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Siliceous sediments

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Zeshan Ali
Zeshan Ali

Flint contains a small amount of water and should not be exposed to excessive heat, as it may fracture violently due to differences in how its molecular structure expands. Heating flint and other siliceous stones slowly at low temperatures can improve their knappability by changing the material from a poorly ordered structure to an equigranular, better crystallized structure. Different types of siliceous stones have different optimal temperature ranges for heat treatment. While heating can improve some material properties, it may reduce fracture toughness.

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Preparation, Properties, Problems By Zeshan Ali and Rachael Pung
Flint contains a few percent of water Should not be exposed to excessive heat, as from a fire. Due to the stone's molecular makeup and the presence of water, flint may fracture, sometimes violently Brown Chert Red Jasper Vanport Flint
Desirable changes, in terms of knappability, takes place in chert and many other siliceous stones if they are heated slowly, at relatively low temperatures and out of direct contact with intense heat. Poorly ordered, strongly interlocking cryptocrystalline fabric of the unheated samples Equigranular and better crystallized with thermal treatment
Types of Siliceous Sediment Temperature Range for Different Rock Treatment Flint and Hornstone 350 500属F (typically 200属C) Agate and Chalcedony 400 600属F (typically 230属C) Jasper 400 750 属F (typically 290 属C) Chert 400 700 属F (typically 315属C)
Properties Changes in Properties Elastic Constant Slight, consistent increase in the Youngs modulus of elasticity when heated at 300, 400 and 500 属C Compressive Strength Did not show consistent changes, vary substantially with the raw material Tensile Strength Did not show consistent changes, vary substantially with the raw material Fracture Toughness Marked reduction after heating to 300 and 400 属C Material tested: Flint, chert, jasper, chalcedony and agate
Made up entirely of cryptocrystalline silica Presence of pore spaces Size/Thickness Temperature Holding time
Physical Properties Description Lustre Waxy, dull for freshly broken surfaces Transparency Translucent to opaque Colour Grey, white, black, brown and other colours due to staining Streak White or lightly coloured Hardness (Mohs) 6.5 - 7 Tenacity Brittle Cleavage None observed Fracture Splintery, Conchoidal, sub-conchoidal Chert Specimen from Mojave Desert
Geotechnical properties Density Chert SpG range from 2.24 2.74. Flint which has about 1% water within it has a SpG that varies from 2.57 2.64 Porosity 0.59 - 3.46% Permeability 0.001 - 0.033 亮D Compressibility (for rock) Extremely strong, with a uniaxial compressive strength of >250 Mpa and a point load index of >10 Mpa
Same piece of rock may have different responses to heat treatment. Outer surface of flint, jasper and chalcedony nodules: high quality stone, require little or no heat treatment Interior portion: opaque, more coarsely grained and may require high temperatures for heat treatment. The outer edges of a stone may be very Knappable following a moderate application of heat, while the interior remains tough. Higher temperatures may treat the interior of the stone effectively, but create potlidding in the outer portion of the rock.
Siliceous sediments
While flint may be used in fire-lighting, it should not be exposed to excessive heat, as from a fire. Due to the stone's molecular makeup, flint may fracture, sometimes violently, when different parts of the stone expand to different degrees. This tendency, to fracture, is enhanced by the fact that most samples of flint contain impurities that may expand to a greater or lesser degree than the surrounding stone.
Deeply weathered chert develops surface pop-outs when used in concrete that undergoes freezing and thawing because of the high porosity of weathered chert. To solve the pop-out problem of weak cherts, the percentage of the low density materials (SpG less than 2.45 is kept at a minimum.) Reducing the maximum allowable size of the coarse aggregate. Smaller pieces are less prone to freeze thaw as they are able to expel all the water prior to freezing
Cherts undergo an alkali-silica reaction with high-alkali cements. This reaction leads to cracking and expansion of concrete and ultimately to failure of the material To prevent the alkali-silica reaction problem, 3 procedures can be followed: (1) use low alkali cement (<0.65 alkali as Na2O). (2) Use nonreactive aggregate or add non-reactive aggregate to reudce the reactive portion by dilution to a level below the percentages mentioned in the previous paragraph. (3) Add pozzolans, which include certain volcanic rocks and fly ash or ash collected from smoke stacks where coal is burned. These finely ground high-silica materials yield a non-expansive reaction product with the alkali in the cement.
http://www.springerreference.com/docs/html/chapterdbid/76579.html http://firearchaeology.com/firearchaeology/Direct_Effects_files/Gregg _Grybush_1976.pdf http://www.rocscience.com/help/rocfall/webhelp/baggage/Rock_Den sity_Table.htm http://www.pugetsoundknappers.com/how_to/Heat%20Treating%20 Guide%20with%20Table.html http://geology.com/rocks/chert.shtml http://geology.about.com/od/more_sedrocks/qt/About-Chert.htm
Siliceous sediments

