That is, it tends to vaporize quite easily from melted glass with a resulting color that is rather washed out. It also has a tendency to settle to the bottom of the tank. A nice trick is to gather the boiling manganese glass. The resulting glass surface is pitted purple, which can be quite effective under certain circumstances. In the early days was used as a decolorizer.
Copper is very reliable and easy to use.
When used in combination with iron oxide, it is possible to produce a nice green. Start with equal quantities of copper and iron. Although cobalt is very expensive, it should be used in minute quantities to create a strong blue. It is best to mix cobalt with some fritted cullet in order to disperse it before charging. The depth of color produced by nickel depends on the character of the alkali in your cullet.