Post 709 -by Gautam Shah
Earlier article in the series CLAY MATERIALS for SURFACE FINISHES and PRODUCTS MAKING > Part -I (https://interiordesignassist.wordpress.com/2019/11/02/clay-materials-for-surface-finishes-and-products-making-part-i/)
Clays and soil materials are universally and abundantly available with negligible cost of procurement. Nearly one-half and two-thirds of the world’s population still live or work in buildings made with raw clays, baked into brick walls and floors and use several other baked products like firebricks, storage and cooking utilities like pots and vessels. Variety of mixed soils are used as natural raw materials for structural, building, surface finishes or craft items.
The soils or clay materials display high organic contents as top soil, to nearly mined soils with nearly zero organic component and washed or ground residual products containing mixed organic and mineral substances. Soils have adjustable plasticity, mould-ability, insulating qualities, high thermal capacity, non toxicity, eco friendly nature and simplicity of application. Soils have besides plasticity and shrinkage on drying, issues of deflocculation, coagulation, dry and wet strength of clays.
‘Soil colloids are the most active constituent that determines the physical and chemical properties of the soils. These are very small particles which are one-thousandth of a millimetre (0.0001 mm, 0.0004 in). Like other soil particles, some colloids are minerals, whereas others are organic. Mineral colloids are usually refined clay particles. When these particles are mixed with water, they remain suspended indefinitely, turning the water murky. Organic colloids are tiny bits of organic matter that are resistant to decay. Colloidal particles are always in motion because of charge particles. Colloidal particles are transformed from a liquid into a soft semisolid or solid mass by adding an opposite charged ion. Colloidal particles have ability to absorb gases, liquid and solid from their suspension. Colloidal particles never pass through a semipermeable membrane. Colloidal particles have the properties of cohesion and adhesion’.
Generally, such soils have numerous problems due to the low strength, high compressibility and high level of volumetric changes. Clays need to be improved before these can be used even for soil-based structures like roads, dams, embankments, landfills etc. Improved mix and layering can solve issues of plasticity, swelling, angle of repose, load-bearing behaviour, stability and workability of the clays.
Soil Erosion is the displacement of the upper layer of soil, it is one form of soil degradation. Rainfall, and the surface runoff which may result from rainfall, produces four main types of soil erosion: splash erosion, sheet erosion, rill erosion, and gully erosion.
Clay is a fine-grained soil, but not all fine-grained soils are clays. Clays are distinguished from other fine-grained soils by differences in size and mineralogy. Silts are fine-grained soils tend to have larger particle sizes than clays. Mixtures of sand, silt with less than 40% clay are called loam. Loam. Loam makes good soil as a building material.
Clay and Soils are used for forming raw or baked products. Such materials have fewer problems such as shrinkage on drying (cracking), less of homogeneity in dry state, high water permeability -hygroscopic, low weatherability, poor bonding to a substrate -peel off, vulnerability to white ants and insects and colour.
Clay products forming processes are, both corrective and additive, unlike wood working, which is basically a deductive process (unless one uses joinery techniques). Clay earthenware processes, at a later stage suffused the stoneware, porcelain and glass making, due to involvement of ‘earthy’ minerals and the heat treatments. In building construction clay products competed against stones, and metals for household items. Stones are not available in all locations and metals need higher technology, compared with the universal material, the Clay.
Plasticity of clay is one its plus quality that is available in no other materials except the flour dough. Clays nominally attain plasticity by addition of moisture, but for high-end ceramics shape forming is through densification by pressure. The plasticity is not a critical criterion, as ceramic soil materials show flow properties at high temperature forming inter-particle bonding.
Soils are exploited by tackling issues with fillers and additives that are local, low cost and technologically simpler. Fillers and additives are primarily natural materials such as other clays, sands, granules, pozzolana, minerals, crushed baked products (like surkhi, ceramics, coal ash, etc.), dried-rotted agriculture wastes, hairs, natural gums, etc. At the other end, fillers and additives are processed materials like pigments, synthetic fibers, polymeric compounds and resins, oxides, carbonates, Portland cements, calcined lime, etc.
