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FEATURE

HackSpace Learns... Glass-blowing

“To make colour, they add metal oxides to clear

glass,” says Josh, the other 50% of Hen Ogledd.

“The pink colour is made with gold oxide – it’s really

expensive. White used to be made with arsenic but

it’s now made with tin; blue comes from iron oxide.

When you add the colours, it changes the quality of

the glass, so the stuff we buy to add colours has to

have guaranteed compatibility.

“Each different type of glass has a different

chemical composition,” explains Josh, “so it

expands and contracts at a different rate. We work

with glass that’s 96 coefficient of expansion (COE),

which is 96 × 10-27.

“If we were to mix this with bottle glass, that

resultant mixture would break, no matter what the

rate at which we cooled it down. Because the glass

we use contracts as it cools at 96. This contracts as

it cools at something like 30, so as the bottle glass

molecules contract, they move; these stay in the

same place, and that causes internal stress. The

forces get so much that it will just explode. If you

hold a piece of glass up to a polarising light, you can

sometimes see the stresses inside, like petrol on the

surface of a puddle of water.”

The granules stick to the molten glass, but they

need to go back into the furnace so that they

themselves can melt onto the surface of the glass

blob. Still turning it round so that the glass doesn’t

drip, the iron goes back into the furnace, then comes

out when the coloured granules have reached the

same consistency as the rest of the glass.

GLASS OF MANY COLOURS

At this point, all the yellow is on one side, and all the

green is on the other side; now it’s time to swirl the

colours around to get a pattern that will look good on

your Christmas tree.

Hen Ogledd uses a chair with a flat arm, which

can be used to roll the iron back and forth at a

constant speed. Keeping the glass moving, we spin

the iron with two hands and grab hold of the end

of the glass with a pair of tongs (yes, this takes

three hands – beginners always need help with

everything). The twisting motion gives the glass

EACH DIFFERENT TYPE OF GLASS HAS

A DIFFERENT CHEMICAL COMPOSITION,

SO IT EXPANDS AND CONTRACTS AT A

DIFFERENT RATE

“

”

Right

The furnace reaches

1200°C, but it’s cool

enough to touch on

the outside – perfect

for warming up a

pasty for breakfast

Below

There are other ways

of adding colour,

other than granules.

Solid rods of different

colours are also used,

but, as they’re so

dense before they’ve

been worked, they all

look the same – black