Post 709 -by Gautam Shah


Earlier article in the series CLAY MATERIALS for SURFACE FINISHES and PRODUCTS MAKING > Part -I (

5 Mixing Hay for Adobe

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.

10 Gully Erosion of Soil Dead Sea CoastalErosion

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.

15 River Silt

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’.

9 Peat Blocks

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.

11 Rummu aherainemägi2

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.

8 House made of mitti

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.

12 38293897802_c842ec3ece_c

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.

13 Silt on the

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.

6 Extruded hollowed Clay blocks being air-dried before going to furnace

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.

4 Caesarea_Concrete_Bath

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.

14 stacked soil for wall

Fillers and Additives often serve mutually supportive as well as mutually un connected purposes, for one or many of the following reasons.

  1. to improve the quality of basic soil material
  2. to reduce or enhance the moisture content
  3. to control moisture removal
  4. to control plasticity
  5. to achieve a desired colour / texture
  6. to produce specific type of castings / mouldings
  7. to improve weatherability of the final product
  8. to improve upon insect vulnerability
  9. to improve substrate adhesion in wet and dry states.

1 Clay Extracting and mixing water to produce bricks

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.

3 Sun dried Adobe Blocks 2815570468_023f24cda7_c

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.

Nukus - Khiva, view from Ayaz Qala

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.

2 Soil mixing, raw bricks casting and stacking for drying Image by Bild von Siva Nanthan auf Pixabay

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.

17 Cow Dung Soil mix allowed to mature for few days before using it surface Finish Village women Bangladesh

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.

16 Volcanic Ash Yogyakarta_eruption of Kelud

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.




Post 180 ⇒   by Gautam Shah 


Matrix-Filler Interface in Composites

A composite material is a complex entity. Understanding its constituents and their role helps in understanding of its strength and weaknesses. Here in very simple terminology this has been explained.



Plasan Sand Cat light (5t) military vehicle featuring integrated composite armoured body


In a composite material, the Filler in the form of particles, fibres and sheets, is expected to take up the stresses in unison with the Matrix, due to the strong interface provided by the later.


Composite materials with weak interfaces have low strength and stiffness, but high resistance to fracture, On the other hand materials with strong interfaces have high strength and stiffness but are brittle. The bonding between the matrix and the filler is dependent on the atomic arrangement and chemical properties of filler and on the molecular conformation and chemical constitution of the matrix.


A crack that starts in a monolithic material generally continues to propagate until that material fails, whereas the filler-matrix combination reduces the potential for a complete fracture.


Bonding at the interface: In a simple system the bonding is due to adhesion between filler and the matrix.


Adhesion can be attributed to following FIVE main mechanisms:


1 Adsorption and wetting: When two electrically neutral surfaces are brought sufficiently close, there is a physical attraction. Most solids, have surfaces that are rarely perfectly in level, blemish, and without any contamination. So a wetting agent that substantially covers all hills and valleys, displaces all air, and overcome the effects of contamination, is required.


2 Interdiffusion: It is possible to form a bond between two polymer surfaces by the diffusion of the polymer molecules on one surface into a molecular network of the other surface. The bond strength will depend on the amount of molecular entanglement and the number of molecules involved. Interdiffusion may be promoted by the presence of solvents and Plasticizing agents.


3 Electrostatic attraction: When one surface carries a net positive charge and the other surface, a net negative charge, electrostatic attraction occurs (as in acid+base reaction). Electrostatic attraction has no major role in contribution of bond strength, but has importance on how things initially begin to get mixed.


4 Chemical bonding: It is formed between a chemical group of filler and a chemical group of a matrix. The bond formation or breakage usually involves thermal activity.


5 Mechanical adhesion: Some bonding occurs by the mechanical interlocking of two surfaces (e.g. fibre shape-section).


Shocks, impact, loadings or repeated cyclic stresses can cause the Individual fibres to separate from the matrix, e.g. a fibre pull-out. In case of laminated or layered construction there could be a separation at the interface between two layers, a condition known as de-lamination.


Composite material: Radar absorbent material for stealth air craft



Post 156   by Gautam Shah ➔

Natural and manufactured raw materials are invariably compounds, made of many materials. These raw materials are formed into products by a method of assimilation -the COMPOSITE or the raw materials are organized into a geometric form -the COMPOSITION.


