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 706 -by Gautam Shah

Part -II will deal with ADDITIVES for CLAYS



Surface finishes and Products composed with CLAY as the prime raw material have been used, for every conceivable purpose and in all parts of the world. Clay is preferred for : Abundant supply, cheapness, universal availability, insulation qualities, ecological value and simplicity of application. Clay finishes and products have some drawbacks like: shrinkage on drying, i.e., cracking, poor weathering qualities, lack of homogeneity in dry state, high water permeability -hygroscopic, poor bonding to a substrate peel-off, vulnerability to white ants and insects.


The quality of the clay-based surface finishes and products depend on:

  1. Quality of soil
  2. Fillers
  3. Additives
  4. Manufacturing processes


Quality of Soil

Soil is a product, formed mainly from the decomposition of a rock and ashes of lava origin. The decomposed product may remain at its place of origin or get transported to other places by natural forces like water, wind etc. The product, which remains at the place of origin the Residual clays, are comparatively pure, but have less uniform particle size distribution. Materials that after being transported get deposited somewhere else are the Sedimentary or secondary clays. These are generally contaminated by other materials and have smaller but uniform particle size distribution.

Red Iron rich Earth

Principal constituents of clays are Alumina and Silica. Alumina provides the plasticity, and Silica, if free, reduces the shrinkage and warping. Composite silica, though increases the warping on baking. Other elements of clays are Calcium, Magnesium, Iron, Manganese, Potassium and Sodium. Various compositions of these elements and their crystalline structure affect the quality, colour and texture of the soil. Kaolin is the chief constituent of clays used for Ceramics production.


Clays used for products making and surface finishing, are either Top-organic soils or Virgin-non organic soils.


Top-organic soils have substantial amounts of organic matters from the decomposition of vegetation and human, and animals excrete. The presence of organic matters makes a soil light in weight and dark in colour. Organic soils usually show high workability and low shrinkage characteristics. When organic soils are found below an existing layer and are old, contain gallic acid and tannin in small proportions but sufficient to act as fungicide and mild insecticide.


Virgin or non organic soils have negligible amounts of organic matters, and so reflect the basic characteristic of the predominant constituent element, i.e., lime, silica, or alumina. Non-organic soils, however, do take-on the personality of the other minor minerals present in it. Iron oxide as ferric and ferrous is the most important colourant. Other important colourants are quartz, kaolin, mica etc. Soils show a wide range of colours from off-white to yellow, light brown and chocolate to reddish tones. Non-organic soils unless constituted by colloidal particles show very little plasticity. Some mineral constituents of such soils are reactive to water resulting in swelling and leaching.


Residual or sedimentary materials available at the top of the surface, or below a certain depth, can be classified as: Clays, Sands, Silt, Shale, Colloids, Hard pan, Hoggin, Loam, Peat-Muc, Humus.


Clays are fine albuminous products formed by decomposition of igneous rocks (lava activity). Clays are tenacious and plastic when wet. Clays are highly cohesive, have high capillaries and no internal friction. Clays are smooth to touch, sticky and plastic. Clays can also be classified according to their plasticity, or silt content. Hard clays or stiff clays have low sand content, and are difficult to excavate. Fine clays have medium sand content, and can be excavated with slight effort. Soft clays have coarse texture and are easy to excavate. Pure clays are mostly useless because of the high plasticity and excessive shrinkage on drying. Plastic clays are called fat clays, and less plastic clays and are also called lean clays.

Ball ClaysClays are black, white, red, brown and yellow in colour. China Clay is a residual material, contaminated with silica, mica, feldspar and decomposed feldspar. Ball clay is a sedimentary material of fine grain size and some organic contents. It is finer than china clay. Fire clays are formed from feldspar as residual and sedimentary deposit. Brick clays are high in iron content, and impurities of calcium compounds and organic matter.


