a good fit.

I have a large commercial Hobart mixer so I simply mixed the glass with Highwater Clay's White Earthenware. I needed to add a bit of water during the mixing to keep things blending.

The resulting mix felt like a heavily grogged clay but not as abrasive. I used some of it within 48 hours and it was not holding together well. It was like working with concrete. I worked with it again about a week later and it was a joy ... behaved well and held together. I would recommend waiting at least a few days before using it.

This is the work from day one when the clay needed to be supported and the seams required a thin roll of clay for support. After allowing the clay to rest for a week, the next group of pieces joined easily with a bit of scoring and some slip.
Notice that the clay took texture very well.I will have to wait until after it is fired to know how well it will accept color from oxides and under glazes. 
I mixed some of the clay with paper to make paper clay slip ...I<

the bottom with a damp cloth, covered that with plastic a

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 On Monday it was firmly attached with no cracks.

I was also able to build two more villas within about 2 hours. This is two hours from wet clay out of the bag to stable structures. I was impressed with how easy they were to handle a nd build with in such a short time. I left them to dry in the open air and they did so overnight with no cracking.
They dry as hard as concrete! Easy to move around and handle but very difficult to trim or alter since they are almost brittle. I will load them into the bisque kiln and see what happens!


The three villas after low firing.
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The surface is open and ready for color. The textures did not shift or change.There are no large, obvious cracks.There are small ones, but they make the surface more interesting.<

I bisque fired them to Cone 08, approx 1737 F. I fired them very slowly so it took over 11 hours to get to temperature. Right now, they look and feel like any other heavily grogged clay. Maybe a bit harder, but very strong. 


This image shows the inside and the glass melts.

Several small pieces of glass settled at the bottom of my slip container, so I dropped them onto the roofs to see what would happen during the final firing.


The next step in the process was adding color using underglazes and oxides. I knew the results I could obtain with my usual methods on my regular clay body and hoped they would translate fairly well onto this clay/glass mix.

I did a wash on the roof tiles with iron oxide and a wash on the stones of yellow ochre with rutile.

Then I sponged off the raised areas leaving color in the crevices.
More color washed off than I was used to and there did not seem to be much residual color deposited on the high spots.

I then applied brown and green under glazes with about the same results. The pieces were very pale and washed out. I believe this is because the colors are only adhering and soaking in to the clay, not to the glass. So, I applied a second wash, trying not to wipe off as much.
I fired the pieces to Cone 06. Still not Fab!

Image on the right is a villa made from regular clay while the image on the left is the re-cycled glass/ clay. Below are the older villas in my garden.


I don't have an opinion yet on the whole process. It is too early in the 'life' of this glass to have a steady supply for experiments. Once large industries start using it perhaps supplies will be more readily available to small businesses. It can also be used as mulch in landscaping so maybe one day we will be able to grab it at the neighborhood Ace Hardware Store.

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