Rutile is a naturally occurring mineral containing a high percentage of titanium (90%) and a small amount of iron (10%), and it is the titanium which seeds the variation of colours in the glaze. Most people will use rutile as part of their glaze or slip recipe, however I have been freely brushing a rutile wash over the glaze, being mindful of the strength of the wash according to the range of colours I want. Even so, there is a lot of unpredicatbility to decorating with a wash as the kiln temperature also affect the results.
This is how ceramic arts network describes rutile:
Rutile is that unbelievably beautiful glaze additive that produces colors ranging from light and dark blue, to tan, gold, yellow, and even purple. It also produces a range of crystal formations. It seems to behave however it chooses, depending on the glaze base and the firing conditions.
I’d like to find an attractive blue glaze to use the rutile with – one which the rutile reacts with in just as impressive way. However, as explained in the above quote, the results from rutile are also dependent upon the glaze base, as the rutile brings little change to some glazes.