Fillers mainly change the physical quality of the soil by adding to the bulk, altering the plasticity and changing the economics profile of raw materials. Additives play an effective role in changing the chemical properties of the mix. Majority of fillers are inferior clays or earth-based products. Additives are proportionately of small volume or weight, like gums, binders, cement, asphalts, pozzolana, lime, bitumen, alkali-acid controllers, colourants (pigments), constituted minerals, baked clay crushing, etc.
Fillers and Additives often serve mutually supportive as well as mutually un connected purposes, for one or many of the following reasons.
- to improve the quality of basic soil material
- to reduce or enhance the moisture content
- to control moisture removal
- to control plasticity
- to achieve a desired colour / texture
- to produce specific type of castings / mouldings
- to improve weatherability of the final product
- to improve upon insect vulnerability
- to improve substrate adhesion in wet and dry states.
Clays can take large amounts of water to achieve a fluid, watery mass or pasty form. It can be moulded to any shape, massive, thin wall or with intricate details. The formed clay, when dries out, must still retain the shape and its surface can be modifiable to different finishes, by way of dry engraving, polishing, coating and colouring.
The air-dried forms of clay, on firing becomes permanent and the mass achieves greater soundness. All clay forming processes are energy efficient as use less energy and labour for conversion than the metal shaping-forming processes.
Clay items can be made by poring in, strip or coil stacking, moulding, wet engraving, shaping on a wheel and casting and deductive (carving-engraving) or additive processes. Clay can be liquidized and poured into moulds with very fine details such as hair, costume, drapery or facial features. Such details are difficult with metal castings. Compared to stonework, the finished products of clay are far lighter in weight, and easier to paint. Terracotta products shrink on drying, which is both an asset and drawback. Shrinkage on drying allows easy removal from casting moulds (like bricks, cups, saucers, toilet-wares), but the same in heavy mass items can causes cracking. Clay products on drying have porous mass due to the cavities left out with water evaporation. Such cavities provide light weight mass, greater heat retention, insulation and bonding with joint materials and external surface finish. But for electric and electronics products greater density is achieved by dry mass, greater compaction casting and non plastic raw materials. Clay is considered the most sustainable and eco-friendly material.
Broad classification of Fillers for clays are, 1 Addition of bulk, 2 reinforcement, 3 adjustment of viscosity for shape forming, homogeneity, 4 Bonding, 5 moisture resistance and 6 substrate bonding and workability for surface applications.
Clays have been used from palaeolithic age or earlier. These were earth structures formed, repaired or improvised by dressing, slope forming and shape contouring the lands for forming terrains, flood protection, burial sites, fishing, and water management. These were in the form of embankments, dykes, canals, bridging paths, etc. Various grades of soils and clay were exploited or the purposes of alteration of the angle of repose, drainage, safety from colloidal clay spreads, dousing of bush fires, etc.
For building of walls, for homes and protection structures clay blocks were cast. For casting viscosity and to prevent cracking on drying reinforcements fillers were required. For both of these purposes’ husks, fine chopped hay, grass and stems, dried leaves, animal excreta, bird droppings, ashes from fires, jute or coir like vegetable fibers, human and animal hair were used. Hay, grass and dry leaves are vulnerable to white ants, but rice-husk due to presence of toxic oils is almost immune from it.
