FIREBRICKS

By R. Finch

I am sure that every one of my fellow Bradford MES members has a brazing hearth somewhere in their workshop. Do they have the right fire-bricks or did they scrounge some likely looking bricks from somewhere not really knowing whether they were actually firebricks? If they are, are they the right sort of firebrick?

It is generally known that standard house bricks (the red ones that are made of clay) are totally unsuitable, but do you know why? The answer is that the clay brick absorbs some water (about 10% by weight) and this is held inside the brick all the time. If such a brick is heated quickly, the water turns to steam and, as it cannot escape easily through the small pores of the brick, the brick will burst. The reason it doesn't burst when it originally being being manufactured is that the brick is fired in the kiln very slowly – typically taking two days to be heated up.

There are three other ‘firebricks’ commonly available. The most commonly thrown-out brick is the type used in domestic storage heaters. These are very

heavy and are designed to store heat. They are rated for 600 deg C maximum service temperature and as they store heat, they take a long time to heat up. This type, whilst apparently a ‘firebrick’, is not really suited to brazing as the flame from the brazing torch will be heating the brick up rather than the job. Consequently the long time taken to heat the job will probably exhaust the flux.

Next on the list is the typical firebrick used for building into a domestic fireplace. These bricks are structural in that they maintain their strength at high temperatures and are about the same weight as a standard house brick. They are typically yellow in colour. While these will withstand repeated heating and cooling and are strong, their weight means that again, the torch is using most of the heat to heat up the firebrick and not the job being brazed.

The best type of firebrick is an insulating firebrick which is designed for use in kilns. These are made from alumina and can withstand repeated heating and cooling, but as they are about one third of the weight of a yellow structural firebrick, they heat up and cool down quickly, so more of the heat from the brazing torch is available for heating the job being brazed.

This type of brick is white, comes in several grades, the most commonly available being Grade 26, and is the same size as an ordinary house brick – 9" x 4." x 3" yet only weighs 1.6 kg, compared with 3.9 kg for a typical house brick. The grade is the maximum service temperature in hundreds of degrees Fahrenheit, so a Grade 26 brick will withstand 2600 deg F which is about 1426 deg C.

The higher the Grade number, the stronger and denser the brick is. The recommended grade for home workshops is Grade 26 which has a low enough weight, yet is strong enough to withstand the rigours of the workshop. Anyone wishing to purchase these should search for "Grade 26 insulating brick" and have a look for “kiln suppliers”.

Finally, there is a block used by some people, which is not a firebrick at all. This is the aerated concrete block commonly known as a Thermalite or Celcon block. These are lightweight building blocks with a high insulation value, but are not particularly strong. While they can be heated up providing that they are dry, they do not take well to repeated heating and cooling and eventually start to spall and fall apart where the flame has been impinging on the surface. If you don't often braze things, then the aerated concrete block may be acceptable.


This article first appeared in the Bradford MES Bulletin.

 
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