ron soyland
The type of glass used to make tubes is dependent on several factors, a major one being how practical is it for YOU to work with the glass. Original tubes were made with soda lime glass, which glass blowers call "soft glass." This glass has a lower melting point and can be worked with an air-gas torch. Here we examine some advantages and disadvantages of several types of glass and why one type of glass stands out as the practical choice for new glassblowers.

SODA LIME GLASS: This is the classic "soft glass" that is used to make light bulbs and a large portion of the original vacuum tubes.
ADVANTAGES: Low softening point makes it practical to work with an air-gas torch. Oxygen is not needed.
Very easy to make vacuum tight seals through the glass. The materials for making the seals are easily available from old vacuum tubes or even light bulbs. New seal wire can also be purchased for reasonable prices. A new glassblower can learn to make a good seal in a few hours.
DISADVANTAGES: The most serious disadvantage of the soft glass is the extremely critical annealing process that is REQUIRED to make a tube that will not crack when cooled, or worse, crack a few days later when there is a slight temperature change. This annealing requires the use of a temperature controlled annealing oven, which is another piece of equipment to buy or build. Even properly annealed, some shapes of glass are impossible to re-heat with the torch. They MUST be re-heated in the annealing oven and then immediately be kept hot with the torch when being worked on. This is ridiculously painstaking and inconvenient.
Soft glass is difficult to find in large diameters that are needed for tube making. Suppliers typically stock sizes up to about 20 mm (.75 inch) but rarely if ever stock any sizes larger. Thus, to acquire larger sizes, you typically must buy from the manufacturer. This will require buying a minimum lot, which is typically one case. This will be over 200 U.S.D. for 1 inch diameter tubing, plus shipping on a box 5 feet long and weighing fifty pounds. This availability problem alone is enough to eliminate soft glass from consideration.

LEAD GLASS: This is another soft glass, made with lead oxide as a major component. It softens at a temperature even lower than soda lime glass.

ADVANTAGES: Low melting point, lowest of glasses other than solder glass. Easily worked with air-gas torch.
Annealing can be done relatively successfully with the torch alone. At least on simple tube types.
Easy to make vacuum tight seals through the glass. Dumet seal wire is commonly available and cheap.
DISADVANTAGES: Lead glass has the peculiar property of turning black when it is heated above a certain temperature. It is very easy to slip and overheat the glass and end up with a black splotch in the glass. This is not reversible in areas where the flame cannot reach and makes an ugly tube.
Large soft range. The glass becomes very soft and remains soft over a large range before reaching the melting point. This makes it difficult to control sagging and collapsing of larger pieces.
Lead glass is, like soda lime glass, difficult to get in large diameters. Lead glass is used solely for neon sign work so distributers carry only the sizes used in sign making. This is a maximum of about 15 mm, (.625 inch) This gives the same problem with buying large diameters since you have to buy a case minimum. Also, a new problem has lately arisen: Some glass producers are no longer making large diameters since the vacuum tube industry has stopped buying it. It may only be available by special order in the near future. Thus, it is not very practical to use lead glass in tube making.

PYREX: Pyrex is borosilicate glass and is called "hard glass" by glassblowers. It takes a much hotter flame to work it successfully, making an oxygen-gas torch mandatory.
ADVANTAGES: No annealing required for most shapes used in tube making! In almost all cases just a few swipes of the flame across the joint as it cools is all it takes.
Very sharp break between soft and liquid state. The glass stays firm up to close to the point of melting. This makes it easier to support the glass. It doesn't have as much tendency to sag and deform as soft glass does as it is heated to its softening point.
Pyrex parts can be easily re-heated as many times as necessary using only the torch, without cracking. This is almost impossible with soft glass.
7740 Pyrex is the most common of all glasses used in glasswork. It is used for all kinds of laboratory apparatus so it is stocked by many distributers in many sizes. Thus, it is possible to buy any quantity of the glass at cheap prices without having to buy a case minimum.
DISADVANTAGES: Pyrex is solid to a temperature beyond that achievable with an air-gas torch. Thus, it requires an oxygen-gas torch to work. This can be a problem because of the cost of oxygen and in some countries it is prohibited to have oxygen tanks in the private residence without a difficult to get permit.
The vacuum tight wire seal through the glass is extremely critical to make. It can take months or longer of practice for the new glass blower to successfully make seals reliably. Indeed, seal failure is the single most reason for my tubes to fail! After years and years of doing it!

However, even with the disadvantage of the critical nature of the seals, the advantages WAY outstrip the disadvantages. Failure from cracking of the glass on cooling is practically eliminated. Re-heating of the tube, if a leak is found after the tube is put on the vacuum system, is easily done to repair the leak, without using an annealing oven. The seals can be learned to do reliably, especially with instructions from this website to help. The obvious choice for the glass used in amateur tube making is thus pyrex.

Another suggestion was to use old TV tube glass envelopes for the glass to make tubes. These old tubes are made of soda lime glass and thus have the critical requirement for annealing. The parts once made cannot be successfully repaired without a time consuming preheat in the annealing oven. Also, it is difficult even with a fine grit diamond saw to cut the tubes open without damaging the envelope glass. Another problem with that is that small diameters of glass must still be purchased and used along with the TV tube envelope glass. This makes the annealing even more difficult due to different batch consistencies between the old glass and the new. Not practical.