STONE CRAFT

Post 464 –by Gautam Shah

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Stone craft consists of many distinct trades, like quarrying, handling-transporting, sizing, cutting, dressing, finishing and masonry work. The first stone craft was using the stone to make tools for working with materials. A right choice of stone quality and appropriate size-shape were important then, and continue to be so today.

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Stone is used for many different purposes.

  1. as industrial raw materials for minerals,
  2. as a constituent material in various composites,
  3. in buildings, for masonry, flooring and applique work,
  4. as an art and craft material.

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Strength of a stone is checked for following types of stresses:

Compressive stresses, tend to decrease the volume of the material, causing breaks with a shattering effect.

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Tensile stresses, produce cracks and fissures, and torsion (or twisting). Generally, fine-grained rocks are stronger than coarse grained. Rocks with interlocking between the crystals are stronger than rocks with poor interlocking. Stratified rocks have poor strength along the plane or strata. Stratified rocks as a rule have lower strength than igneous and non-stratified homogeneous rocks.

Shear stresses, which move one part of a stone with respect to another, under certain conditions, inducing a permanent change of shape. These are best avoided by appropriate angle of extraction and cut, by careful orientation during coursing a masonry.

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Torsional stresses are important for structures of stones such as piers. Heat induced stresses were once critical for structures like fire places and hearths, but optional materials have obviated that as the criteria of design.

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The general requirements for stones used in Buildings can be summarized as follows:

■ Sound, uniform rock material.

■ Presence of rifts to facilitate workability by hand tools.

■ Porosity advantageous for cementing, provided it does not decrease the resistance to weathering.

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■ Inherent chemical stability to prevent fluorescence.

■ High strength (as required in certain cases).

■ Low specific gravity (necessary for easier handling and in light weight structures).

■ High abrasion resistance (an important factor for flooring, steps).

Palazzo dei diamanti, facciata principale. Wikipedia Image by Nicolò Musmeci

Masonry walls of stones require specific methods of construction such as:

1 Heaviest and thickest of pieces should be used for lower courses.

2 Small pieces of stones should not be used on outer face.

3 Best flat face with a smallest area should form the wall face.

4 Each stone must rest on a flat surface, if required flat face should be achieved preferably by dressing of the stone, by bedding material or mortar, or by use of splinters and wedges.

5 Wedges should be placed with their wider face on the inside and narrower face on the outside.

6 All loose particles, cleavages, layers should be removed before using a stone.

7 Joints must be staggered.

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8 In case of very thick walls, if more than two stones form a width, several full width stone should be employed for keying.

9 For all walls especially random masonry, the corners should be made of long rectangular stones of even thickness (preferably dressed).

Opus Reticulatum Pompeii Roman stone facing pattern Wikipedia Image by Jensens

10 Stratified stone materials should be used for compressive loads to occur across the section or strata.

11 For tension bearing areas stratified and sedimentary stone material should be avoided.

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STONE WORK – Quality Parameters

Post 228 – by Gautam Shah

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Quality of stonework depends on not only the technique of dressing, cladding and fixing, but also on the stone material itself. A building stone material to be useful requires specific type of extraction, handling skills, seasoning and curing processes.

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Most of the stone materials are extracted from a depth. Such materials, since their age of formation, have remained buried under heavy over loads. In many circumstances these have been devoid of oxygen, other gases, moisture, and light or radiation exposures. They may have stayed with entrapped moisture and gases.

 

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On extraction the stone material is brought to a totally different environment, and certain inevitable changes set in. The lessening of pressure allows the entrapped moisture and gases to escape, such as in sedimentary deposits. Some materials absorb fresh moisture and atmospheric gases to fill in the voids. Due to reduced pressure, and exposure the mineral structures begin to alter, causing expansion or contraction stresses in the mass.

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The environmental changes continue long after mining, sizing and dressing of stones. Rain water, is slightly acidic, and causes a calcium carbonate to change into bicarbonate. In industrial areas atmospheric sulphur and carbon dioxide enhance the acid action on stone. Chlorides on sea front and in industrial areas can get converted into weak hydrochloric acid and dissolve the carbonate rocks. Nitric acid produced from oxide or Nitrogen also corrodes stone faces. All corrosive mediums depend on supply of water or moisture, so care of fresh-cut stone is essentially a moisture management exercise.

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Seasoning: Seasoning involves both the drying and wetting on one hand, and airing of stone, on the other hand. There is an increase in strength due to the re-deposition of percolated minerals, surface carbonation, transmission and deposition of minerals on the surface of a stone, by both the evaporating moisture, and addition of water. Dehydration, during seasoning, is more or less an irreversible process and subsequent artificial saturation of stone with water, often lowers its compressive strength. Seasoning is more relevant to soft limestone whereas hard limestone seems to be less affected by it. Immediately after quarrying, a stone is soft, and it is easy to work with it, but polishing to glossy finish requires a fully seasoned and hardened stone.

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Weathering: The term weathering denotes both desirable and undesirable changes. It may even enhance certain qualities, like colour, texture, strength, etc. Following are the main sources for decay or the unwanted effects, on a stone:

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  • Penetrating water: Effect of acid in air and rain water, mainly sulphur dioxide derived from the combustion of the sulfur constituents of fuel.
  • Surface action of water, gases: Bleaching of colour, water soluble salts’ crystalline deposits, erosion.
  • Effects of temperature variations: Frost, Cracking,
  • Effects of foreign deposits or organisms: Bird droppings, roots of climbers and other vegetative growth weaken the stone and accelerate the decay. Fungus and mildew are other destructive agents.

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Applied Surface Finishes on Stone: Water repellent or finishes that completely seal a stone surface should be used very judiciously. A sealed surface is likely to allow crystallization of salts beneath the surface instead of its natural leaching out. Water repellent finishes once applied are difficult to remove, and in many cases not re-coatable. Better design techniques, correct type of stone use and use of a biocide treatment give more satisfying result for problems such as algae, fungi, lichens etc. The decay of stone can be minimized by laying the stone with natural grain in horizontal orientation.640px-Olandalbymedbridge

Porosity, has no direct relation to the weathering resistance of stone materials, such as limestone. It is the shape, size and nature of pores, especially the degree of micro porosity that plays an important role in weathering. The action of carbonic acid, increases with greater micro-porosity, and capillaries prolong the dissolving action. Small cavities on the surface and rough finished textures trap dust, bacteria and retain water for longer period. Sand-stones containing colloidal minerals as a cementing medium has the most pronounced expansion due to weathering.

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Efflorescence (also called wall white, stack white or wall or stone cancer), is the appearance of crystallized salts on the stone’s surface, and the cryptoflorescence, denotes crystallization of salts within the pores. The crystallization of the salts and their re-crystallization from a lower to a higher hydrate within the range of mineral stability may develop stresses of high magnitude with quite an appreciable qualitative change.

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Strength of a stone is affected by three types of stresses:

Compressive stresses, decrease the volume of the material, causing breaks with a shattering effect. This is more common in non-homogeneous stones, and stones with inclined grains or stratification.

Shear stresses, move one part of a stone with respect to another, under certain conditions, inducing a permanent change of shape. These are best avoided by appropriate angle of extraction and cut, by careful orientation during coursing a masonry.

Tensile stresses, produce cracks and fissures and torsion (or twisting). Generally, fine-grained rocks are stronger than coarse grained. Rocks with interlocking crystals are stronger than rocks with poor interlocking. Stratified rocks have poor strength along the plane or strata. Stratified rocks as a rule have lower strength than igneous and non-stratified homogeneous rocks.

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