Large Tiles for Kiln Shelves

Pizza stone in use

People frequently wonder if other materials than mullite can be used for kiln shelves.  Mullite is used for its strength and very small expansion, even at high temperatures, as used in ceramics firing.  There are other materials that can be used in kilnforming of glass such as refractory fibre board, and ceramic pizza stones,  the best of which are made from mullite.  This post is about using ceramic floor tiles.

An unglazed floor tile, 11 x 11 inches

Structural Soundness

A major element in obtaining and using a floor tile is how sound it is.  Tapping the tile to determine whether the sound is a low toned ring or a dull thud is important.  There may be invisible cracks within the tile.  A dull thud is an indication that the whole tile has one or more cracks in it, or that it has not been fired high enough to completely vitrify the clay. A low frequency tone indicates there are no cracks and that it has been fired sufficiently high.

Flatness

The first thing you need to do is make sure the ceramic tile is flat and without undulations before using it. To test this, get a straight edge and move it along the tile to look for any slivers of light coming through underneath the straight edge. Any light or variation in the amount indicates depressions that can produce bubbles during the firings. Do this test at least twice at right angles to each other.  Take note of the depressed areas (or even possibly high areas) to know where these uneven areas are to work them out of the tile. 

You can do the above test in the showroom.  Another more accurate means of checking is more difficult to do in a sales area.  Place a line of dark powder, say black glass powder, and with a straight edge held vertical to the shelf, drag the powder across the shelf.  Where there are dark patches is an indication of depressions.  The area and depth can be seen from the spread of the visible powder and to some extent the density of the colour.

Making Shelf Flat

If you buy two of these large tiles, you can rub them together face to face in circular motions. The abrasion marks will show the high spots, with the low spots clear of those marks.  This will indicate the amount of work needed to get the whole surface even.  The smaller the unmarked areas, the less grinding will be required. You can add an abrasive with some water to form a slurry and continue to grind until everything is even. The use of water with the abrasives is important to eliminate dust which might be harmful, and to ease the grinding process.

The above is a manual process.  If you have a large enough flat lap, you can mechanise the flattening process.  Using decreasing grit sizes, you can grind the shelves level with a high degree of smoothness. You do not have to use a grit of less than 200, as the tile structure is even more coarse than that.

If you can't find unglazed floor tiles, you need to look at the back of the tiles.  Many floor tiles have a grid pattern on the back to ensure sufficient adhesive is used.  This makes getting the back, unglazed side flat more difficult or time consuming, because they will need to have the grid ground down to the lower surface.  In this case, it may be that you need to sandblast the glazed side before making sure it is flat.  The sandblasting can make a flat tile uneven by unequal times spend on various parts of the tile, so you have to check after sandblasting for the flatness.