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Science, Maths & Technology

Does glass flow downwards?

Updated Friday 8th April 2011

Is it true that glass is a liquid, slowly trickling downwards - and is this why old stained glass panels are thicker at the bottom?

In brief, it’s a myth. I mean people often say this and what they mean is that within a window there are individual pieces of glass which are thicker at the bottom.

But then the assumption is that they were all uniform before, and I don’t think this is correct.

First of all, you can think about how glass was historically made, and a lot of sheet glass was made by the so-called Crown method which is where, at the end of a rod, you attach a globe of molten glass and spin it faster and faster until this globe of glass is flattened, and it’s thinner at the outside of the sheet and thicker at the inside.

But what it doesn’t explain is when you go into a church and look at old windows, why so often, and people are quite right to say this, there are little bits of glass which are thicker at the bottom than they are at the top.

Stained glass image of St John The Evangelist, Downside Abbey Creative commons image Icon Lawrence OP under CC-BY-NC-ND licence under Creative-Commons license

And the reason for that is a reason to do with the way in which stained glass windows are actually made. If they’re well made, that’s how they’ve been put together.

Now, when you’re putting together a window, if you’ve got a piece of glass that’s thicker on one end and thinner at the other, you obviously put the thicker piece of glass at the base, into the lead, because then it’s more stable; it’s got a bigger base.

And, secondly, it gives you better protection against the weather. If the glass is thicker at the bottom, there is very little space for cement which means there isn’t any space for the rain to corrode cement and go in.

Stephen Byrne was talking to the Naked Scientists for an episode of Breaking Science orginally broadcast on BBC Radio Five Live in Sprin 2009. Listen to the full programme online.

CORRECTION: Due to an error in editing, the introducion to this piece originally asked if it was true that water is a liquid. Apologies for the slip, and thanks to David Clarke for alerting us through the comments section.


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