Chipping edges on handmade terra cotta tile. Why?

Long after installation, handmade clay tiles can be susceptible to chipping at the corners and edges. This is more of an issue when the tile is glazed. As the temperature of the tile increases from the heat of the sun, the dimensions increase and they can begin to press upon each other. This can create high compressive stresses at the bearing points. If the gap between the tiles is not sufficient, the stress at the bearing point can continue to build until a piece cracks out of the corner like this. Terracotta tiles are most susceptible because they have much lower strength than vitrified ones. Since such handmade tiles have been in use since ancient times makers have always needed to compensate for this issue.

Of course the installers of the tile bear the main burden of avoiding this problem. However, in the manufacture of the tile itself, various things can be done to minimize the issue. For example, if an engobe is used, pay attention to its compatibility to the body (e.g. it needs a similar firing shrinkage and thermal expansion). Likewise with glazes, they should fit (not be under excessive compression or tension). Rounded corners are better than angular ones. Another option is to fire the tile to a higher temperature to get more strength - with caution since this can significantly increase firing shrinkage.

Related Pictures

Bi-Clay strips test compatibility between engobe and body

Slips and engobes are fool-proof, right? Just mix the recipe you found on the internet, or that someone else recommends, and you are good to go. Wrong! Low fire slips need to be compatible with the body in two principle ways: drying and firing. Terra cotta bodies have low shrinkage at cone 06-04 (but high at cone 02). The percentage of frit in the engobe determines its firing shrinkage at each of those temperatures. Too much and the engobe is stretched on, too little and it is under compression. The lower the frit the less the glass-bonding with the body and the more chance of flaking if they do fit well (either during the firing or after the customer stresses your product). The engobe also needs to shrink with the body during drying. How can you measure compatibility? Bi-body strips. First I prepare a plastic sample of the engobe. Then I roll 4 mm thick slabs of it and the body, lay them face-to-face and roll that down to 4 mm again. I cut 2.5x12 cm bars and dry and fire them. The curling indicates misfit. This engobe needs more plastic clay (so it dry-shrinks more) and less frit (to shrink less on firing).

How to test if an engobe fits a clay body

This is part of a project to fit an engobe (slip) onto a terra cotta at cone 02 using the EBCT test.
Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way!
Right: SHAB test bars of the white slip and red body enable comparing their drying and firing shrinkages.
Center back: A mug with the white engobe and a transparent overglaze. The slip is going translucent under the glaze because it is too vitreous. Its higher fired shrinkage curls the bi-clay bars toward itself. Reducing the frit will reduce the firing shrinkage and make it more opaque (because it will melt less).
Front: A different, more vitreous red body (Zero3 stoneware) fits the slip better (the strips dry and fire straight).

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