I did some research on lime because I was having a hard time understanding the industrial processes by which hydrated lime is manufactured.
Most of the lime products available in the building supply stores are dolomitic and type S. I searched the Graymont web site and asked questions on the lime forum, for more information on dolomitic and types S and N lime.
According to Harry Francis on the lime forum (May 2001), type N hydrated lime (normal) is slaked under regular atmospheric pressure while type S (special) is slaked in autoclave and therefore is more fully hydrated than type N. The process of pressure hydration was developed in the 40’s to solve the problem of hard burnt magnesium lime particles in the firing of dolomitic lime, and to make possible the use of dolomitic lime as a building material. Dolomitic lime is a mixture of calcium carbonate and magnesium carbonate. Calcium carbonate needs to be fired ( in the CaO manufacturing process) at higher temperatures than magnesium carbonate causing the magnesium oxide to become hard burnt. Those hard burnt magnesium particles are not easily slaked (hydrated). If not properly slaked they will eventually pop in the mortar (plaster, stucco, etc) weakening the bonds and/or disfiguring of the surface.
Dolomitic lime has not traditionally been used in the past for frescoes and stuccoes. Specialized literature only mentions the use of high calcium lime for these trades. Even though I have no evidence that the type S dolomitic lime would be suitable, I have no indication that it would not be. Therefore industrial type S dolomitic lime may be an avenue worth investigating. Harry Francis writes: “Soft Burned limes generally slake nicely at normal atmospheric pressures, resulting in fat, plastic putty, if the slaking temperatures are kept hot, but below boiling temperatures.” He also writes: “Most hi-calcium limes are now made at very high temperatures, into dry hydrated lime powder, resulting in a very low plastic putty, not suitable for fine finishes” (May 2001).
Because of the high temperatures in the industrial firing of lime, even high-calcium hydrated lime might have to be type S in order to be usable for frescoes and stuccoes. According to Graymont (Genlime, May 2001) a type S, dolomitic lime is best used for mortars while high-calcium lime usually classifies as type N has poor workability and is only suitable for plasters.
The type S Putty sold by Graymont under the name of “Niagara” is not available for retail in my area.
In my local building material store I found Ivory which is a type S sold for gauging plaster. Indeed nobody buys it for frescoes or stuccoes. I am grateful that somebody is buying it for gauging plasters (and mortars?).
I am not sure what the Niaguara lime putty is made of (N or S?) but I suppose that it is a dolomitic lime because that what the Queries are, there in Ohio ( they claim to have the purest best quality dolomitic lime)
they would not consider shipping buckets of lime putty. They do not function that way. They only sell truckloads.
From Missippi lime, with the help of a sale rep I got a bag of high calcium lime (vertical lime, he shipped it to me). When I have the time to work at it I should be able to assess how the high calcium lime fares compared to the type S dolomitic. Phew! Hope you can sort this out!
Myriam