Combining Black and White

Black and white are at almost opposite ends of the viscosity spectrum in glass terms. Black is the runniest, and white is the stiffest. Black transmits heat more quickly to the lower layers than white.  White is the glass that absorbs and transmits heat most slowly.

an example from Pintrest

A lot of care is required when combining the two.

If the white is on top of black, the white shades the heat – more than other colours - from the black underneath, so a lot of stress can build up in the black.

You need to give a lot of time for the two to adjust to each other. A slower rate of advance than normal is advisable. A significantly longer soak at annealing temperature is required. The annealing cool needs to be much slower than for other glass of the same dimensions.  Consider slowing the rates to half your normal firing rates.  Also double your soak times.  After some experience you will be able to alter these cautious rates to those more suitable for you.