Glass in the living space: wall thickness, colour and what harms it

Wall thickness is a question of time, not of material

An object with a wall thickness of 9 mm (0.4 in) costs many times as much as an object with 2 mm (0.1 in). The obvious thought is: more material, higher price. But the value of the material is the smallest part of the calculation.

After shaping, glass has to be annealed, otherwise internal stresses remain in the piece and it cracks later for no apparent reason. And the permissible cooling rate is inversely proportional to the square of the wall thickness. Double the thickness therefore means roughly four times the cooling time and so four times as long in the oven for the same piece. With wall thicknesses under 2 mm (0.1 in), the holding time is in the range of minutes; with every further millimetre it grows disproportionately.

Anyone who buys a heavy glass object is therefore paying above all for oven time and not for sand.

Why the edge shows colour

At the cut edge of a thick glass there is usually a greenish shimmer, while the same pane looks completely clear from the front. The cause is iron, which is present as an impurity in the sand.

Iron occurs in glass in two forms: trivalent, it colours slightly yellowish; divalent, distinctly blue-green. The mixture produces the familiar green tone. What matters is the amount: ordinary glass can contain three to eight tenths of a per cent of iron oxide, hollow glass for drinking vessels and tableware only one to four hundredths of a per cent.

The colour only becomes visible over the distance that the light travels in the material. Through 2 mm (0.1 in) of surface it is not noticeable, through 9 mm (0.4 in) of edge it is. Formulations with particularly little iron are sold as white glass or Extra Clear and cost more, because the sand has to be purer.

Incidentally, this is a good test on your own shelf: hold the edge of a glass object against a white sheet of paper and you can see the quality of the glass immediately.

Mouth-blown, pressed, machine-made: how to tell

Three processes, three signatures. They can be told apart without tools.

Mouth-blown means: slight asymmetry, visible tool marks, occasionally tiny bubbles in the material. On the base there is often a rough spot or a ground-off point, the so-called pontil mark. That is where the piece hung on an auxiliary rod during shaping. This mark is not a flaw but the proof that someone had the object in their hands.

Pressed glass is made in multi-part metal moulds. It can be recognised by the mould seams, which run along the parting lines of the mould and are usually ground over. The wall thickness is very even, the pattern equally deep everywhere.

Machine-blown glass lies in between: as even as pressed glass, but without seams and without a pontil mark.

None of the processes is fundamentally better. For drinking glasses meant to stand in the cupboard as a matching set of six, evenness is an advantage. For a single object that stands on its own, it is not.

Crystal glass is not an advertising term

The term is regulated in the EU. A directive from 1969 sets out four categories, and the boundary lies in the lead oxide content: from 24 per cent a piece may be called lead crystal, from 30 per cent full lead crystal. Lead increases the refractive index, intensifies the splitting of light into colours and produces the long ring when glasses are clinked.

Lead-free crystal glass replaces the lead oxide with barium, zinc or potassium oxide. The refractive index remains similarly high, the glass becomes lighter, and the splitting into colours is somewhat weaker. For vases and objects in which no drink stands for a long time, this is a question of weight and brilliance, not of safety.

Heat, cold and the crack out of nowhere

Glass expands when warm. If one part of a piece heats up quickly and another stays cold, the two areas work against each other, and at some point the material gives way. That is exactly what happens when boiling water runs into a cold vessel or an object from the fridge is put on a warm surface.

Thick glass is not at an advantage here but at a disadvantage: the greater the distance between the inside and the outside, the greater the temperature difference between them. Special glasses with very low thermal expansion are made for this case; ordinary soda-lime glass is not.

For the living space, this means little more than: use lukewarm water for flowers, do not place objects on the windowsill above the radiator, and never put a glass vessel in the oven.

Care: the difference between limescale and damage

A cloudy glass has two possible causes, and they lead to completely different answers.

If it is limescale or detergent residue, the film can be wiped off – with a cloth, a little vinegar or citric acid. If the cloudiness remains after wiping, the glass surface itself has been attacked. This is called glass corrosion, and it cannot be repaired.

It occurs in the dishwasher when ions are leached out of the surface and a dull, silica-rich layer is left behind. This is encouraged by high temperatures, long programmes and, surprisingly for many, by very soft water: water low in minerals takes the missing minerals from everything it comes into contact with, including the glass.

To keep glass objects, wash them by hand or choose a short programme at a low temperature, and do not set the water softener any harder than necessary. Check new cloudiness early with a cloth, while something can still be saved.

Where glass works in a room

Glass has a property that no other material in the living space shares: it takes up space without blocking the view. An object can be large and still not make the room smaller. That makes it useful where mass would be disruptive: on a narrow sideboard, in front of a window, in a small room.

Conversely, glass needs light to have an effect. In a dark corner, a glass object is barely there. It depends on backlight, from a lamp behind it, from a light-coloured wall. And it depends on the edge: the thicker the material, the more the light gathers there and the more colour becomes visible.