Ultramarine,
Prussian blue,
Smalt.
Ultramarine is perfectly good, and every body that likes to use it may
do so.
Prussian blue, equals ultramarine in encaustic, for all intents and
purposes; there is no other blue required for crayons neither.
Smalt may be used, but I think it rather too gritty; its particles are
too transparent for parts where a solid mass of colour is required.
For crayons it does very well mixed with Prussian blue to bind it, both
together make a beautiful colour, the grittiness of smalt will there be
of advantage. This colour will not grow black fixed with wax as it does
in oil.