Cow dung is the most popular filler for clay type of surface finishes, in India. Typically dry season fresh cow dung consists of, 33% solids and 67% of water+gases etc., by weight. The solids in a cow dung are as follows:
- Soluble organic 7.5 parts
- Insoluble organic 76.0 parts
- Soluble inorganic 4.5 parts
- Insoluble inorganic 12.0 parts
- Total 100.0 parts by weight
A matured or rotted dung is better filler then a fresh one. Rotting and consequent decomposition leaves an odourless mass that does not leach out with the addition of water. Rotting also generates fungicidal and insecticidal agents like gallic acid and tannin. Best way of maturing dung is to mix it thoroughly with 1/3 of all the soil to be used and then allow the slurry to remain in a dark, warm, impermeable pit for at least 72 hours. The clay to cow dung proportion vary according to the type of use such as:
- Quality of dung dry of summer or wet of monsoon
- Type of soil organic or mineral
- Type of plaster plain, decorative, mural
- Substrates smooth or rough
Dung to clay ratios of 1:4 to 1:8, is common for plaster work, but 1:1 ratio is often used for flooring and art work. Cow dung provides homogeneity, improves workability, retards shrinkage on drying. Clay+cow dung surfaces are fairly impermeable to water.
Dungs of other animals, like horse, donkey and other domestic and wild animals are drier and more fibrous due the quality of diet. For this reason such dungs are more suitable as fillers for excessively plastic clays. But such dungs do not rot or decompose as readily as cow dung.
Scrapping of old Clay+Dung floor and wall surfaces, are added to clay to control the plasticity. Such scrapping from Chulhas and Tandoor are fire baked products, and dust of bricks (Surkhi in India) have cementious siliceous compound. Surkhi is added to clays for floor and wall daubing besides being used for clay tennis court, country cricket pitches, paths and low traffic country roads. Surkhi may need addition or presence of lime in clay to form a cement like compound. Properly rotted, Clay+Dung mixtures have been found to be low-cost eco friendly water seepage resistant base for freshly dug pits and canals.
Pozzolana is volcanic ash. It is an active siliceous material that reacts with hydrated lime to form a gel, which on drying becomes insoluble and stable. Slag is a siliceous waste taken off from the molten ores of metals. If slag is quenched immediately on its removal from a furnace, crystallization of silica into glassy structure is stopped. Slag also needs hydrated lime to harden. Slags however contain sulphur and can be used to neutralize alkaline soils. Surkhi is a manufactured siliceous compound to which addition of lime is not required. These materials are used with organic plastic clays (which tend to be acidic) to achieve initial setting and with mineral soils, for greater homogeneity.
Fly ash, a fine residue of from pulverized burnt coal, collected from chimney stacks and boilers. It contains 55% SiO2, 30% Al2O3, 5% CaO and 7% Fe2O3. These crude forms of tri calcium silicate and tri calcium aluminate in the presence of water bind particles of mineral types of soils. Mineral coal ash, if fine and free from u-burnt coal and sulphur can be used as filler provided black colour is not objectionable.
Portland cement 5% to 18% on dry clay weight basis is used for quick setting, better wear-tear properties and overall mass strength. Sandy or mineral soils require lesser amounts of cement then organic or silt soils.
Additives like, protein glues, vegetable gums and chemical binders are used as binding agents to improve the workability and fast setting. Such additives are of little use with plastic clays but are more suitable for sandy soils. These are water thinnable, hygroscopic and so soften up every time they come into contact with the humidity. But some chemical binders, though is water thinnable, on drying harden into a water insoluble matter. Typical agglutinates are guar gum, arabic gum, casein, soluble starches, cooked starches, molasses, sodium alginate, acrylate and other polymeric resins, amino resins etc. For optimum results the quantity of agglutinate required is small, but their high costs prohibit the use.
Sand stone dust, shell and lime and other kankers provide ‘body’, improve workability and to an extent reduce the shrinkage. Calcined, hydraulic, non hydraulic limes and calcined gypsum (plaster of Paris) are used for better initial setting and overall strength. Whiting and china clay are mainly used to impart lighter colour tones. China-clay, because of its hydrophilic nature helps the mixing of water and `false’-initial setting of the mass.
Other clay fillers include partially ‘digested’ paper-pulps, paper shreds, lint (of cotton seeds), staple fibers, viscose, glass wool, hairs, carding waste of wool and cotton. These mainly reduce the cracking on drying.