1 Composite (Cloth+Gypsum) (Wikipedia image by JanSLWC)  2 Composition -geometric arrangement (Wikipedia image by Viapastrengo at En Wikipedia)

Many of the raw materials are naturally compounded materials, in the form of Composites. The composite materials come into being, by putting together natural and manufactured materials in such a special way that the strength and other qualities are different from the constituents, individually and cumulatively. The term different, is considered here as an improved quality, because man-made composites are designed and created towards specific performance requirements only.



Man-made composition -Howrah Bridge Kolkota

Similarly natural and man-made materials have an inherent organization of geometric arrangements which endow unique structural behaviour capacities. Some of the simplest examples are hives of honey bees or birds’ netted nests. At a complex level a truss, bicycle frame, a hull of a boat, are all Structural Compositions or geometric arrangements.

Natural and manufactured materials, and their composites, all are further shaped, re-formed and geometrically integrated to create secondary components as well as Structural Compositions.

1- 640px-PCCB_Wiki_9949


A composite is a natural or designed material entity with potential utility, but has no operational functionality. On the other hand, a component is a configuration of many materials into a utilitarian product. Component manufacturing involves processes that are many times similar to a composite formation. As a matter of fact for component manufacturing, the ‘composite formation’ and the ‘component creation’ both occur simultaneously. Structural compositions (trusses, bridges, buildings) are geometric-configuration of materials, often assisted by components (nuts, rivets, pins, bearings, etc.). Structural compositions use composites to form the constituent elements.

A Layered composite -Plywood

Various Definitions of Composites:

# Consisting of two or more physically distinct and conceptually separable or visually identifiable materials.

# Products that can be made by mixing separate materials, so the dispersion of one material in the other can be done in a controlled way to achieve optimum properties.

# Products with properties that are superior and possibly unique in some specific respects compared to the properties of their individual components.

Particulate composite


Cement Aggregate composite -Mozaic tile

Classes of Composites:

Natural composites               Wood, Bamboo, Bone, Muscle and other tissues

Macro composites                 Galvanized steel,

Engineered products           Reinforced cement concrete (beams, etc.), Helicopter blades Skis, Tennis rackets.

Microscopic composites     Metallic alloys, Toughened plastic (impact polystyrene, ABS), Reinforced plastics.

Nano composites                   Electronics circuits, diodes, transistors

Toys Object Sport Ping Pong Ping-pong Table Tennis

Some natural composites are easy to identify, such as: wood, bamboo, bones, muscles, etc. First man-made composites related to the bronze, as man tried to fix natural stones and ceramic pieces by hammering into the bronze. Layered wood composites have been used by Egyptians. Mud bricks reinforced with hay, hair, and rice husk have been used in prehistoric times. Cow-dung is also reinforced with granular sand particles for wall plaster. Gypsum (Plaster of Paris) has been applied on a lattice of jute, papyrus and such other fibres.

1 - 640px-Textolite

Macro, Micro and Nano Composites: Composites can be categorized in terms of the size of constituent particulate matter. Ingredients of macro composites can be distinguished by naked eye, whereas one may need an electron microscope to understand the constituents of micro composites and nano composites. Nano composites are created by introducing nano-particulate, which drastically add to the electrical, thermal, and mechanical properties of the original material.

1 -Composite_laminate_specimen

Basic Constituents of a Composite:     MATRIX and FILLERS

The constituents of a composite are ordinarily classified as Matrix and Filler. It is the nature of relationship between the filler and matrix, or the Interface that defines the composite. Fillers serve to resist stresses, mainly tension, and the Matrix serves to resist the shear, and all materials present including any aggregates, serve to resist the compression.

Fibres -used as FILLER in composite

Categorization of Composites on the basis of strengthening mechanisms.

Composite materials can be distinguished into three categories based on the strengthening mechanism.

These categories are:

1. Dispersion strengthened,

2 Particle reinforced

3 Fiber reinforced.

Dispersion Strengthened Composites have a fine distribution of secondary particles (fillers) in the matrix of the material. These particles impede the mechanisms that allow a material to deform. Many Metal-matrix composites would fall into the dispersion strengthened composites’ category.

