There are two common types of glass used while torch-working which vary depending on what type of "fluxes" (chemicals) are added to the silica. These include: Borosilicate glass and Soda-Lime glass. Each has a different coefficient of expansion (the level of expansion and contraction of the glass while in and out of the heat) and melting temperature, thus why the glasses cannot be mixed. I have worked with Borosilicate glass, "Boro" or "hard glass", and the Soda-Lime, "soft glass", in the past and find that each has their own unique traits and characteristics.
Boro glass has a fluxing element of Boron Oxide. A common brand name for Boro glass is Pyrex. This glass has a low coefficient for expansion but a need for a higher temperature torch to efficiently melt the glass. I find this type of glass is best when making sculptures, marbles or larger glass pieces due to the fact it can be out of the flame for longer periods of time and not crack. In torch-working, this glass is bought in tubes for blown work (vessels, beads, etc.) and rods for solid work (beads, marbles, sculptures, etc.). Currently, I work with this type of glass because I have a higher temperature torch and want to create larger pieces.
Soda-Lime glass has fluxing elements of sodium carbonate (soda), lime, dolomite, and aluminum oxide. Soda-Lime has a higher coefficient of expansion and thus must be kept in the flame almost at all times to avoid cracking. This type of glass is great for bead making, small sculptural work and blown work. Soda-Lime melts faster and at a lower temperature than Boro, so a high-temperature torch is not needed and would actually boil the glass if it became too hot. I began creating beads with this type of glass and found it useful to learn how to create shapes and detailed designs with the glass continually in the flame. This type of glass is usually bought in rods for solid work.
When choosing what glass to work with remember to consider the type of torch you will be using and the glass pieces you are creating…there are strengths and weaknesses to each!