LAST EDITED ON 30-Sep-01 AT 01:12 PM (PST)
>Thank you for the detailed instructions - this
>site is tremendously helpful. With lime putty
>mixed in this way, how long will it take before
>it is suitable for use in fresco? Preferably you let it "sit" for a week or two, however you can use it after 48 hours.
And, (this is
>a question which i have been curious about for
>some time, and betrays the fact that I am still
>new to fresco)why does the lime not set in the
>bucket or trough?
It does not set while covered with layer of water in the bucket because a chemical reaction must occur in which calcium carbonate is formed as a result of carbon dioxide from the air combining with the calcium hydrate in the wet lime plaster, slaked or hydrated lime that has been mixed with whater into a paste.
>And is this process the same thing as slaking,
>or are they different?
Gary is talking about Hydrated lime - that is a quick lime that has already been slaked then dried and crushed into powder. You get "second generation" slaked lime when mixing hydrated lime with water into a paste.
Making "first generation" slaked lime or original process of Slaking lime is a different process of mixing QUICK LIME (calcium oxide, CaO) with water. To make slaked lime quick lime must be mixed with water, a lot of energy and heat gets released, so you should be carefull.
Slaked Lime - calcium hydroxide
Slaked lime is white hexagonal solid which dissolved slightly in water.
Formula: Ca(OH)2.
calcium oxide
calcium oxide,
chemical compound, CaO, a colorless, cubic crystalline or white amorphous substance. It is also called lime, quicklime, or caustic lime, but commercial lime often contains impurities, e.g., silica, iron, alumina, and magnesia. It is prepared by heating calcium carbonate (e.g., limestone) in a special lime kiln to about 500°C; to 600°C;, decomposing it into the oxide and carbon dioxide. Calcium oxide is widely used in industry, e.g., in making porcelain and glass; in purifying sugar; in preparing bleaching powder, calcium carbide, and calcium cyanamide; in water softeners; and in mortars and cements. In agriculture it is used for treating acidic soils (liming). It is incandescent when heated to high temperatures; the Drummond light, or limelight, provides a brilliant white light by heating a cylinder of lime with the flame of an oxyhydrogen torch. Calcium oxide is a basic anhydride, reacting with water to form calcium hydroxide; during the reaction (slaking) much heat is given off and the solid nearly doubles its volume.
