Borosilicate Material Properties

Most characteristics of any given glass, such as colors or hardness, are attributable to metallic oxides, and the "boro" in borosilicate is a reference to boron. Our products are made from these borosilicate glasses: Pyrex 7740,  DURAN® , Kimax KG-33,  or Kavalier Simax,  or , all of which are functionally equivalent. Borosilicate glass has been a primary choice for research and industry  because of its low thermal expansion, high surface strength, and it's high resistance to acids, salt solutions, and organic substances. Low thermal expansion allows us to heat the glass locally to construct elaborate glass apparatus without it shattering. This also means that you can heat and cool it without damage.These borosilicate glasses have a linear coefficient of expansion of 32.7 x 10-7 cm/cm/deg. C. There are other borosilicates having higher expansions, for other specific purposes, such as vials and syringes. 


The blue images on the right (below), viewed in crossed-polarized light using a polariscope, shows what happens when we try to seal glasses of different expansion rates together. These components were sent in by a customer to be fused together. We were able to do this, but when viewed in polarized light, the stresses produced at the intersections of the two different glasses are revealed as multicolored fringes, as well as a black fringe. These stresses are states of tension and compression which compromise the integrity of the seal. This optical phenomenon is known as stress birefringence or double refraction. Another example of this is shown here. Though these components are both technically borosilicate glass, they are not considered compatible

incompatible seal
250ml media bottle with Balch tubes fused on as ports.


Much of the tensile strength of borosilicate glass resides in the condition of its surface - if the glass is scratched or abraded, its strength goes way down. These flaws are sometimes referred to as "Griffith Flaws". We routinely capitalize on this aspect of glass to scribe-cut tubing accurately and cleanly; it's the cylindrical equivalent of scoring flat glass for windows or stained-glass art. For a graphic illustration, check out our video of large diameter tubing being scribe-cut this way using a diamond point, a small flame, and a drop of water. Tubing and apparatus can withstand vacuum well, within the constraints of good design, because the glass is under compression. Under internal pressure, the surface is under tension, so there is greater danger of explosion. Apparatus for pressure require heavier walls and care in use. See a nomogram of pressure versus diameter of tubing (use the markup line tool to draw on the nomogram), or download the pdf, courtesy of Kimble Glass. This is because the strength of glass is very high in compression, but much less so in tension.

Even though a feature of borosilicate glass is its high surface strength compared to other glasses, it is inevitable that glass will get abraded and scratched during use; a colleague refers to this as "glass accumulating defects". We can coat glassware with a thermoplastic covering to protect the surface, and to contain the contents and shards in case of disaster. We do this commonly on apparatus for vacuum, such as receivers and condensers for rotary evaporators.

Much of our "raw" material comes in the form of glass tubing, or as "blanks" in a number of shapes such as round bottom and erlenmeyer flasks, carboys, media bottles, etc. These we modify and transform, most commonly by heating, using glassblowing lathes. Our page of tubing end finishes shows some simple examples, including firepolishing, lapping, and tooling to specific form configurations.

glass tubing w box
Above, stock tubing comes in 5' lengths

We purchase
Some examples of borosilicate "blanks".

You can find more detailed technical information listed at the websites of the major manufactures of borosilicate glass, including optical and chemical characteristics;
DURAN®   a registered trademark of the DWK Life Sciences GmbH
This Youtube features borosilicate glass manufacturing.

The Corning Museum of Glass has good descriptions of other glass types such as soda-lime glass, lead glass, etc., as well as lots of other info on Art & History, Science & Technology.


There are a number of surface treatments for glass such as;
Etching and sandblasting
Organometallic coating; see a sample of gold coating.
Amberizing for light sensitive compounds is becoming difficult to source domestically.
Some sizes of borosilicate amber tubing are available as well.
We can coat glassware with a thermoplastic covering to protect the surface, and to contain the contents and shards in case of disaster.