BLOCK PRESSING
It has already been mentioned the mixture to be pressed should not be too wet. You cannot
press water and if you try your press machine will probably break. Another simple test is to
take a small amount of your mixture and roll it between your hands until you have a roll
about 6mm in diameter and 50mm long. Now, on a smooth surface, slowly roll it down to
3mm diameter. If the sample is too wet it rolls down to 3mm or less without breaking. If it is
too dry it will break when it is still thicker than 3mm. The perfect sample will break into
pieces longer than 10mm when the diameter is 3mm.
Normally you might start by adding 10% of water (by volume) slowly to your mix. So if you
have a dry mix that is 20 buckets in volume, you will probably start by adding 2 buckets of
water a little at a· time. You should keep testing the mix as you wet it until it is right.
Experience is the best way to know if the mix is right. When you have made a lot of blocks
you will know when the mix is right for pressing. But still do the tests from time to time to
prove that you are right.
There are two types of manual presses that you could use. The first is called a "constant
pressure press". This type of press will press any quantity of mix that is put into it. One or
two people apply their strength to a lever until they cannot compress the block any more. One
problem with this type of machine is that it can produce blocks of different thicknesses
depending how much material is put· into it. It is very important with this sort of machine to
have a gauge box so that the same mount of mix goes into every block. In this way most
blocks should be the same thickness.
For this first type, the volume of the compacted block compared to the empty press chamber
should be about 1 to 1.65. So if the empty press chamber measures 32cm long x 16cm wide
x 14cm deep (volume = 7,168 cubic cm) then the pressed block should measure 32cm x
16cm x 8.5cm (volume = 4,352 cm). As a very rough guide, the finished block should have
a height slightly more than 3/5 of the empty chamber height.
The second type of machine is called a "constant volume" press. This type has a definite
"stop" when no more pressing can be done. As long as this stop is reached, the blocks
produced will be all the same thickness. If the stop point is not reached, there is too much
mixture in the press and the block that comes out will be too thick. Blocks like this should be
broken up and put back into the mix. With this type of machine you still need to make sure
that the same amount of mix goes into every block by using a gauge box, or by filling the
chamber exactly the same each time. The strength of a block comes from the amount it has
been pressed. If there is too little mix in the press, the block that comes out will be less
dense and so less strong than it should be.
BLOCK CURING
From the press the blocks can be carried by hand to the "curing" area. It is best to use two flat
boards to lift and carry the block between. It is easier to lift blocks in this way and it helps
prevents damage to the block.
Curing is the process of the block getting stronger and setting hard. SSB's need to be cured in
the shade on a clean and flat area. If no building is available shade can be provided by cut
thatch grass in a thick layer over the blocks.
The blocks can be stacked together after 24 hours, still in the shade. They should not be
stacked tightly together or too high or the bottom layers will start to crack. Perhaps blocks
should be stacked no more than 5 high until they are fully cured. Spaces should be left for air
to pass between the stacked blocks.
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