You can make a very simple glaze just mixing the ash out of your fireplace with clay in the right proportions and making it hot enough to melt. Sadly, it takes a lot of wood to make ash and I just don’t generate enough to make buckets of glazes, despite having a woodburner.
This glaze takes a well known ‘fake’ ash glaze and reformulates it to work with the materials I have to hand – Cornish stone and clay. As you can see it, quite runny, and has the characteristic ‘rivulets’ of an ash glaze despite not having any ash in it at all. This glaze, and many more of mine can be found on the excellent glazy.org.