Changing The Furnace Pot

It was the end of August that disaster struck the Crieff factory, the working day started as normal, glory holes were fired up, colours set out and the makers ready to take the first gather of glass from the furnace, that’s when we noticed it……..there was no glass, a small scrapping at the bottom of the pot and that was about it! We had filled the furnace with raw batch on Friday and Saturday so there should have been glass, but there wasn’t. This only meant one thing – our furnace pot had cracked and all the melted glass was now floating around outside the pot eating away at all the insulating bricks! This wasn’t meant to happen this early, the pot should have lasted until the middle of Ocotber. We knew we would have to prepare for the pot set but thought we had better limp on for as long as we could before asking all the makers to take an early holiday. We lasted another week or so then the decision to switch the furnace off and start the pot set was taken. In Perth the pot set was done when the furnace was still lit, they do the same in Dartington Crystal, in Crieff we have to switch the furnace off, but before we can do this we need to clean all the glass away from the inside of the furnace brick work. This is done hot.

Fraser and Ian pull molten glass from the side pocket of the furnace. When as much glass as possible has been cleaned out of the side of the furnace, the front of the furnace has to be opened so that any glass that has fallen down the front can be cleaned away. If we leave the furnace without cleaning the molten glass away we would never be able to remove the damaged pot or get the front open as when the glass cools it sets solid and will stick like glue.

Fraser, Ian and Robert unscrew the huge furnace door and then force it open. They check the front of the furnace to clear it of any molten glass. The crack in the pot is clearly visible in the photograph below – the diagonal white line running from the mouth of the pot down to the bottom left side. Once everyone is sure no more glass remains, the door is closed and the furnace left to cool.

The new pot is made from clay that has not be fired in a kiln to make it go solid, it is very fragile at this time and is still quite damp so it needs to be dried out, we can’t dry it out too quickly as this will cause it to crack, the most effective and best way to apply a little heat to the pot is to hang a lit light bulb inside! Doesn’t seem like much but this method has been used for years and seems to work.

After 3 days the furnace is cool enough for work to start on the inside. The damaged pot is removed – or in this case the rubble of the pot is removed and the inside of the furnace brickwork tidied up and all the glass chipped and knocked off. Martin and Ian have to be very careful as the remaining glass and broken bits of pot are very sharp.

Once the inside of the furnace has been cleaned the new pot is gently manoeuvred inside the furnace. It has to be positioned precisely in order for it to heat up evenly.

Once the pot is in position Scott carries out the last of the repairs to the furnace door before it is shut and front bricked up ready for it to be switched on.

After heating up gently for 7 days the pot is ready to be filled with new glass and ceramic rings. The rings are just what they are – a circlular ring that sits on top of the glass keeping all the imperfections and impurities away from where the makers gather from. These need to be placed into the furnace when they are hot.

The temperature of the furnace is around 1150 degrees, and it is Calum’s job to put the rings in place – protective clothing is a must, heat resistant gloves, leather jacket and a welders mask are needed. The rings are heated to 500 degrees in the kiln before being taken out and placed on a metal carrier. Now Calum has to run as fast as he can to the furnace without dropping the very hot rings, a bit like the egg and spoon race, but with intense heat and obstacles! Once he reaches the furnace he slides the rings into the mouth of the furnace.

Once the rings are inside the pot the glass is ready to use. Production can once again start and the countdown to the next pot set begins!

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