Casing glass is one of the most basic of all hot glass operations. Simply put, it is the act of layering glass over glass whether it is color over clear, or clear over color. For a furnace glassblower, casing is a relatively simple matter. He simply dips his gather, marvers, and continues to work. For a lampworker, limited to flame and cane, the problem is a bit more dicey.
Lampworkers case glass for pretty much the same reasons that furnace workers do. Color casing gives an illusion of solid color, even though the core may be clear, as well as providing a background for decorations like frit, trailing, and shards. When selecting colors for casing, you must bear in mind the relative nature of color. Pick pastel opaque as backgrounds for brightly colored decorations and darker translucents for lighter, more opaque decorations. It takes practice to develop a feel for color compatibility and experience to discover which colors work best for what. Due to the sometimes unpredictable nature of commercial borosilicates, accurate labeling and careful note-taking are strongly recommended.
There are two basic methods to casing glass in a flame. The first is to coil the colored cane around the clear core in a continuous stripe. Care must be taken to keep the edge of the colored stripe against the previous edge so the casing will not have any gaps. This method is quick because the casing is accomplished in one long continuous move. This is a very efficient way to case glass. The disadvantage to this method is that air will tend to get trapped between the coils and there is no way to get it out. The second method is to stripe the casing over the core in parallel bands. The principle is exactly the same. Each stripe is butted against the last so that there are no gaps and the rod is heated and marvered to complete the casing. This method takes significantly longer to do than the coil method. The major advantage to parallel striping is that the air trapped between the stripes is completely eliminated as the stripes are melted in. The bubbles are literally 'chased' out as the stripes melt together. Each method has its proper application and it is up to the individual lampworker to judge which method is best for each situation.
However, for applying clear casing over color, the parallel striping method is preferable because you don't want any trapped air visible in the finished piece. After the color casing and decorations are complete and marvered smooth, draw a series of parallel stripes with 8mm, or 10mm, clear rod over the entire surface. Be careful not to overheat the decorated surface so it doesn't distort as you wipe the clear glass over it. Once the entire surface is cased, concentrate your heat on the center of the piece until the ridges soften and disappear and the long skinny bubbles that separate each stripe begin to retreat to either end. "Chase" the bubbles out slowly, first one way and then the other, pinching off any that remain at the tip of the rod. Marver until smooth, blemish-free surface with a feeling of depth, much like furnace glass pieces have.
All of the previously described methods can also be applied to tubing, but a whole series of difficulties can arise which directly affect the control a lampworker needs to blow vessels. First, the casing must begin and end on the thick part of the tube. Do not attempt to case down into the point! With all the intense heating involved, particularly if you case first with color, then decorate, and then case with clear. Your points are sure to melt and buckle. I remedy this by replacing the points with 12" lengths of 10mm, or 12mm, heavy-wall, or capillary tubing. Careful attention must also be paid to wall thickness. Once a tube is cased in colored glass, you can no longer see the wall thickness and must rely on the way the piece behaves to tell what is going on. When applying the colored casing, concentrating on drawing straight; even stripes will definitely help with control later on. If you choose parallel striping, it helps to paddle flat the little "tail" left at the end of each stripe. If you choose to coil the color casing, care must be taken not to buckle the surface of the tube as you go. Another helpful hint for working cased tubing is to shrink the entire tube down to half its original size for marvering. The thicker glass holds the heat longer and allows for more radical surface decoration like combing and twisting. Once the casings, decorations, and marvering are complete, blow the tube back out to a workable wall thickness, marvering it to keep it cylindrical, before blowing it into its final form.
Working cased tubing is definitely for the advanced lampworker only. I would strongly recommend becoming adept with clear tubing and casing solid rod before trying it. The colored borosilicate are very expensive, and these methods are very labor-intensive, so failures are costly. But the reward for success is beautiful, state-of-the-art lampwork, so don't be intimidated.