Backyard glass blowing

This one is legitimately dangerous.  Do not try at home.  Glass is an amazing substance, but I recommend finding a studio where you can work on it.

A couple of years ago*, in the time of great projects, before visas, work and life got in the way, we tried our collective hand at glass blowing.  Why? Because high temperature physics makes for lots of trial and error, and great fun.  Also,  it fits into my theme of trying to recreate old technologies.  But mostly to see if we could.

It started with a not uncommon problem: we had accumulated so much glass for recycling that actually getting it to the recycling centre became a logistical challenge.  Add some gin, and a bit of youtube, and we became convinced that we (Rob, Khilan, I) had a great opportunity at hand.  But first there was much to learn.

Some physics

Most container glass is soda-lime glass, which consists of approximately 75% silicon dioxide (silica), sodium carbonate (soda), lime and smaller amounts of other minerals.  The silica comes from sand; this is fired with the other minerals at up 1675 degrees Celsius to fuse.  The composition of ingredients determine the physical properties of the resulting glass:  colour, softening temperature, chemical durability, and hardness (to some extent).  This is a very energy-intensive process, luckily the resulting glass can easily be recycled by heating it and reshaping it.  “Easily” still requires temperatures near 1000 degrees, however.

Glass-blowing, as it should be done

A simplified view of the standard glass-blowing process looks as follows:  a crucible is filled with glass, this is gently heated.   (Real professionals might start with raw materials, to precisely control the composition.) Between 500 and 800 degrees the glass softens, and slumps together. As the temperature is further increased, the viscosity decreases, and around 1100 degrees, it is right for blowing – imagine the viscosity of honey, and you’re close enough.   A long tube, the blow pipe, is inserted into the molten mass of glass, and a small amount picked up: the “gather“.   To prevent the glass from sagging to one side, the blow pipe is constantly rotated.   The gather is initially shaped into an even form by rolling it on a steel plate (marvering).

The glass blower then proceeds to inflate the vessel by puffing small amounts of air into the pipe.   As the glass is very hot, it heats the air in the middle. As this air expands, it pushes the glass out further.  So it is very different from inflating a balloon.  Here a beautiful negative feedback loop comes into play:  as the glass thins in one part, it cools faster, increasing the viscosity in that area.  Further expansion will therefore happen in the thicker areas.  As a result, we can form vessels with even wall thickness without any internal support  (which we would require when a technology such wheel-throwing ceramics would require).

When the wall gets too thin, more molten glass is gathered, and the expansion continued.  The blown vessel can be shaped with various tools while still hot – the viscosity slowly increases as it cools down.  If the glass cools down too much, it is reheated in a very hot oven  (~1400 degrees): the joyfully labelled “glory hole”.

When the vessel is complete, the bottom is attached to a new rod, called the punty.  A small blob of molten glass is gathered on the punty, and stuck onto the base of the vessel.  The top attachment can then be grooved and snapped off.    This allows finishing of the top of the vessel, to shape and polish the neck and rim, for example.  Once the top is finished, the punty attachment is also removed.

Once complete, the vessel needs to cool down very slowly to prevent the thermal stresses from fracturing it.  This annealing phase happens between 380 and 480 degrees, over several hours to days.

DIY glass-blowing

Nowadays, all of the above is done using gas furnaces, temperature control, and modern materials, but the Romans were blowing glass more than 2000 years ago without any of that.  This forms the basis of our approach: do it like it’s 100 BC, with some applicable short-cuts, and more accessible knowledge on our side.   Needless to say, we adjusted our definition of success accordingly.

The first major challenge we had to overcome, was getting a furnace to be hot enough.  To estimate temperatures, we looked at the colour of the hottest part of the furnace: we really needed a white colour to ensure the glass got hot enough.  This requires three things: fuel, oxygen, and insulation.   Insulation was easy – we bought a high-temperature ceramic blanket, as is used in kilns.  A “salon tested” hair dryer acted as bellows, to provide the oxygen.   For fuel we started on charcoal, but soon upgraded to house coal, because it has a higher energy density, which means more heat.

A stainless steel tube served made a good blow pipe – the thermal conduction is low enough that you can hold one end while the other is placed at a crazy temperature.  As we couldn’t source an affordable ceramic crucible in time, we simply used a cheap stainless steel saucepan.

