To gain experience in plastering at a minimal cost it is possible to prepare a "Practice Lime Putty" from common Type-S Hydrated Construction Lime.Type-S Hydrated Lime is manufactured from Dolomitic Limestone (Dolomitic limestone contains 35 to 46 percent magnesium carbonate).
Being a great lime to practice fresco plaster application techniques, this lime is NOT suitable for the actual painting in fresco due to fast setting and poor adhesion/binding of colors and efforescence on the color layer. However it is widely used in construction and is available at very low cost almost at any building supplies retailer.
Making of Practice Lime Putty Video from the Fresco School:
http://www.youtube.com/watch?v=z5ZkgcxDqYc
Contemporary construction lime expert, Margaret Thompson (Ph.D., Technical Manager, National Building Construction, Chemical Lime Company) states:
"...In the today's plaster and masonry industries in the United States, Type S hydrated lime is used almost exclusively. In 1977 in the United States, no more than 2% of the building market used pulverized quicklime (Boynton, 1980) and the remaining 98% used hydrated lime. Of the 98% that used hydrated lime, over 95% are estimated to have used Type S hydrated lime. These values are not much different today...
Lime putty was progressively being displaced by hydrated lime, which was more convenient and already slaked (Lovewell, 1975). For commercial use, quicklime slaked to hydrated lime was more attractive than quicklime slaked to lime putty. Transporting water was and is expensive.
There were probably 20 producers or more at the time, working one regionally-extensive geologic unit: the Lockport (Dolomite) Formation of the Niagara Group, which is middle Silurian in age.
...In 1918, Charles and Irving Warner of Willmington, Delaware submitted a patent titled "Method of Hydrating Dolomitic or Magnesian Lime", which states:
"Our invention relates to a process of hydrating dolomitic or magnesian quick-lime and has for its object to secure, as far as possible, the hydration of a material or part or all of the magnesium oxide, in addition to the hydration of all the calcium oxide in order to secure additional strength in mortars made from such hydrated limes. A further object is to maintain the plasticity and for working under the trowel of mortar made from magnesian lime...
Although the Warner (1918) process was available, it was the Corson Pressure Hydrator, patented in 1943, that was the most significant technological advance. It gave the lime industry the ability to make sound and highly-plastic hydrated lime (when a putty) from dolomitic quicklime in a continuous process. This innovation drove the definition of Type S hydrated lime as being "special". Hydration did not occur at atmosphere, but rather at high pressure.
-- Margaret Thompson -"WHY IS TYPE S HYDRATED LIME SPECIAL?" International Building Lime Symposium 2005 Orlando, Florida, March 9 -11, 2005
It is possible to locate Type-N Hydrated Lime - "N" stands for "Normal", nowadays it is mainly produced for agriculture as a mineral supplement for the fields. Type - N Hydrated Lime is very high in calcium (95%) which would be ideal for fresco, however the resulting plaster is weak and takes ages to fully cure.
For your convenience, a short video from the FrescoSchool.org demonstrating the process of making "Practice Lime Putty" from Type-S Hydrated Lime is available at Fresco School's Video Channel at http://www.YouTube.com/FrescoSchool free of charge.
The best for fresco painting - High Calcium Slaked Lime Putties are difficult to find locally (in USA), at the FrescoSchool.org we use imported from Italy "Calce Florentine" - High Calcium Lime putty sold by the www.FrescoShop.com at the www.TrueFresco.com/frescoshop