BOTTLE OVENS and BOTTLE KILNS
Gladstone Pottery Museum, Longton.
Photo: Julian Read Collection Date: April 2017
The word kiln is the generic term for the structure or equipment in which pottery and other ceramics (and other materials) are fired.
To most of us, the terms, bottle oven and bottle kiln mean the same thing - a complex brick-built, bottle-shaped structure for the firing of pottery or associated materials. Most of us assume them to do the same job. And although both ovens and bottle kilns have that curious bottle-shaped chimney, and both were fired with coal, there is an important technical difference between the two.
BOTTLE OVEN In the North Staffordshire pottery industry, the term bottle oven meant the potter's biscuit or glost oven which was fired with coal to produce long flames that passed from the firemouths directly into the firing chamber. Pottery inside the chamber needed to be protected from the flames, smoke, sulphur fumes, ashes and dust in fireclay boxes called saggars.
There were other types of bottle ovens which included two-tier structures and salt glaze ovens. Temperatures of around 1000°C to 1400°C were reached in bottle ovens.
BOTTLE KILN In the North Staffordshire pottery industry, a bottle kiln was built completely differently from an oven. There were several types.
A muffle kiln was constructed in such a way that the flames and products of combustion were prevented from entering the firing chamber by being circulated through enclosed flues which surrounded it. The products placed inside the chamber were thus kept away from the filth of fire and did not need to be protected in saggars. Muffle kilns were used for the firing of onglaze enamel decoration and for 'hardening-on' underglaze transferware. Temperatures typically around 700°C to 850°C (1300°F to 1560°F) were reached in muffle kilns.
A calcining kiln was used by potters' materials suppliers. For example flint stones or animal bones were calcined in kilns to make them friable and able to be crushed and ground ready for use in pottery recipes.
SOME OTHER TYPES OF KILN but not with a bottle shaped chimney
DOWN DRAUGHT BEEHIVE KILN
Kilns for the firing of 'heavy clays' (bricks, tiles, drainpipes, chimney pots and sometimes large pottery) and 'domestic ware' (salt glazed or dipped pottery) were not bottle shaped at all but were described as 'downdraught beehive kilns'. They were characterised by their separate tall chimney. These were found in great numbers in the Potteries, but all of them have been demolished. They were also found in other parts of the UK but just a few now remain.
SIMPLE (or BASIC) UPDRAUGHT KILN
The simple or basic updraught kiln has a short (low height) conical
chimney built on top of the kiln chamber. The kiln was sometimes built inside one of the buildings of the factory, the short chimney poking through the roof. A good example of this can be seen (April 2020) at the Wetheriggs Pottery in Cumbria. Sometimes they were free standing.
FRIT KILN
This particular design of kiln was used for the production of 'frit'. This material is created from a mixture of ceramic materials blended together and then heated and melted in the kiln. The resultant 'molten glass' is quenched in water to make it shatter and become small granulated friable particles. These are then are milled to a powder for use in porcelain enamels, and fritted glazes. Used to reduce the melting point of a glaze.
LIME KILN
A kiln used for the calcination of limestone (calcium carbonate) to produce the form of lime called quicklime (calcium oxide). Similar to a calcining kiln.
NO TWO ALIKE No two bottle ovens or bottle kilns were alike. They were all built differently, the vast majority without architect's drawings. Many were built 'by eye' and based only on the experience of the oven builder and the requirements of the factory owner. An old bottle oven builder, Tom Clewes of Longton, explained, during the Last Bottle Oven Firing in 1978, that "They just went up. You won't find any of these ovens drawn on paper since you just built them. We did it on a day-work basis and got on with it."
The decorative brickwork at the top of the hovel chimney was created on the whim of the builder and owner.
In their heyday, the different types of bottle oven and bottle kiln were not specifically listed or classified. People in the pottery industry knew exactly what they were, so why bother listing them!
Of the 2000 or so coal-fired bottle ovens and kilns which once littered the skyline of the Potteries only 50 remain today (47 standing complete with their bottle shaped chimney and 3 more which have collapsed). Firing them is no longer permitted, the Clean Air Act of 1956 signalled their decline. They were replaced with kilns using the alternative fuels of electricity, gas and oil.
Source:
1) Alfred Clough, the 'fireman' responsible for the Last Bottle Oven Firing in The Potteries in 1978. He was a local pottery manufacturer and at one time owned over 30 pottery factories.
2) In 1921 Ernest Sandeman described the various types of oven and kiln in his book 'Notes on the Manufacture of Earthenware'