The Complete Guide to Mid Range Glazes has a reciepe for a wash using yellow ochre. You could use the same and substitue the cobalt.
Yellow Ochre 33.33
Frit 3124 33.33
It also states: "Washes are coloring oxides combined with a flux and mixed with water, usually in a 50/50 blend.....most potters just measure them with a spoon."
Copper Wash 1
1 teaspoon copper carbonate (or Mason stain)
1 teaspoon Frith 3110 (or Gerstley borate)
Copper Wash 2
1 teaspoon copper carbonate
1 teaspoon frit 3110
1 teaspoon EPK