ISO 3585 · DIN ISO 3585

Borosilicate 3.3 — the glass, and its limits

Almost every piece of glassware we sell is made from it. Here is what the 3.3 actually refers to, what the material will and will not survive, and the numbers worth knowing before you heat something.

The name

Why 3.3

The number is the glass’s coefficient of linear thermal expansion: 3.3 × 10⁻⁶ K⁻¹, measured between 20 and 300 °C. It is not a grade, a quality tier or a version number — it is a physical constant, and it is the reason this glass behaves the way it does.

When glass is heated unevenly, the hot part tries to expand while the cold part holds it back. The resulting stress is what cracks it. Halve the expansion coefficient and you roughly halve the stress for the same temperature difference. Ordinary soda-lime glass expands at about 9.1 × 10⁻⁶ K⁻¹ — nearly three times as much.

That single property is what makes the difference between a flask you can take straight from a heating mantle to a cooling bath, and a jar that has to be treated gently.

The composition behind it is high in silica and boron oxide, with very little sodium:

Component	Share by weight
Silica — SiO₂	80.6 %
Boron oxide — B₂O₃	13.0 %
Sodium oxide — Na₂O	4.0 %
Alumina — Al₂O₃	2.3 %

Interactive

Thermal shock, side by side

Drag the temperature jump and watch what happens to each glass.

Sudden temperature change — 120 K

0 K260 K

Borosilicate 3.3

Soda-lime

Thermal shock resistance is not one fixed number. It falls with wall thickness, rises with good annealing, and a sudden chill is harsher than a sudden heating because it puts the outer surface into tension. The thresholds used here are the figures commonly quoted for laboratory glassware of ordinary wall thickness, and they are a guide, not a guarantee.

Reference

Properties to ISO 3585

Thermal and physical

Linear expansion, 20–300 °C	3.3 × 10⁻⁶ K⁻¹
Density at 25 °C	2.23 g/cm³
Strain point	525 °C
Annealing point	565 °C
Softening point	821 °C
Maximum working temperature	500 °C, short periods only

Chemical resistance classes

Hydrolytic resistance (ISO 719)	Class HGB 1 — highest
Acid resistance (DIN 12116)	Class S1 — highest
Alkali resistance (ISO 695)	Class A2

Note the asymmetry. Borosilicate 3.3 sits in the top class for water and for acids, but only the middle class for alkali. That is not a defect — it is chemistry. Hot alkaline solutions dissolve the silica network itself, and no silicate glass escapes it.

Where the temperature limits sit

Honest limits

What borosilicate 3.3 will not survive

The material is remarkably inert, which makes the exceptions worth memorising. Each of these attacks the glass itself, not whatever is on it.

Never

Hydrofluoric acid

Dissolves silicate glass outright, at any concentration and any temperature. There is no safe exposure and no glass grade that resists it.

Never hot

Phosphoric acid

Tolerable cold and dilute; aggressive hot and concentrated, where it etches the surface and progressively thins the wall.

With care

Strong hot alkali

Hot caustic soda or potash dissolves the network steadily. Brief cold contact is routine; prolonged hot contact frosts the surface and eventually weakens it. This is why base baths destroy ground joints and sintered discs.

Damage is cumulative and invisible at first. Alkali attack does not announce itself with a crack. It frosts the surface, widens the pores of a frit past its grade, and loosens the fit of a ground joint — all of which show up later as a failed separation or a joint that will not hold vacuum.

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