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Preliminary Notions
Cast:
Sunday, December 6, 2009
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• In its commercial range formula is: 2 (SO4 Ca ½ H2O) (before use, when we bought it) is a calcium sulfate hemihydrate. 
• The natural Plaster or Gypsum, or calcium sulfate bihidrato (CaSO4 • 2H2O), is composed of calcium sulfate with two molecules of water of hydration. Contains 79.07% calcium sulfate anhydrous 20.93% water and is considered a sedimentary rock, colorless or pure white. Preparation
once extracted from quarries: • The natural Plaster or Gypsum, or calcium sulfate bihidrato (CaSO4 • 2H2O), is composed of calcium sulfate with two molecules of water of hydration. Contains 79.07% calcium sulfate anhydrous 20.93% water and is considered a sedimentary rock, colorless or pure white. Preparation is dehydrated by increasing the temperature until the total water release, strongly combined, we obtain during various plasters used in construction, which according to rising temperatures of dehydration include: • Temperature ordinary : gypsum, or calcium sulfate bihidrato: CaSO4 • 2H2O. • 107 ° C: formation of calcium sulfate hemihydrate: CaSO4 • ½ H2O.
• 107 - 200 ° C, drying the hemihydrate, with faster set than the previous commercial gypsum plaster.
• 200 - 300 ° C: light gypsum waste of water, setting very slow and very resistant.
• 300 - 400 ° C: fast setting plaster apparently, but very low resistance • 500-700 C: Anhydrous plaster or extra cooked very slow setting or no: plaster dead.
• 750 - 800 ° C begins to form hydraulic plaster. • 800 - 1000 ° C: normal hydraulic plaster, or pavement.
• 1000 - 1400 ° C: hydraulic plaster greater proportion of free lime and faster setting. Main
varieties used in ceramics:
Plaster Paris or Type II
is a bit more compact and hard as Type I. Baking is generated in closed autoclave at 120 ° C. Its particles are smaller, regular type I, therefore, less porous and fragile. Also called "Stone." It is the most widely used ceramic molds that will be used to make castings.
extra hard plaster type III or Stone:
is heated above 120 ° C and mineral salts are added. It is even harder than the type II, with regular and fine particles, so it needs less water to set. Is much less porous, less fragile, so using models or matrices. (Of a matrix identical results in countless molds)
Hardening:
Basically the setting reaction is to reverse the process of calcination the cast. This reaction will give us energy provided by calcination and will therefore be an exothermic reaction (releases energy as heat). The hardening of plaster
occurs because contact with water, commercial plaster [2 (½ Ca SO4 H2O)] recovers the "parts" of water lost at time of his firing, making these molecules form again the original mineral or Algez, regaining their tiny molecular structure of crystals (the crystals formed cause a moderate but noticeable expansion). The chemical reaction that occurs can be written as follows: 2 (SO4 Ca ½ H2O) + 3H2O → 2 (Ca SO4 2H2O)
Why
used to make plaster molds for ceramics? • Because besides having hygroscopic properties (hydrated and dehydrated to water or moisture or lack thereof) plaster is porous and water particles in the slurry are absorbed into the pores by capillary action as the water molecules are much smaller than the molecules of clay.
plaster after setting presents a microscopic structure consisting of acicular crystals (needle shaped) calcium sulfate dihydrate, which intersect in three dimensions, leaving between them a series of micro and macroscopic pores. Microscopic pores have two origins: structure of the crystals, leaving a space between them, and the waste water does not react and the evaporation also occurs. Preparation
cast:
For every 1 ½ kg of gypsum is added 1 liter of H2O.
Directions: Place first
water in a container and then go throwing the cast in the form of rain until the surface water to form "islands" of plaster. Let stand 2 'and then stir gently with your hand without forming bubbles, until it has consistency of a milk that covers the skin of your hand evenly. Only then pour into the formwork and the cast (previously greased with release agent) stick tapping on the table to escape any bubbles that may have formed. Let set and then proceed to stripping. molds can be made for pressed or strained when we pressed for use with wet paste, here we must bear in mind that all walls of the piece must have an even thickness.
For slip casting is when we pour into them.
Materials needed for mold-making:
• solid clay model or original piece, if it is empty we fill it with newspaper and mouth to make a fresh layer of clay plaster casting that it does not enter it. • Formwork
• release agent (a mixture of tallow with kerosene)
• Water • Plaster • Lamas metal
• Sierras • Sandpaper • Shellac (if we make an array)
release agent: 1 part
March stearin kerosene parties
Dissolve stearin double boiler the fire withdraw add kerosene. Mix well. Save
tightly capped glass jar. Apply with a brush.
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