A wood ash glaze is not generally something you mix from a recipe. It is something you develop from a batch of ash (see the article below for more information). This ash glaze contains no iron but does have a little cobalt (about 0.25%). The ash is about 50%, with 20% EP kaolin, 20% Custer feldspar and a little rutile (2-4%). If your attempted glaze does not melt well enough consider substituting some of the feldspar for balanced boron frit. If it melts too much and crazes add some silica at the expense of the feldspar.
It makes sense to maximize the percentage of wood ash. This glaze was the product of preparing a large ash batch and a project to develop a glaze specifically from it (this is important because every batch of ash is different). This recipe contains ~2% iron oxide. Wood ash generally contains very little Al2O3, a critical oxide needed for stable glass development. I thus added 20% EP kaolin (any kaolin should work), it supplies the Al2O3 and suspends the slurry. The ash usually contains lots of fluxing oxides, but it still needs help to melt a glaze at cone 6 (so add 30% feldspar, it also supplies more of the critical Al2O3). For better melting consider substituting some of the feldspar with an Al2O3-containing borax frit (like Ferro frit 3195, even better, frit 3249 since it will prevent crazing). To ease development it is often easier to add more powdered materials to an existing slurry (e.g. it is likely that more frit will be needed), this will push down the total percentage of ash possible in the recipe - thus I often end of with 40% ash.
Wood Ash Glaze
Common washed wood ash can supply important ceramic oxides when melted, so it can comprise significant percentages in a recipe. Plus it can produce unique visual effects.