11.1.5 Superstructures

Hard evidence concerning the form of kiln superstructures is extremely scarce. None of the known kilns from the seventeenth and eighteenth centuries survive above the level of the fire box. In surviving kiln bases red brick, stone and tile all feature; brick being the most common material. The outer shell and chimney are likely to have been constructed from similar common building materials. The likely form of these kilns can be inferred from the limitations imposed by functional requirements. The need for a chimney on an updraught kiln will vary according to the proximity of other structures. As can be seen from du Monceau's account of the small kiln from Rouen, the most simple type need have no chimney at all; the top of the kiln being demolished each time the kiln is opened. The natural tendency of heated gases to rise is sufficient to carry them from the fire up through the chamber to the exit point above. In the proximity of buildings a chimney may be required to carry smoke and embers clear, minimising fire risk and nuisance. The chimney must clear the roof line to prevent adverse effects from winds eddying around the structure.

In the case of a downdraught design the chimney serves a more specific function. The hot gases rise naturally to the crown of the chamber from which point they must be drawn back down through the setting to the exit flue in the kiln floor. To create the necessary pull a chimney is required. The relevant areas of fire boxes to chamber floor and exit flue, together with chamber height, flue length and chimney height are critical factors in the performance of a downdraught kiln. Although the chimney must be tall enough to create the necessary pull, if it is too tall it will increase the gas velocity inside the chamber lowering rather than increasing the temperature. Leach quoting R. W. Baker, the then pottery advisor to the Rural Industries Bureau, propounds three feet of chimney for every foot of downward pull plus one foot for every three and a half feet of horizontal pull (Leach 1940, 191). Cardew, a man of considerable practical experience, repeats this formula (Cardew 1969, 183). Olsen gives a similar formula, three feet for every foot of downward pull plus one foot for every three feet of horizontal pull (Olsen 1983, 60). Cardew measures downward pull from the springer course whilst Olsen measures from the crown. Cardew measures horizontal pull from centre to centre of the chamber and the chimney whilst Olsen uses an overall measurement from front wall of chamber to back wall of chimney. Rhodes skirts round the problem stating that each design is something of a law unto itself (Rhodes 1969, 117). If these rules are applied to the King Street conversion, Cardew's formula requires a chimney 9.9 metres (33 feet) high whilst Olsen's one of 13.5 metres (45 feet). The chimney at King Street is in fact twenty two feet high. The effect of a short chimney would be to reduce the speed of gases through the chamber and protract the firing period. Unfortunately there is no record of when the conversion was made or how effective it proved to be. It is possible that, falling short of the ideal, a saving of fuel over the original updraught concept was still effected. In any event the kiln could still be made to function as it had previously by sliding back the covering bats over the crown vent and blanking the flue to the second chimney.

The majority of the kilns encountered are of updraught design. It was not until the second half of the nineteenth century that downdraught principles, developed by porcelain manufacturers, were adopted by some potters and pipemakers. The benefit to these trades lay not in the possible attainment of higher temperatures but in fuel economy and more even temperature distribution throughout the kiln chamber. Of the eighteen plans illustrated in Chapter 8 only that from Broseley displays features consistent with a downdraught system. Of the fifteen kilns illustrated in Chapter 9 again only that from Broseley is a downdraught. McLardy, at his Manchester works employed downdraught kilns as early as 1881. Pollock also of Manchester was still using a downdraught kiln in 1967 when Walker visited the factory. Walker published photographs showing the floor of chequered bricks (Walker 1977, 1659 & 1663). Walker also records that McDougall's kilns at Glasgow were of downdraught design (ibid, 1660). After a visit to the factory in September 1967, two months before it closed down, Walker wrote of the kiln:

The kiln had six coal fires round it fired from the outside, a flue in the floor and a vent in the roof which was opened if a rapid cooling was required. Coal was used simply because it was the cheapest fuel. In 1920 four such kilns were being used by the firm. (Walker 1970, 136)

