We have used granitic top soils for making mud bricks at Murphys Creek, Victoria, and it proved very satisfactory. Generally most of the bricks have been made from heavy clay soils and while it was sticky and hard to use it made good bricks. Cracking in mud bricks can be a problem, particularly in clay soil and in such substances a fair quantity of straw, preferably about eight inches in length needs to be added to the mix. Straw appears to reinforce bricks where the material tends to crack due to uneven drying and to spread the cracks so that they do not break apart. These bricks might not have such good appearance at first but by the time they are quite dry the cracks have reduced in size again.
For this reason slow drying bricks are better than quick drying ones. Against that advantage must be set the time element and the risk of damage from rain. In my own house I started off using grids that could be set up and knocked apart and filled by using a Ferguson tractor and scoop. The grids were the width of three bricks wide and about ten bricks long. Five inch by one-and-a-half inch timber cut to fit into each other like long egg crates were formed up on level ground and the Ferguson with its scoop load would drive over them and drop the mixed earth, onto the grids. It had then to be pressed into the moulds. It worked to some degree, but ramming the earth on them and breaking open the moulds and reforming them made it only a little faster than working by hand. Admittedly the tractor mixed and lifted the prepared soil and reduced the physical labour. Sometimes it would run over the edge of the timber boxing and break it and the relative advantage of the method decreased again. In addition, the bricks were set too close together for speedy drying and unless the the dimension of the site is unlimited it soon gets hard to manoeuvre with the tractor at this comparatively large scale.
Mike Calder at Cottlesbridge has been making mud bricks professionally for some years, and he has developed a satisfactory system of working. He is fortunate in having a soil that can be graded so that it generally requires no addition of straw. He works on a long level area using a Ferguson tractor and a trailing grader blade. The material is spread thinly along the top of his working level, wetted down and then the wet soil is turned over with the tractor blade. It is left in this condition and men follow up scooping the wetted soil and placing it in single mud brick moulds. These are left until dry then picked up and stacked ready for cartage.
Charles Stevenson, who enjoys the pseudonym of McGillicudy, is also from the Eltham area. He has evolved a complicated machine for making bricks. It certainly makes strong clay blocks quickly and efficiently but the finished results have not quite the same personalised appearance of hand made bricks. This machine represents the first attempt I know of in Australia to produce mud bricks in quantity on a commercial basis. In good conditions he should produce well over 1000 per day - I am confident this style of manufacture will increase as people discover the special qualities of mud brick and they become a popular structural medium in Australia. The high price and lack of availability of standard sized bricks, particularly hand made sandstock bricks, will cause adobe building to be increasingly sought after.
The whole system of an alternative society implies a doing it yourself process, so any method which can produce building material by labour alone has distinct advantages.
In Central Australia Dr. Holly, a Czech soil scientist, made building materials out of compressed spinifex and cement which although I have not yet seen the result, appeals to me as a fully practical idea and it would cost no more than mud bricks if done in an adequate way. It is light weight and could therefore become an indigenous material for the vast hinterland.
I have never found it necessary or worthwhile to stabilise earth for bricks.
The only soils that will not work are self mulching soils of the Wimmera or the first material that was used at Murphys Creek. Bricks can be proved for adequacy by making a few from available soils. It is obviously better if they do not have to be intermixed but can be used as they come.
All mud brick soil has two major ingredients. Clay or colloidal material and sand or granular material. These require to be mixed in proportional balance. Generally mud bricks employ more than 50% colloidal and less than 50% granular material to pise de terre 70% granular material and 30% clay.
Variations in soil or moisture make it distinctly possible for faults to happen in these pise walls and it is very difficult to heal them later. They can fret at small points indefinitely. Pise is the more technical method and its quality is not certain until the wall is complete, whereas the mud bricks are fully stabilised before they are used. The actual ramming process is now done with jack hammers, which has made it more reliable than the old hand ramming method. Mud bricks are laid into the walls with top soil mortar that has been screened or is without lumps. It should not be a mere friable loam but must have that admixture of colloidal binding that makes it hold together. It is a general practice to render mud brick walls after they have been erected. This can be done by simply wetting down the walls first and putting on a thin layer of top soil with the hands, using rubber gloves. (The material will rub into all the hair cracks and irregularities yet finish up still expressing the character of the brick, but giving it a more durable finish.)
If one experiments by pressing coarse sand into a lump of wet clay held in the hand you can feel the resultant mass forming into a substance as hard as rock as it dries. This is the ideal to which adobe soils should aim although there are a tremendous number of dimensions between possible and perfect.
Where bricks can be made from different mixtures it is obvious that exposed top soil bricks will not withstand water as well as clay bricks. On the other hand they will not crack to the same -degree either. So, somewhere between these extremes it is generally possible to find a happy medium that will produce apractical result.
Pise de terre, on the other hand, is a much loamier mixture. It has eight or nine per cent water added and is laid into the form work in three inch layers and rammed rhythmically until solid.
In treating mud walls the golden rule is 'like to like', that is use materials that have similar characteristics. Earth walls are very flexible and can withstand considerable movement without cracking so that any waterproofing application should have similar qualities. Boiled linseed oil with some sand rubbed into it gives a waterproof seal to mud brick that will not crack. So do silicone finishes. And of course cow dung previously mentioned, does the same thing.
Externally it is probably advisable to use a waterproofing agent that may vary from oil paint to cow dung. Internally it is preferable to keep the walls as natural as possible, and this can be done by painting on a silicone solution to which some powdered clay of selected colour is added. This mixture will give a soft natural quality and variation throughout.
The hardest waterproofing render I have seen was evolved by Fred Day who has built the majority of buildings for the Wycliffe Bible Translators at Kangaroo Ground. This interesting complex houses the Australian Headquarters of that Institution. All the buildings are constructed in adobe. His recipe is to prime the walls with a one to one mixture of Lokrete (a concrete binding agent) and water. He then applies a mixture of equal parts of fine top soil and cow dung mixed with a one part Lokrete to four parts water solution. So far this treatment has proved indestructable.