This chapter discusses all about grain and liquid enamel. The painting enamel information is in the Supplements and Add-Ons chapter. This page lists some infomation that did not make it into the book.
This enamel chararteristic of soft, medium and hard is dicussed: soft enamels fuse at a lower temperature than medium and medium fuse lower than hard. If you put a soft enamel under a hard enamel, you will probably get pull-through, which may not be what you want. The book mentions that this characteristic is dependent on two things: the enamel's fusion flow number and its softening temperature. Here I give a bit more about this...
Normally, but not always, a low softening temperature and a high fusion flow makes an enamel a soft enamel. Coversely, but not always, a high softening temperature and a low fusion flow makes a hard enamel.
The fusion flow number was obtained in an interesting way. Tom Ellis related that a small pill sized piece of fused enamel is fused to a steel plate. Then the steel is set in a kiln at a certain temperature, say 1450°F, in a verticle position, for a standard period of time. The amount of dripping/flow the enamel causes on the plate is used to determine the fusion flow number.
Enamels with a low fusion flow number (ie: they didn't flow as much) were ones that have a heavier viscosity. Viscosity being the thickness of the enamel when it flows.
All these characteriscs affect how one enamel interacts with another. See the manufacturers specification for these numbers. For Thompson Enamels, see The Thompson Workbook.