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 |
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.
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