What’s causing the rough, pitted surface on my decal transfer/on-glaze enamel/lustre decoration?

The glaze surface appears to have a layer of sugar stuck to it. In actual fact it is a mass of tiny bubbles. In industry this fault is known as ‘spit-out’ and the bubbles are formed by moisture being driven out from within the clay through the glaze layer. All ware absorbs water from the air naturally through the unglazed areas of the piece. Whilst this normally causes no problem, if the ware is then decorated and refired to a much lower temperature as in the case of on-glaze work, the moisture is forced through the glaze layer by steam pressure within the clay. The glaze is not sufficiently fluid to flow back and so the surface disruption becomes ‘frozen’ in place. Sadly, the only effective treatment for affected ware is to refire to the glaze temperature which will often burn away or destroy the on-glaze work. The accepted way to prevent this is to refire any glazed ware which has been out of the kiln for any length of time immediately before applying and firing the on-glaze decoration.