Sometimes your clear base has bubbles, or as the trade calls it, seeds. When capped with opalescent glass, in certain circumstances, these tiny bubbles can become larger and rise to the surface, pushing the opalescent aside as it rises. This leaves a clear spot in the midst of the opalescence.
One way of reducing this problem, is to avoid it altogether. This can be done by placing the clear on top of the opalescent as a cap. This way the bubbles, if any, are rising through the clear.
Flip and Fire
If you can’t, for one reason or another, cap the piece with clear, you can fire upside down. Again, the bubbles are rising through the clear. When the firing is complete, you can flip it over to the right side. You will need to clean thoroughly and take to a fire polish temperature to get the shiny surface back.
Another way is to fire to a lower top temperature with a longer soak. This means the glass can take up the profile you want without becoming so soft that the bubbles can rise through the glass. You will need to observe to determine when the glass has the right profile, and then advance to the cooling and anneal phases.
Low and Slow
This last way of reducing the possibilities of bubbles rising through toward the top is based on the characteristics of glass. As glass becomes hotter, it becomes less viscous and so allows the air to rise toward the top of the glass surface. Using a low temperature gives a more viscous glass to resist the bubble movement. The long soak at the chosen lower temperature allows the surface of the glass to take up the profile you want, as the surface is hotter than the bottom of the glass, therefore reducing the possibilities of bubbles rising. It does take a longer soak at the top temperature, but it also reduces the marking on the bottom of the piece.
This low temperature process is using the principles of heat work. The effect on the glass is a combination of temperature and time. The higher the temperature, the less time is required. The longer the time, the less heat is required. The heat work put into the glass to achieve the effect you desire is determined by the combination of temperature and time used in firing the glass. This principle of heat work is why you can achieve the same effect at very different temperatures, depending on the length of time a piece is soaked.