By Patrick McAfee

Tomas Lipps asked me to run a two day lime workshop as part of the 2008 Stone Foundation event in Barre, Vermont. A lime kiln seemed to be a good idea if the materials to build and fire it could be found. Norm Akley of Trow and Holden was invaluable when it came to finding what was needed.

With all the materials in place I built a mini kiln the day before the workshop to check out the coal, burn rate and final product. It was successful but slow.

The workshop began the next day with 12 people who worked hard to make the workshop a success despite the rain. We mixed mud (earth/subsoil) for the building mortar and for lining the kiln. Paul Janusz brought 300 pounds weight of sea shells (oyster and barnacle) from Pennsylvania (don't ask). 

The lime kiln was built with concrete block on edge (4 inch thick walls) with a mud mortar and parged (plastered) internally with the same. This was done course by course, all the while filling the kiln with alternate layers of coal and limestone (Indiana). The shells were stuck into the mud parging.

At an earlier Stone Foundation workshop in Charleston, South Carolina, shells had only been used in alternate layers between the coal. They bonded together with the heat reducing the updraft and slowing the burn until broken up with a pinch bar. To avoid this happening again the shells were fixed into the mud parging on the internal walls of the kiln.

A fire was lit in the firebox at the base of the kiln using paper, kindling and wood. The purpose of a lime kiln is to convert limestone and shells (calcium carbonate) to quicklime (calcium oxide). To do this the kiln must reach 900 Celsius in temperature. From experience and without use of a thermometer I knew that this was possible in as little as four hours. This kiln took far longer, possibly twelve hours. The reasons for this is probably accounted for by the weather, blocks, coal, stone and shells being wet from the rain. Also the mud mortar and parging had insufficient time to dry out before firing. In the past, faster burns occurred when the blocks were laid dry without mud. Some of the participants on the course were camping out beside the kiln and thanks is due to them for taking care of the kiln during the night.

The next day the kiln was dismantled. It was a success. A highly reactive quicklime (therefore probably non-hydraulic/high calcium lime) was extracted from the kiln. There was little waste with no over-burning and little under-burning. The mud parging (3/4 inch thick) was burnt to a red biscuit. This was crushed finely to a pozzolan to be added to the lime to give a pozzolanic set (ability to set in wet/damp/cold conditions and to a higher compressive strength). Proper testing in a laboratory would be needed to check out the reactivity of the pozzolan.

We produced a lime putty by slaking, a hot lime mortar, a pozzolanic lime mortar and with natural hydraulic lime from a local contractor produced NHL mortars. The group then built a small double wall and set out a circular and spiral pier and some sample panels of lime plaster including mud.

Commercially a kiln like this is not viable because it uses too much fuel (approximately 50% coal to 50% calcium carbonate). It is also too small. An improved version could easily be built by building a cavity wall type kiln using clay brick (solid) on the inside and concrete block on the outside. The cavity between could be filled with vermiculite or perlite for insulation. Long lengths of steel angle iron could be fixed vertically at the four corners and connected tightly with tying wire to prevent the kiln expanding and disintegrating as it will do with heat. To prevent the vermiculite or perlite draining out of open joints it could be mixed with a small amount of mud. A kiln like this would use far less fuel.

Why would anyone go to the trouble of building a small kiln? Well from a learning point of view it is indispensable. Also it could have a place in small repairs to historic structures using local stone/shell and fuel. A contractor specialising in conservation could tender on the basis that he/she would replicate the lime and sand in an existing mortar using local matching source materials. What is needed is a small, efficient, transportable kiln, possibly made of steel for small burns. What about carbon dioxide? Unlike cement, lime reabsorbs the emitted CO2.

Working with lime will hopefully continue as an important element within the Stone Foundation and further kilns and experiments and discussions and drinking beer late at night will raise the awareness and further our knowledge of a material handed down to us from the past that has a place today in the repair of old buildings and in new work.