Sands are of small granular particles, usually of stones. Sands are gritty to touch, with little cohesion. It has high internal friction and very little capillarity. Silts are soils that are somewhere between a clay and sand. Silts are slightly gritty to touch and are darker in colour than clays. Colloids are gluey matter found with clays but of ultra fine particles. The colloids absorb moisture and remain suspended, rather than settle down in water. Shale is a compressed and laminated clay with or without organic matter. Shale is plastic when wet but disintegrates when dry.

Volcanic ash deposition

Hard pan is a very dense accumulated mass of soil, consisting of clay, sand, gravel, etc. held together in a rock like but layered formation. Hard pan does not soften when wetting. Hoggin is a natural deposit of a mixture of clay with small stones, grit and sand. Loam is a soft mixed deposit of silty clay and sand in different proportions. Peat-muc and Humus, have fibrous or spongy organic matters formed by the decay of plants. These deposits are black or dark brown in colour, varying compressible in presence-absence of water and so unsuitable for heavy loads. The decomposition of organic material is more advanced in muc than in peat.




Post 565 by Gautam Shah



Terracotta is one of the oldest ‘converted materials’# used by humans. By firing a shaped clay object, it not only gets a permanent form but attains superior strength and properties. Terracotta or baked clay ceramics have been used for cooking pots, storage utilities, artefacts, monetary units, toys, adornments, statuettes, sarcophagi, masonry units like bricks, roofing tiles, and other architectural elements. The art and crafts of terracotta have been practised in almost all regions of the world. Terracotta items of small sized beads to human size statues and jars have been produced. The qualities of local clays have contributed to the colour and density (porosity), whereas the firing techniques and fuel have exploited the baking temperature to impart unique properties.

# ‘converted materials‘ = change their state, but cannot be reconverted back to their original state versus re-convertible materials, which can revert to their original state.


Roof fragment of the Roman bath, at Bath, UK, Wikipedia image by Heinz-Josef Lucking

In archaeology a terracotta is a baked clay product formed by processes other than on a potter’s wheel. Baked clay items used in buildings are called Earthenware goods, whereas pottery items formed on potters’ wheel are popularly known as earthenware pottery or ceramics. Terracotta items are unglazed and created through single firing process. Faience is made from a vitreous frit (baked powdered of ceramic clays), and also called white-earthenware or lighter terracotta when created with the self-glazing process. Fine ceramic beads, figures and other small objects were made in Egypt (000 BC), Mid-East, Indus Valley and elsewhere.


Earliest known ceramics are the Gravettian figurines dating bet 29000 to 25000 BC Image by “Petr Novak, Wikipedia”

Clays show Three transformation stages. First: Clay can take large amounts of water achieving a fluid or watery mass to pasty form. It becomes so plastic that can be moulded to any shape. Two: The formed clay, when dries out, still retains the shape, and its surface can be fashioned to different finishes. Three: The dried form on firing becomes permanent, and the mass achieves greater density. These processes use less energy and labour than the metal forming. The clay 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 architecture clay products competed against stones, and for household items formed with metals. Stones are not available in all locations and metals need higher technology, compared with a universal material, the clay.


Harappa miniature Votive images or Toy models Image by Trish Mayo from New York, US

Plasticity of clay is one its plus quality that is available in no other materials except the flour dough. Clay items can be made by strip or coil stacking, moulding, wet engraving, or shaping on a wheel. 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 bronze casting. 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, but the same in heavy mass items, causes cracking. Clay is considered the most sustainable and eco-friendly product.


Terracotta pottery fragment from Mathura sites, Now at Govt Museum Wikipedia image by Biswarup Ganguly


Baked clay products or Terracotta have a tough surface that can last for years in buried or open conditions. It is, however, vulnerable to moisture and salts. Fired terracotta is water absorbent, but surface-burnishing before firing compacts the surface reducing the porosity. Raw clay tablets were inscribed with a cuneiform script, and fired for indelible record keeping. The use of terracotta or earthenware nearly died with the Roman empire. Throughout the world, though terracotta continued to be used for building-brick, roof and wall cladding tiles. This began to change in Europe and at other places past 14th C, with high temperature firing to produce the stoneware. Italy and Germany began to produce moulded and carved terracotta for architectural friezes, column capitals and medallions. Palladio extensively used terracotta friezes, column heads, architraves and other decorative elements replacing costly marble. The use of glazed or unglazed terracotta for free sculpture was revived.




