From "Glassblowing, The Technique of Free-Blown Glass" by Frank Kulasiewicz:
p86
Boro-silicate glass:
Boro-silicate glasses are commonly referred to as "Pyrex," and their compositions vary. These glasses substitute boric oxide (B2O3) for some of the silica. They are used for applications that require rapid changes in tempature without cracking caused by thermal shock. Although some craftsman have used these glasses for hand work, you find they are difficult to melt and require high tempatures. One "Pyrex" type glass has proportions of 80 silica to 13 boric oxide to 4 sodium oxide to 2 aluminum oxide. Surface melting tempatures are in excess of 3000oF.
p89
Sand/Soda/Lime Glass:
By far the largest percentage of glass melted today is refered to as sand, soda, and lime or soda/lime. The silica is the glass former, the soda the flux (melter), and the calcium the stabilizer. There are great differences in the working properties of these glasses, and in the finished glass. This is because the proportion of the oxides are changed, and because soda/lime glasses are seldom melted without the addition of other materials. Some of these glasses set-up (harden) very fast with a temperature drop of only a few degrees. These fast-setting glasses are developed to use with fast machine production-such as bottle-making machines. Others have a slow setting-up time, making them "easy" or "long-working" glasses suitable for hand processes. The latter groups are these we are interested in. Most soda/lime glasses have proportions of about 70 silica to 15 soda to 15 lime. The surface melting temperature for such glasses is about 2700oF., however changing the proportions and adding other oxides can lower the melting tempatures.