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Siliceous sediments

  • 1. Preparation, Properties, Problems By Zeshan Ali and Rachael Pung
  • 2. Flint contains a few percent of water Should not be exposed to excessive heat, as from a fire. Due to the stone's molecular makeup and the presence of water, flint may fracture, sometimes violently Brown Chert Red Jasper Vanport Flint
  • 3. Desirable changes, in terms of knappability, takes place in chert and many other siliceous stones if they are heated slowly, at relatively low temperatures and out of direct contact with intense heat. Poorly ordered, strongly interlocking cryptocrystalline fabric of the unheated samples Equigranular and better crystallized with thermal treatment
  • 4. Types of Siliceous Sediment Temperature Range for Different Rock Treatment Flint and Hornstone 350 500属F (typically 200属C) Agate and Chalcedony 400 600属F (typically 230属C) Jasper 400 750 属F (typically 290 属C) Chert 400 700 属F (typically 315属C)
  • 5. Properties Changes in Properties Elastic Constant Slight, consistent increase in the Youngs modulus of elasticity when heated at 300, 400 and 500 属C Compressive Strength Did not show consistent changes, vary substantially with the raw material Tensile Strength Did not show consistent changes, vary substantially with the raw material Fracture Toughness Marked reduction after heating to 300 and 400 属C Material tested: Flint, chert, jasper, chalcedony and agate
  • 6. Made up entirely of cryptocrystalline silica Presence of pore spaces Size/Thickness Temperature Holding time
  • 7. Physical Properties Description Lustre Waxy, dull for freshly broken surfaces Transparency Translucent to opaque Colour Grey, white, black, brown and other colours due to staining Streak White or lightly coloured Hardness (Mohs) 6.5 - 7 Tenacity Brittle Cleavage None observed Fracture Splintery, Conchoidal, sub-conchoidal Chert Specimen from Mojave Desert
  • 8. Geotechnical properties Density Chert SpG range from 2.24 2.74. Flint which has about 1% water within it has a SpG that varies from 2.57 2.64 Porosity 0.59 - 3.46% Permeability 0.001 - 0.033 亮D Compressibility (for rock) Extremely strong, with a uniaxial compressive strength of >250 Mpa and a point load index of >10 Mpa
  • 9. Same piece of rock may have different responses to heat treatment. Outer surface of flint, jasper and chalcedony nodules: high quality stone, require little or no heat treatment Interior portion: opaque, more coarsely grained and may require high temperatures for heat treatment. The outer edges of a stone may be very Knappable following a moderate application of heat, while the interior remains tough. Higher temperatures may treat the interior of the stone effectively, but create potlidding in the outer portion of the rock.
  • 11. While flint may be used in fire-lighting, it should not be exposed to excessive heat, as from a fire. Due to the stone's molecular makeup, flint may fracture, sometimes violently, when different parts of the stone expand to different degrees. This tendency, to fracture, is enhanced by the fact that most samples of flint contain impurities that may expand to a greater or lesser degree than the surrounding stone.
  • 12. Deeply weathered chert develops surface pop-outs when used in concrete that undergoes freezing and thawing because of the high porosity of weathered chert. To solve the pop-out problem of weak cherts, the percentage of the low density materials (SpG less than 2.45 is kept at a minimum.) Reducing the maximum allowable size of the coarse aggregate. Smaller pieces are less prone to freeze thaw as they are able to expel all the water prior to freezing
  • 13. Cherts undergo an alkali-silica reaction with high-alkali cements. This reaction leads to cracking and expansion of concrete and ultimately to failure of the material To prevent the alkali-silica reaction problem, 3 procedures can be followed: (1) use low alkali cement (<0.65 alkali as Na2O). (2) Use nonreactive aggregate or add non-reactive aggregate to reudce the reactive portion by dilution to a level below the percentages mentioned in the previous paragraph. (3) Add pozzolans, which include certain volcanic rocks and fly ash or ash collected from smoke stacks where coal is burned. These finely ground high-silica materials yield a non-expansive reaction product with the alkali in the cement.
  • 14. http://www.springerreference.com/docs/html/chapterdbid/76579.html http://firearchaeology.com/firearchaeology/Direct_Effects_files/Gregg _Grybush_1976.pdf http://www.rocscience.com/help/rocfall/webhelp/baggage/Rock_Den sity_Table.htm http://www.pugetsoundknappers.com/how_to/Heat%20Treating%20 Guide%20with%20Table.html http://geology.com/rocks/chert.shtml http://geology.about.com/od/more_sedrocks/qt/About-Chert.htm