Vaishnavite temples of Bishnupur and other places in Eastern India were entirely built of Clay products like bricks and faced with terracotta figurines and panels.



Modern Terracotta Pottery, India, Wikipedia image by McKay Savage from London UK



Post 426 – by Gautam Shah 


9 Koyama_3

Architecture and structures of mud or clay, for every conceivable purpose, exist in all parts of the world. In hilly regions of the world clay sediments have been used for packing the joint and as a masonry course leveller. Mud or clay is used because of the abundant supply, near zero cost of procurement, wet plasticity, mould-ability, insulating qualities, high thermal capacity, non toxicity, ecological friendly nature and simplicity of application. Mud as a forming material for architecture, structures or ceramics have some drawbacks like, shrinkages on drying, i.e., cracking, poor weathering qualities, lack of homogeneity in dry state, high water permeability -hygroscopic, poor bonding to a substrate -peel off, vulnerability to white ants and insects.


Mud has seen renewed interest during the last Six to Seven decades. First interest in architecture was for its abundance and simplistic technology. Later, the material was favoured for its insulative qualities. During the last 4o years the mud buildings are being favoured for their Eco-friendliness, chiefly the recycle-ability aspect of it. The ideology of sustainability, with its varied interpretations, has supported experimentation for different uses.

2 Wattle_and_daub_construction

8 Berber village Near Ait Benhaddou 5600152155_532c99cc67_z

Some basic techniques of Mud construction are identified. These are: Sod, Rammed earth, Cob (cobb or clom), Adobe, Wattle and daub Compressed earth block methods. These techniques differ in details, from region to region, type of soils, natural moisture content and availability of additional water, additives, reinforcements and support form-work within reach. The mix design and forming techniques also depend on building elements (wall, slab, etc.), architectural elements and surface finish or applique decorations.

11 st_stephens_church_at_acoma_pueblo1

Mud architecture presents fascinating forms. The quality of space formation, the suitability for range of basic architectonic elements, adaptability to different usages, and the universal availability, make mud a very coveted material. The love affair is very poignant during the academic period of designers. The passion, however, gets muted over the years, for variety of reasons, such as lack of the clientele, the place, scope and sponsorship for experimentation and the irrelevance of the technology at locations where the educated designer will operate. There are many other reasons for a failed take off for ‘low technology and eco-friendly’ endeavours. Mud, is reckoned to be a sustainable material, of very relevant (‘green’) technology, non toxic, universally available and completely recyclable material.


1 House_in_Toteil_002

These concepts remain valid so far as one can use the mud architecture concurrently with matching ‘lifestyle’ if one is conducting. A personal habitat of mud and to maintain (sustain) it for a long period, are two different things. A mud building is a very fragile entity and needs day to day care. Such concerns cannot be assigned to any outsider or agency. The cost of daily upkeep can turn out to be very high. And even if one can afford the cost, (which could be equal to the cost of a new structure), takes lots of time, practically a full time vocation.

7 Siwa mud Homes2009

Mud built-form cannot be conceived as a drawn plan or scheme. A person who constructs it must improvise it on own. The execution of such form cannot happen quickly, and during the period whatever that has been constructed will need updating and improvisation. Some of the key elements of built form, material behaviour, form and space organization exist in the society that has been using mud for generations. These innate capacities can be reinforced by being not only an active participant on the site, but by being an inhabitant of the entity. Only an inhabitant of the mud architecture can sustain it.

4 Mud_plaster_over_straw_bales_wallDesigners cannot, and must not meddle in mud architecture design or execution. A design student may be asked to design one and perhaps execute it, as a learning exercise. The fashionable word coined by teachers who never practice, or have never done, is “hand on experience” in material-form-and the technological implications.

3 Annual_repair_of_the_world's_largest_mud_brick_building_the_Great_Mosque_of_Djenné_in_Mali._(32088227574)