Rub them over with butter, and then apply potash moistened in a little water, and rub the spot until all traces of it disappear; then wash in plenty of water to take out the potash; or the moldy spot may be wetted with yellow sulphide of ammonia, by which it will be immediately blackened. After a couple of minutes wash it off, and remove the black stain with cold, weak chlorohydric acid; then wash well with warmish water.