What is the ‘Firing’ process?
Clay brick slips start their life in the ground as raw clay. Once it is dug from the ground, prepared into powder, mixed with water and pugged it is formed into shape and slowly dried until it is hard.
Once dried it is ready to be ‘fired’ – heated up until glowing red hot (c. 1100ºC) – which fixes (vitrifies) the clay into a permanent ceramic form.
You can think of brick as being essentially ‘dirty glass’.
The art of firing the brick is quite a complex process often perfected over many trials. There is no set temperature a particular clay body will take before melting into a puddle (literally) or simply not being fired and therefore not frost resistant.
The Firing process
At Matclad we have natural methane fired intermittent kilns capable of firing upto 1200ºC but the clays we use fire between 1000ºC and 1150ºC.
These kilns are made of a steel frame with a lining of ceramic fibre that insulates the walls and ceilings. The product is loaded into the kiln on kiln cars (like miniature railway cars) that are decked out with heat resistant carriers.
Once the kiln is loaded and the doors are sealed the burners are lit and a control panel monitors the temperature whilst controlling the fans that drive the air and methane mixture into the kiln. The combustion by-products are exhausted away through the chimney stack.
The firing profile of a product usually consists of several sections which if not correctly achieved can affect the product appearance and performance. Different products require different profiles but the profile usually consists of the following;
A slow ‘ramp’ at the beginning to final dry the ware, a ‘hold’ to burn out carbon in the body if required, a ‘ramp’ to top temperature and then a ‘hold’ to make sure the heat has penetrated all the product in the kiln. Once completed either a fast cool if the product can take the thermal shock or a slow cool to stop ‘dunting’ – cracking due to thermal shock.
As if that was not complex already another way to vary the appearance of the brick is by controlling the atmosphere of the kiln.
Usually a kiln is setup so that the mix of air and methane means there is enough air to burn all the fuel. This is an ‘oxidised’ atmosphere. There are times and products that require a technique called ‘flashing’ the kiln – that is where we restrict the air (and by implication oxygen) to create a ‘reduced’ atmosphere.
This can create a multi-coloured brick slip such as our Hadley Brindle Smooth. The areas you see the bluey/silver colour ‘flashes’ is where the excess methane stripped oxygen atoms from the iron oxides present in the clay body.
Purpose made brick slips have major advantages compared to making full bricks and cutting them using a masonry saw when it comes to the firing process.
Due to the thinner profile of a brick slip we can load more into each kiln firing than if making full bricks. This means each slip uses less energy to dry and fire than cut slips.
It also means our factory can be more compact than a traditional brick works because the amount of clay we need to process is lower per unit, the amount of water decreases (which needs to be driven off at a later time) and we can ship the product further before the economics of haulage start to play a factor in the cost. Cut brick slips are usually made in a factory, transported to the brick cutting operation and then shipped to the site.
We ship reasonably economically to Europe, the middle east and as far as Japan!
As you can see in our product range the average cost of a square meter of purpose made brick slips is about 30-50% more cost effective than cut slips. This is all down to processing, transport and energy savings made during manufacture.
Frost Resistance and the firing process
The firing process largely determines the frost resistance of the brick slip.
The temperature we fire our products to affects the final water absorption (WABS) – that being the amount of water the brick will absorb if immersed or saturated. The higher the WABS the higher the relative likelihood of failure due to frost.
The more heatwork the product sees the lower the WABS. Heatwork not only means the temperature but also the length of time it is exposed to that temperature.
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