Fuels and Kilns

Richards Tiles was burning a miscellany of fuels in 1948. In descending order of dirtiness they were:

A Coal for: a) bottle ovens.

                   b) in-house production of producer gas.

                   c) steam raising for drying stoves.

                   d) steam raising to drive machinery.

B 96 Seconds oil.

C Town’s Gas.

D 32 Seconds oil.

Coal firing necessitated saggar protection around the ware and so did 96 seconds oil and producer gas, unless used in muffle kilns. Town’s gas was cleaner, but more expensive. As the gas works at Etruria expanded and became more efficient and competitive, so did our in-house made producer gas gave way to town’s gas.

96 seconds oil was very viscous and needed heating to enable it to be pumped around the piping system. It was used to fire some of the muffle kilns.

32 seconds oil was our cleanest fuel but we used it only intermittently for experimental open-flame (unprotected) firing.

The cleanest industrial fuel known was natural gas (methane) and was widely available to our competitors in the USA. It was later discovered in the North Sea and arrived in The Potteries in 1966. A new era of kiln technology became possible.

However, we shall first return to 1948. The UK was almost entirely fueled by coal. Some petroleum products were being imported, but on a relatively small scale. Two hundred and thirty million tons per year of coal were being mined. The North Staffordshire coal-field contributed seven million tons per year to that total. The Potteries was both a producer and a user of coal on a very significant scale.

Ceramic tile has a low financial value per unit of weight relative to, say, a china dinner service or a Doulton figure. Consequently, fuel represents a larger item of cost for tile. Price and efficient use of fuel have, therefore, always been very important in tile production. Protection of ware by saggaring, the use kiln cars and kiln furniture necessitates the heating up of bulky-heat-hungry items that are not to become saleable product. A technology known as “roller hearth” now provides the latest generation of kilns. Tiles are conveyed through the kiln as a single layer on a bed of rotating rollers. Only the tiles need to be heated, there are no kiln cars and no supports. Clean natural gas enables burning to take place immediately adjacent to the ware, both above and below, with very precise temperature control. This precise control, together with powerful hydraulic presses, has made possible the manufacture of a new product: the “homogeneous” or “porcelain” fully vitrified tile. Homogeneous tile is impervious, very strong and suitable for very heavy-duty purposes. Roller hearth single layer technology necessitates much shorter firing cycles to achieve required output. One hour, or less, is now typical. Tunnel kiln firing cycles were measured in days and the turn round time of a tile bottle oven was at least ten days.

The roller hearth kiln is well suited for linking to fully automatic systems. Most of the development of modern tile technology has taken place in Italy, which now leads the industry.

It is interesting to reflect that the coal fired bottle ovens, still firing in the 1950’s, operated with technology that had been in use, relatively unchanged, for hundreds of years. The old rule of thumb statistic: “four tons of coal for one ton of clay” equates, in modern technical parlance, to 36Kwhr of energy for one Kg of product. A roller hearth kiln uses about 1/60^th of that at 0.6Kwhr for one Kg of product.

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