20 February 2010

The Hoff

At the foot of Llanymynech Hill is the Llanymynech Heritage Area, the most notable feature of which is the Hoffman kiln, one of only four remaining in Britain. The others are at Armadale, West Lothian; Carluke, Lanarkshire; Langcliffe, North Yorkshire; and Minera, near Wrexham.

Traditionally, limestone was burnt in draw kilns such as those pictured above, on the hill, wherein layers of stone and coal were loaded from the top and the resulting lime drawn from the bottom. From time to time operations had to be suspended in order to clear from the chamber any 'unburnt' stone. The Hoffman kiln, by contrast, was operated on a constant basis, using a continuous tunnel divided into a number of chambers. In such kilns, the fire can burn without surcease for years. The design was developed in Germany by Friedrich Hoffman, who first patented it in 1857 for the firing of bricks. Early Hoffman kilns were circular, but later versions were elliptical or rectangular, which enabled them to have more chambers.

The Llanymynech kiln, built in about 1899 of brick in battered section, had 14 chambers. At any one time one chamber was empty, one was being filled, five were pre-heating, two were firing, four were cooling, and one was being emptied. The limestone was brought alongside the kiln on the tramways that ran down from the quarries above, and stacked within the chambers in temporary walls. Between these were left gaps aligned with the numerous coal feed holes in the tunnel roof. The coal was brought straight from the nearby railway siding onto the kiln's roof, again by tramway.

Once a chamber had been loaded, its entrance was temporarily sealed with stacked bricks. Coal was fed from above into the spaces between the limestone stacks, and burned in the hot air that circulated from one chamber to the next, drawn down into and along the central flue that leads to the 140 foot chimney. This circulation was controlled by using the flues and dampers alongside each chamber entrance. Once the lime in one chamber was properly burned, the flue was closed and coal was fed into the next chamber, the flue for which was opened, and so on round the ring. Given the caustic nature of lime, and the need to keep out the damp, the kiln was covered with a corrugated iron roof. This has been reinstated, in mild steel, as part of a sensitive and informative restoration.

Hoffman kilns are still used in 'developing' countries; in Iran there are kilns that have been working continuously for 35 years. That at Llanymynech continued in use only until the outbreak of WWI, in 1914. The introduction of chemical fertilizers, and the reduction in the use of lime mortar, made production of lime uneconomic.


abijsmith said...

AMAZING photography, yet again!

YMGW said...

Thank you!