As a first try, we constructed a small furnace from bricks in the garden.  We lined the inside with ceramic wool, and filled the spaces between bricks with building sand.  The space inside was split in half, with a shelf for the crucible in front, and space for the coal at the back.

This heated up really quickly, due to the low thermal mass of the system.   Also, that much fire in a confined space is more exciting that anticipated:  it rumbles like a frustrated rocket, it wants to push out and escape.  The pressure of the fire burning in such a small space is very noticeable. And the radiated heat is intense!

And our pesto jars gradually turned into a crucible of molten glass.  Not just slumping, but quite liquid.  Still not quite soft enough to gather and blow, but close.


However, the ceramic wool isn’t really meant to be used like this – all the poking and fiddling around it at high temperatures caused it to disintegrate.  And then the rather angry fire leaked though the bricks,  with jets of flames reaching 50cm out,  and shutting us down for the rest of the night.


The second iteration

Although not successful on the first round, we learned many things.   We needed to lift the crucible to be heated from below, not the side, for better heat distribution.  So we constructed a steel grid to put the saucepan on.  The door needed a redesign, so we put more steel in there as well.  And the furnace itself was adapted to have a layer of bricks on the inside of the insulation as well.  In addition the mechanical protection this provides, the bricks increased the thermal mass of the inside which helps keep the temperature more stable.


The heavier construction meant it took much longer to get the furnace up to temperature.  This time we encountered new issues: The extended exposure to very high temperatures did not play well with the metal in the furnace.  For example, the steel grid disappeared completely.  The stainless steel crucible became less stainless, slowly oxidising in the heat.   Things are very different at 1000 degrees.

We were also emitting a lot more smoke, primarily due to not getting enough oxygen into the furnace to satisfy the fuel.  At this point I suddenly understood what the industrial revolution must have felt like:  we smelled of smoke, we coughing up soot, and everything was dirty.   Needless to say, the neighbours were not amused.

Again, a couple of gathers happened, but we couldn’t get beyond the marvering stage.

The third iteration

Finally, we admitted defeat on the fuel front.  We just couldn’t get clean enough glass and sufficiently well-distributed heat from the coal.  One friend provided his propane cylinder, another brought his kiln for annealing –  a problem we had not considered at all yet.   It pays to have the right friends.

This worked quite well.  With a team of suitably protected volunteers, we melted a pot of glass and tried to blow some.  One person would manage the door to the furnace, protected with goggles and welding gloves – this was a taxing place to be.  Blowing was a specially choreographed dance:  The door would be opened, and the blow pipe be inserted for the first gather.  Wait for the pipe to start glowing yellow-white, then gently dip into the crucible.  Pipe out, door closed, marver, keep spinning, a gentle puff, cap the pipe with your thumb to let the air expand.   If you blow too much, you’d simply burst a bubble of glass, spraying thin shards everywhere.  Sometimes, the viscosity would by just right, the blower would stay calm, and we’d get a gentle inflation from the glass, at that point glowing bright orange.  Door open, reheat, gather some more, spin, retreat, close door.  Spin the pipe, blow again.  Now for detachment:  roll the vessel along a stand while someone tries to shape a groove into the neck with shears.  Then attempt a thermal stress fracture by carefully dripping some cold water on the groove.  At this point, the doorman would be present again, to catch the still hotter than 500 degrees glass in his gloves, for transfer to the annealing kiln.  The still hot blowpipe can then be quenched in a bucket of cold water, accompanied by dramatic spitting and steaming.  If you’re lucky this would crack the remaining glass off the pipe, cleaning it for the next round.  And repeat.

However, after 2 hours of direct heating, the crucible would burn through, causing a lava-like flow of molten glass to flow out, usually directly at the gas flame.   This would require swapping out the crucible, and starting with fresh pots, and new pesto jars.  One of the crucibles was made of steel, but not stainless.  All the (clear) glass melted in this pot came out green, due to the iron oxides contributed by the steel.

But did it work?  Well, we managed a couple of mediaeval looking hollow glass baubles.  Not something that I’d put in my mouth, and not particularly fine and crafted,  but they are definitely made of blown and spun glass.


It is a decidedly non-trivial process. (And if you made it this far, you will enjoy this video.)



* I’ve just counted: four.  Still, the world deserves to know about this. Also, many words.