This lime putty issue can become very complicated or it can be very simple. I choose simple. Pure lime is 100% calcium hydrate, it will cure to 100% calcium carbonate. Unfortunately, other than maybe pharmaceutical grade, there is no 100% pure. This chemical truth does not matter what rocks (dolomite or marble) the lime was made from - it all comes down to the numbers. I asked the different lime companies for their data and advice, there are many (do a web search). In the Northwest a prominent one is Chemical Lime Co. (they are also on the web). They make a product called "Classified" lime, or "Type N" lime, a "high calcite" lime. This lime has been sifted through a air curtain so the particles are more uniform. If I remember correctly it is over 98% pure calcium. The more common "type S" lime, available at nearly every brickyard is slightly less pure and more likely to have some "grits" in it. Both limes are suitable for fresco and much purer than anything that existed in the old world. They might, however, be different in how they spread out. I am used to "Type S", it spreads like soft butter. (by the way, i may have transposed type N and S, I am doing this from memory - not my strongest faculty!)

So what do these "exotic" ingredients cost. I can buy a fifty lb sack of "Type N" lime for $8 (cdn) at a brickyard/stucco supplier. Or I have to go direct to the "mill" for the classified, where I pay $5 a bag. For our purposes, you have to pour the powder through a flyscreen type mesh into a new trashcan with water in it.This helps prevent lumps when you mix it with a stucco mixing paddle and heavy duty electric drill. Let this soak at least 24 hours, I think the Italians say "it must be kissed by the moonlight". Unfortunately, you have to screen the fresh putty one more time before you use it for fresco, making sure there are no little "pips" of unhydrated lime. These are the instructions for a first class job, for very little money. Some lime suppliers sell drums of putty already slaked and seived - I have not seen their lab reports. The one ingredient to avoid for frescos which are to go outdoors is magnesium - it expands slightly different than calcium and used to cause problems in the early 1900's. I doubt if you could find any of this crappy lime today. By the way, when you are at the stucco yard, buy a bag of "00" size white dolomite sand and some "hyrib" mesh. I buy pure white marble sand, quite coarse, from a stone supplier. If you bought a used drill, you would be setup to do 100s of sq. yds. of fresco for under $100. Pigments, are much trickier, I recommend Kremer or Sinopia. There are companies selling concrete pigments. I think I mentioned previously that I purchased Warm Golden Yellow Ochre from Sinopia and the found Davis brand concrete color to be exactly the same and half the price. But these earth pigments are generally inexpensive anyways.

I use white concrete, at $23 (cdn) per bag,to give early strength to a fresco, it is used mostly in the first or scratch coat. The problem is that I feel, buy do not know, that it reduces the absorbtion of the fresco. I have used some on intonaco for simple frescoes, and have used simple putty and sand on the intonaco, both seemed to work well but I do not really have enough experience to make this conclusion. I also feel that ultimately, lime will cure much harder than if adding white cement, but this would take many, many years.

Hope that this helps more than it confuses.