I have always used 2/3 coloring oxide to 1/3 Gerstley Borate as per Val Cushing. It works great as an oxide wash under or over glazes, especially when sprayed. My personal favorite is a Rutile wash on Tenmoku (golden with that movement indicative of rutile glazes) also very nice on Copper Red. I don't know what you fire at but these are at cone 10 reduction. To avoid speckling, ball mill the ingredients or run them through the finest strainer you have. I always mix them quite thinly, like a water color wash, so that you are in control and can go back with more layers for more variation.
For Oxide brushwork I do not use the Gerstley Borate, it fluxes out so you lose the crispness of line. Try it, you might like it. I just use mixtures of coloring oxides mixed with water and vegetable glycerin. I find the the glycerin very helpful in improving the brushability.
Here are some mixtures I have come across in various places. Sorry for the formatting
For Brushwork (have not tried as washes could be nice)
1- Medium Dark Blue: Cobalt Carb. 30%, Manganese Carb. 15%, Talc 10%, Yellow Iron Oxide 25%, EPK 20%.
2-Vivid Bright Blue: Cobalt Carb 50%, Yellow Iron Oxide 20%, Titanium Oxide 20%, Manganese Dioxide 10%.
3- Dark Ink Blue: Cobalt Carb 80%, Manganese Dioxide 10%, Gerstley Borate 10%.
4- Black: Yellow Iron Oxide 60%, Cobalt Oxide 20%, Manganese Dioxide 10%, Chrome Oxide 10%.
1- Rust Orange Tan: Equal parts Red Iron Oxide, Rutile, Gerstley Borate.
2- Grey Blue: Cobalt Carb. 60%, Manganese Dioxide 20%, Gerstley Borate 20%.
3- Orange/ Tan/ Straw: Rutile 50%, Titanium Dioxide 17%, Gerstley Borate 33%.
4- Medium Green: Cobalt Carb. 25%, Rutile 53%, Chrome Oxide 2%, Gerstley Borate 20%.
Another Source Recommends starting with 150 grams Gerstley borate and adding the following oxides. (have not used)
1. Copper wash: 30 grams,2. Cobalt wash: 15 grams Cobalt Carb,3. Iron wash: 30 grams Red iron oxide,4. Rutile wash: 20 grams Rutile and 5 grams Red Iron Oxide
All of the above should be sieved thoroughly (150 mesh if you have it) and thin. Can be sprayed, brushed, poured (I spray pieces that will have oxides poured on top with CMC gum solution to protect base glaze), splattered, etc.. Applied to bisque or over glaze. Also try applying them very thinly and layer different mixes.
Anyway, I hope this is useful and not too hard to read. I think I will go run some more tests, just talking about this has given me some ideas.