Opalescent enamels are semi transparent, usually getting more transparent the higher they are fired. They need a special firing to achieve opalescence.
Remember opalescence cannot be achieved over another layer of enamel. The copper oxides that are absorbed in a 'next-to-copper' firing are essential for developing the opal effect. If opal enamels are fired over other enamels they will never be truly opalescent - but they will be partly clear and can be very effective in cloisonné for creating a semi-transparent alabaster effect with a great deal of depth.
It is recommended that you fire once high to achieve transparency, and then low for 3 to 4 minutes to develop the effect. Opal colours are soft and ACID SENSITIVE Dilatometric Softening Point: 914F Fusion Flow: * Expansion: 341