Borosilicate Glass: Why There Is More to It Than Heat Resistance

What the temperature range means in practice

Material specifications for borosilicate glass usually give a range within which the glass survives changes in temperature undamaged. What matters is less the individual figure than the principle behind it: a piece of glass never heats up everywhere at the same time. The warm spot wants to expand, the cold one resists, and tensile stress builds up between the two. If this exceeds the strength of the material, it cracks.

This is exactly where the boron makes the difference. Because borosilicate glass hardly expands, the difference between the warm and cold zones stays small, and the stress stays below the limit at which glass gives way. Soda-lime glass reaches this limit at temperature differences of just a few dozen degrees. Borosilicate glass tolerates many times that.

In the home this mainly concerns two situations. The first is an open flame that heats a piece of glass locally and unevenly while the rest of the glass stays cool – the classic case in which ordinary glass cracks. The second is a vessel that is filled alternately with hot and cold. In both cases the problem is not the absolute temperature but how quickly it changes.

One side effect of thin-walled glasses is often misread: such glasses quickly feel hot on the outside. This is not due to good thermal conductivity, because glass conducts heat poorly. It is because with a thin wall there is simply little material between the contents and the hand. Grips, handles or sleeves made of another material solve this by design, but do not change the properties of the glass.

The second, less well-known advantage

Borosilicate glass can be made with thinner walls. Because it builds up less stress when the temperature changes, the wall needs no material reserve to absorb that stress. The wall thickness that soda-lime glass needs to avoid cracks can be dispensed with here.

Thinner walls have two consequences. The first is weight: an object made of borosilicate glass weighs considerably less than one of the same size made of conventional glass. In addition, borosilicate glass also has a lower density – both effects work in the same direction.

The second consequence is light transmission. The less material the light has to pass through, the less of it is absorbed on the way. That is why even through-coloured borosilicate glass looks transparent rather than murky: at the same colour intensity, the wall thickness decides whether a glass lets light through or looks dull and heavy.

In the home, this is the real difference in design terms. A thin-walled glass object looks less bulky, casts less shadow and does not appear massive even in small rooms. This lightness is a property of the material, not a question of shaping.

Why it is so often handmade

Borosilicate glass has a higher softening point than normal glass. It has to be worked hotter and stays mouldable only briefly. Automated pressing processes struggle with it; hand-forming and mouth-blowing work well.

The process leaves traces on the finished piece. Wall thicknesses vary slightly, shapes differ minimally from one another, and occasionally fine air bubbles are trapped inside. In an industrially pressed glass these would be faults. In a hand-formed one they are simply part of the process.

Anyone buying such objects should therefore not expect two pieces from the same series to be identical. Dimensions given for hand-formed glass are approximate values, and slight deviations in height, diameter or weight are normal. Anyone who needs exact uniformity, for instance for a strictly symmetrical arrangement, is better served by a machine-made glass.

What the material cannot do

It is not break-proof. Resistance to temperature changes and impact resistance are two different things – a thin-walled borosilicate glass that falls onto tiles breaks like any other glass. Anyone who reads "laboratory glass" and infers that it is unbreakable is mistaken.

It does not belong in the normal bottle bank. Its softening point is too high for the regular recycling cycle: when a batch of waste glass is melted down, borosilicate particles remain unmelted and disrupt the melt. Just a few shards can make an entire batch unusable. Broken pieces therefore belong in general waste or at the recycling centre.

And it is more expensive. The raw material costs more, processing needs higher temperatures and more energy, and handwork is often involved too. For a simple water glass this extra cost is hard to justify. Where, on the other hand, open flame, hot liquids or frequent changes in temperature are involved, it decides how long the object lasts.

How to recognise it

Two clues help when no material is stated. The weight relative to size – borosilicate has a lower density and therefore seems too light for its volume. And the colour when looking through it: looking through a thick spot or across an edge, ordinary glass shows a slight green tinge, while borosilicate glass stays neutral.

This green tinge has nothing to do with wall thickness but with the composition. It comes from iron oxide, which is present as a natural impurity in quartz sand and is not removed from ordinary glass because doing so is costly. The thicker the spot you look through, the more clearly it shows. Borosilicate glass is melted from purer raw materials and therefore stays largely colourless even at thick spots.