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).

Lime & Sand

So, the purpose of this blog is becoming increasingly clear. I hope this might provide a forum in which I can share little bits of information regarding questions we get on a daily basis.

Just a few minutes ago I got a phone call from a homeowner who has recently repointed his home. However, he repointed 70% of the work without realizing that as he did not buy a pre-mixed mortar, he needed to add sand to the mix. The subsequent "dried lime" is cracking and not holding up.

Sand is a important component to mortar. It adds body, it increases durability, and it takes up shrinkage. It provides a vital service to the material. It's tough to make mortar without it.

The type of sand that can be used for mortars/plasters/renders in a perfect world should be clean, sharp, and well graded. Clay or silt will absorb water before the lime causing overdosing with water (hence clean). Sharp and well graded sands will reduce the lime content to "bind" the mix together. Most often these sands have a void ratio of about 33%.

How much sand is often the question most often asked. For more workable and creamy mortars (where you need to have clear and crisp joint profiles) two parts of sand to one part of lime is a often a good mix. For general work 2.5 parts sand is often utilized. For thin joints (under 1/4" or so where the exposed surface area is minimalized) one part of fine sand to one part of lime is often prescribed.

For more information check out the main website at http://www.virginialimeworks.com/