Particle reinforced composites have a large volume of particles dispersed in the matrix. The load is shared by the particles and the matrix. Most commercial ceramics and many filled polymers are particle-reinforced composites.

Fiber-reinforced composites have fibre as the main load-bearing component. Fiberglass and carbon fibre composites are examples of fiber-reinforced composites.



Post -by Gautam Shah


Chinese Style Redwood Wood Bathroom Cabinet

Natural Woods are many different types, such as Soft, Medium and Hard grain. Woods are likely to be Fresh, Seasoned and Old woods. Woods have different types of faces such as the end grains, flat and side faces. The woods have problems of uneven colours, patterns. The variations in textures are both natural and tools or machine made.

Soft Woods Pinus sylvestris

Wood products have similar problems but an average quality due to the mass production processes. The variations however are between near-natural products (plywoods, veneers) and synthesized products (particle boards, MDFs etc.)


For a clear coating application all wood surfaces require surface preparation treatments and post coating application treatments. The surface preparation treatments are over and above the nominal craft processes such as planing, sand papering, etc. These are mainly Filling, Sealing and Staining. The post coating application treatments are not always required, but could be Buffing, burnishing, waxing, etc.

Hardwood tops


Fillers generally consist of an Extender, a Binder and occasionally a Colourant. Fillers are required to fill in the pores in the wood grains of Natural woods and cavities in case of wood products. A levelled surface provides better gloss and integrity of the coating (one continuous surface without breaks). The coating material does not sink into it, and provide an even finish. Transparent fillers (low body NC lacquer, shellac, etc.) are used not only to fill the pores, but provide a sealing coat to the decayable material in the grains and vessels filled with gum exudates.


Extenders are low opacity -reflectivity, fine grade powder of materials like gypsum, chalk, china clay, precipitated calcium carbonate, lime, asbestine, colloidal silica, barytes and talc(unlike the Zinc oxide or Titanium dioxide -the white pigments that adds whiteness and opacity).


Binders are binding agents that hold together the extenders, temporarily or permanently, and also bind the extenders to the wood substrate. A binder could be water, gums, oils, alkyds and poly vinyl emulsions. Solvent bound binders are better compared to water bound binders. The later ones raise the grain or fibres off the surface.


Colourants provide an equalizing tint to the wood surface and slightly colour the white extenders. Fillers, made of low opacity-reflectivity pigments serve a dual purpose, of filling as well as staining.

Wood Stain Brush Delete Planks Wood Wax Wood

Stains as colourants

Stains provide a correct transparent tinge to the surface. Stains are generally dye-material soluble in water, oil or solvents. Water soluble stains though of many different varieties, raise the fibres and are difficult to penetrate. Oil soluble stains are heavy bodied, take longer to dry out and interfere in the subsequent coating application. Solvent soluble stains are costly, dry out immediately and may bleed residual gums and other exudates.

Wood Staining

Stains have one important drawback that they darken the existing colour of the substrate. Where timber surfaces need to be of lighter colour, surfaces have to be bleached or toned with opaque materials.



Bleaching process lightens the existing colour of the wood. It includes a treatment with hydrogen peroxide followed by an alkaline accelerator like lime, caustic soda, sodium silicate or ammonia. Bleaching affects adhesion and toughness of coating. It also provides an amber hue to the coating on aging. Staining is also done by micro spray guns, singeing, burning or carbon deposition from flames.


Imposed patterns

On wood surfaces where there are very irregular grains or no patterns, these are screen-printed, pressed or embossed using stains. Such patterns may emulate a wood grain pattern or just very fine mesh or lines.

Wenge veneer imposed pattern

Post application treatments

Clear finishes often require some post application treatments. These are mainly burnishing and waxing. Burnishing is mainly done to NC lacquer, Acrylic and Melamine coatings, to provide a glossy surface. Burnishing is not done to slow drying coatings, because such coatings, though are dry on outer face, take days to thoroughly dry out. Burnishing is done with a Carborundum like rubbing material with a waxy or oily base. Waxing provides a dull sheen and a protective coating. Waxing is also done to renovate old coatings. Waxing compounds also include a small amount of oils and sometime silicone materials.