History Of The Lime Kiln
History of the Lime Kiln
If you’ve taken our Farm Tour Wagon Ride, you’ve probably heard all about our lime kiln, located out in the meadow beyond the parking lot. Though ours is no longer used today, it was a real working lime kiln back in the early 1800s when it was built. So what is the purpose of a lime kiln? How were they built by our ancestors? What do they look like today? Read on to find out!
Calcination of Limestone
To understand how a lime kiln works, it’s important to know what the purpose is. Lime kilns are used for a process called “calcination,” which is a process of heating to very high temperatures with a limited supply of air or oxygen, to bring about decomposition. In a lime kiln, the calcination process is used to decompose limestone (also called “calcium carbonate) into quicklime (also called “calcium oxide”). This process takes place at temperatures over 900 degrees. The word actually comes from the Latin word calcinare, which means “to burn lime”—because the most common application of calcination is the decomposition of limestone to lime and carbon dioxide to create cement!
Lime is used by farmers to help lower the acidity in the soil. Using a fertilizer like manure helps to replenish the nutrients, but it can raise the acidity level of the soil over time. Using lime and manure helps to keep a balance in the field, allowing the farmer to continue farming in the same fields for many years. These days, farmers can just go to their local quarry and pick up a load of lime to spread on their fields, but years ago they had to make it themselves using a lime kiln.
Lime kilns were usually built into the side of the hill, so that the limestone could be easily shoveled into an opening in the top. The stone was crushed into smaller lumps (not too fine), called a charge, before being placed in the kiln. A fire was kindled in the bottom of the kiln, and after the calcification process was complete, the lime would be cooled and then raked or shoveled out through an opening in the bottom of the kiln. Lime kilns were generally made to be the same size—too small and the work was not efficient enough, too large and the materials would cave in on themselves. The output could be as much as 30 tons in one batch. This may seem like a lot (and it was), until you compare it to modern kilns, which can produce anywhere from 100 to 550 tons in one day. The process of making lime usually took about a week—a day to load the materials, three to four days for the actual process, a day or two to cool, and a day to unload the materials.
Flare kilns vs. Draw kilns
There are two different types of lime kilns, “flare” kilns and “draw” kilns. In a flare kiln, a temporary limestone vault is built over the furnace area, and the charge is loaded on top of the vault. After the limestone has been calcined, the vault is removed, the kiln is unloaded and the process is repeated. In a draw kiln, there is a permanent grate built near the bottom of the kiln, over the furnace area. The limestone charge is loaded into the kiln in layers, alternating with layers of fuel (usually coal or wood). As the limestone is calcined, the lime and ash falls through the grate and is removed. This allows a drawn kiln to be used continuously, since it can be loaded through the top as the material is removed below. The lime kiln at Cherry Crest is a flare kiln.