Tuesday, August 10, 2010

Let's start at the very beginning


When discussing historic mortars, and lime mortars in particular, its helpful to understand where they come from.


The first question we often encounter from someone that hears that we make "oystershell" lime, or handmade limestone limes is "How (or where) do you grind your shells?" The simplest answer is: we don't. When you make lime there doesn't have to be any grinding or crushing in the process.

Today, I'm going to talk generally about making lime and then through the rest of the week I'll talk about the various processes that are involved. There's a great little diagram known as "The Lime Cycle" that shows the whole process pretty clearly.


So, it all starts with Calcium Carbonate. We'll talk about the forms of these more specifically tomorrow, but at Virginia Lime Works we use either limestone or oystershell. We're going to take this calcium carbonate and burn it in a kiln. The burning does two things: First, we're going to drive off the moisture from the stone and secondly we are going to drive off carbon dioxide. This yields a material called "quicklime" or lump lime.


We take this lump lime and we hydrate it. When lump lime comes in contact with water it generates heat and will start to dissolve and break down. If we hydrate with a little bit of water (steam, etc.) we've got hydrated lime (or lime in powdered form). If we hydrate with a lot of water we make lime putty.


Now we've got the material in a "lime" form, ready for use. Sand is added for mortars, plasters, and stuccos, or water is added to make limewashes/whitewashes and the material is applied. While the material cures, water evaporates out and carbon dioxide is absorbed, in a way reverting the material back to calcium carbonate (or limestone/oystershell).

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