Yellowing around cloisonné wires

Hi everyone.

Does anyone know what might cause opaque enamel to go slightly yellow where the enamel meets the cloisonné wire?
This is occurring on both sterling silver and fine silver wire.

The same happens with copper wire except the enamel is dark brown where the enamel meets the wire.

I am using a butane torch/ torch firing method.

Many thanks in advance.