But it is necessary to keep in view that, innocent though it may seem, [washing off surface dirt with water], yet if it be employed upon works painted on a priming of chalk, and if every care be not taken, it may ruin them in a few seconds, by wetting the chalk through the crevices. Luckily an eye that is a little practised may easily discover these chalky preparations, which are scarcely met with except in certain old pictures of the sixteenth century, or the first years of the seventeenth, with are painted on panel.,