Crazing - is fine cracks in the glaze, sometimes one or two long ones but most often a fine web-like network akin to cracked ice or crazy paving. Crazing is not dirt or staining as is gemonly misunderstood nor is it due to intense heat and it is not always readily visible when it is perfectly clean. These errors are most unfortunately reproduced in many books including and most unfortunately Blanchett on page 62, Barnard has the accurate explanation of the reason on page 66 plate 55. Authors usually check their sources and refer to earlier publications, I wonder why this excellent book is oft ignored.
Crazing is a near inherent feature of high gloss tiles, only a few percent of one hundred year old tiles do not exhibit crazing, even today manufacturers of high gloss tiles offer no guarantee that glaze will not craze. Tiles that have not crazed will usually be brown or red clay bodied as the clay absorbs less moisture and expands less - the underlying cause of crazing is the expansion of the tile body when it absorbs moisture. Glaze is in effect a thin layer of glass on the surface of the tile which is impervious to moisture, the clay is porous and can absorb moisture, when it does it expands a little. The glaze can not expand and so cracks finely, this is crazing. Crazing in fireplace tiles is often more easily seen as there was no damp proof course beneath ground floor fireplaces in victorian and edwardian houses so providing a ready source of moisture and there was plentiful soot and smoke to get trapped in the fine cracks. Washstand tiles also craze due to humidity and often have soot in the crazing due to candle and gas lighting.
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