Capping with a clear or tinted top layer is necessary in many cases of inclusions, or desirable when looking for depth or distortion in flat fused work.
Capping inherently has bubble creation potential. The development of a bubble squeeze helps prevent the largest of bubbles. It cannot eliminate all the trapped air that then turns into small bubbles around the inclusions or multiple pieces when covered by a sheet of glass.
An alternative is to do away with the sheet glass capping and instead use enough frit to provide the desired depth, or the necessary material to cover the inclusion. In fusing with two large sheets, a fine covering of powder between the layers will help to eliminate bubbles. However, this will not be enough to successfully cover metal or other inclusions, or provide the amount of glass to give an appearance of depth.
The size of frit to use in a given application can be determined from other styles of glass working. It is known from glass casting that the smaller the frit the greater number of small bubbles will appear in the fired piece. This means that you need to use medium sized frit for cast work. Fine frit is likely to produce many very small bubbles across the whole piece in fusing applications. Large frit is likely to produce larger bubbles, as the pieces themselves trap air as they deform. This means that medium frit is a good compromise between large and small bubbles in capping.
The layer of frit should be at least 2mm thick. This means a lot of frit is required to do the job. To judge the amount, you can measure the area of a rectangle or circle in square centimetres and multiply that by 0.2 to give you the volume (in cubic centimetres) of frit required. Multiplying the volume by 2.5 (the approximate specific gravity of soda lime glass) will give you the weight of frit needed to cover the area.
Alternatively, if the piece is irregular, you can weigh the base and add the appropriate weight of frit on the top. If the base is 2mm, no further work is required to determine the weight. Weigh the 2mm sheet and use the weight of frit to equal the base. If the glass is 3mm, you need two thirds of the weight in frit, and so on for thicker glass.
Using frit to cap is unlikely to eliminate all bubbles, but it will reduce them to a minimum.