Ash is 33% of a Nuka glaze,. And as you can see in the above image. Preparation plays a big part.
Recipe for Nuka glaze, cone 10 Oxidation
Water_1.5 times the weight of dry ingredients.
add 3% bentonite to help keep the glaze in suspension.
The downside of bentonite though is, it takes a lot of effort to sieve the first time.