Caithness Crystal : Real Glassmaking Experience in Kings Lynn, West Norfolk

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The main raw materials for glass making are Silica, Sand, Soda, Limestone and Potash. Arsenic used to be added to this mixture however it is no longer used.
 
As the largest component is sand it is vital that the very best is used. The sand is dried, sieved and weighed and the other chemicals are then added to make-up what is known as the ‘BATCH’.

 

Up to five percent crushed glass called ‘Cullet’ can be added to the batch to prevent what is known as ‘Cording’ this is the name given to the striation or rope like imperfection in the finished product, this can also be caused if the temperature in the furnace fluctuates and is not regulated properly. The batch today unlike years ago is mixed by machines and not by hand, is then sent to us as in 25kg bags

The batch is placed and melted at a temperature of 1300 degrees. This process is normally done during the night. When melting is completed the temperature is reduced in the day to that of 1100 degrees.

The furnace is known as a pot furnace, which is heated by natural gas, inside is a stone crucible which holds approximately 150 kg of molten glass. The crucible in the furnace has a life expectancy of approximately 1 year.

Lets take a moment to talk about glass blowing and ornament making. An iron is dipped into the molten glass and a quantity is gathered at the end of the pipe by rotating it just like gathering treacle onto a spoon. This piece of collected glass is known as the gather.

The tools that are used in the glass making process are, blowing irons, which are hollow stainless tubs, gathering irons, shears, Secateurs, wooden block, tongs and Wet newspaper these tools have remained unchanged for several hundred years.

Then the glass is allowed to cool to about 1000 degrees and is rolled onto an iron slab, this process is known as marvering. This forms a parison, which can then be blown or manipulated into animals, glasses, paperweights and many more objects.

Colour used in glass making when used chipped gives a translucent colour and powdered gives a solid dense colour. There are many colours available, and chemicals are used to make up these colours, such as Cerium for Yellow, Selenium for Red, Copper Oxide for Green and Potassium Pemanganate for Purple.

These colours can be added at many stages during the process, however, great care must be taken not to contaminate the batch when going back for more gathers and this is why cool air is blown onto the glass.

The Glory hole is heated by natural gas at a temperature of 1150 degrees, this can be used once or several times during the production process to soften the glass and remove shear marks created on the object, if this process is used for to long it could also loose the form of the object.

You will see the glassmaker rotating the gather constantly this is keep the shape of the object from distorting.

When the object is complete it then needs to be annealed this is the process of controlled cooling, the item is placed into a kiln by knocking it from the iron onto a tray containing sand. The temperature of the kiln is 500 degrees and this allows the glass to cool at a slower rate, which reduces stress, brittleness and prevents cracking in the finished object.

Once the kiln is switched off and the items are allowed to cool overnight the objects are then removed and sent on to the finishing process this tends to be the first job of the morning. The finishing process is made up of three stages.

Firstly milling the bottom on a flat bed revolving steel wheel.

Smoothing on a linished belt (linisher).

Polished on a cork wheel, using pumice.

This process is done entirely by hand.

We commonly get asked about our glass processing and the environment, glass is an ideal material for recycling and is good news for the environment however we do not reuse the small amount of waste from our process these cutoffs could contaminate the whole batch.


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