The following material is taken from an informational packet published by a glassmaking factory in England. Read through the material below, look at
pictures of the equipment used here, and compare it to the process in America that you may have observed or heard about via videos, written descriptions, blog notes, or class discussion of glass making in our local factories.
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’.
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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.
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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.
Answer the following questions.
1. Compare and contrast the the two processes in the different counties?
2. What are the ingredients of glass?
3. What is the furnace heated by in the article above?
4. How much molten glass does the furnace hold?
5. What is the temperature of the furnace during the day? How does this compare to numbers you have seen before? Explain.
6. Can you name three different tools used in glass making?
7. Where is the item put to soften the glass and reshape?
8. Where is the item put for annealing, the process to avoid cracking?
9. Explain what gather is.
10. Describe how color is put into the glass.
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