Why am I getting hairline cracks in my glaze after firing?
Crazing, as it is called, is usually caused by opening the kiln before it is cool enough to open. Cold air rushes in over the extremely hot pieces and you get this fine network of cracking in your glaze. Sometimes it is caused by too heavy a glaze application.

Can I use underglaze on my high fire clay?
Many brands of underglazes you can use on high fire pieces. In many cases the underglaze will turn color. Dark colors may turn darker and light colors may burn out. It is better to use an engobe as they are made to be fired to cone 5 or 6.

Should I clean my bisque before I apply glaze to it?
Yes. Always wash your bisque under running water before applying glaze. It will dry in just a few minutes and it may save you from some glazing problems.