A further development adopted by some makers in the second half of the nineteenth century is the multi chamber kiln. In this type the chambers constructed one above another can function in a number of ways. That commonly adopted in the potteries utilised two updraught chambers, the lower with multiple fire mouths being used for glazed ware and the upper taking advantage of waste heat from below was used for biscuit. The kiln used at Nantgarw for pipes was most probably of this type. "Minton's Oven", patented in 1873 had a downdraught lower chamber and an updraught upper (Rhodes 1969, 51-2). The Royal Copenhagen factory went even further with a three chambered downdraught kiln (ibid, 52). The two upper chambers each had fire mouths whilst the lower operated on waste heat drawn from above through floor vents to the chimney. Although it is unlikely that anything this complex was used for the manufacture of pipes the practice at McLardy's is not far removed from the Minton model. One other British kiln photographed at Rainford has proportions which suggest a multi chamber design (Figure 95a). This type is better known from the European mainland. Fraikin illustrates a two chamber kiln from Andenne (Fraikin 1978, 36) and a three chamber version from Chokier (ibid, 34-5), both in modern Belgium (Figures 95b-96b). A kiln of this type is still standing at Weert in the Netherlands (Figure 96a) on the site of the Trumm & Bergmans factory (1855-1933). As the founders gained their expertise in Belgium (Engelen 1985) these kilns may represent a fairly localised tradition. The survival of the Weert kiln is probably due to the fact that it was converted to burn garbage. In 1989 this kiln was moved to a new site (25 metres from its original position) and has since been restored. The picture was provided by Johan Hesemans who assisted with the removal of the kiln to its present site (Figure 96a). Little is known about early kilns on the European mainland. It is generally thought that open flame kilns were preferred. Duhamel du Monceau writing in 1771 illustrates open flame kilns used to fire pipes in Northern France and in the Netherlands. Both employ saggars although of different types.

The French saggars are open cylinders (cassettes or boiseaux) in which the pipes are placed, alternating stem up, bowl up, around a prop (chandelier). This alternating arrangement is reminiscent of the saggar from Barnstaple (Figure 27). The saggars from the Netherlands (pijpenpotten) are of more sophisticated design, standing on three small feet, tapering outwards to the rim, with a conical lid. The pipes were placed inside the saggar, bowls downward, around a central prop (trompet). The French arrangement of saggars stacked one upon another with no indicated divisional supports may have paralleled some intermediate British practice. The date of 1771 falls within the period postulated for tiered developments in Britain. Duhamel du Monceau illustrates a small muffle kiln from Rouen, built from brick with the muffle made out of tiles. In this kiln pipes were stacked in the same manner as that described above for the larger open flame kilns. Recent excavations at Breda, in the Netherlands, has proved the existence of pipe reinforced muffle technology dating from the second quarter of the seventeenth century.

It has been noted above that in the latter part of the nineteenth century open flame kilns became the predominant type used by the industry in Britain. Writing in 1979, this author, describing the evidence from a short lived production site in Gloucester, noted material from both a muffle and an open chambered kiln and inferred rather simplistically that the one had given way to the other (Peacey 1979, 74). This may indeed have been the case but it is also possible that the two kilns were contemporary slot serving different functions. There is now a growing body of evidence to show that many manufacturers had more than one kiln and that some of these had specialised uses.

From Rainford:

Other manufacturers had two types of kiln: one especially for the longer churchwarden pipes, which had two walls and was also updraught. The fire, in this kiln, was between the two walls and the saggers were placed on shelves in the inner chamber (Berry 1963, 4).
Berry is clearly describing a muffle kiln with the fire passing between two walls. It is used for the specific purpose of firing churchwarden or long stemmed pipes. He is clearly confused in the final sentence placing saggers (sic) on shelves within the muffle. Saggars stand one upon the other requiring no additional support. The only shelving known in connection with muffle kilns is of the peripheral type, consistent with both muffle use and the firing of long pipes. To place a saggar inside a muffle puts an additional barrier between flame and product requiring more fuel and lengthening the firing cycle.

From Broseley:

Some manufacturers at Broseley used two types of kiln, one specially for firing Churchwardens; Southorn's did similarly (Walker 1977, 144).

Unfortunately this statement is not accredited and no further reference to this point has been found in the literature. It possibly derives from Walker's interview of William Southorn in 1968 (Walker 1977, 135). The saggar evidence from the King Street works suggests that in recent times at least Southorn's did not use separate kilns for churchwarden pipes, rather separate saggars.

From Leeds:

When Samson Strong retired in 1950, clay pipe making finally ceased in Leeds, but fortunately much of his original equipment, one of three kilns, and part of the original pipe works was rescued and re-erected in the Abbey House Museum, Kirkstall, where they are now displayed. (Brears 1991, 3)

This probably derives from the account in Life and Traditions in West Yorkshire (Hartley & Ingilby 1976, 140). This source draws upon the memories of Mrs E Hope, Samson Strong's daughter and Mr J F Strong, his brother. Three kilns are mentioned but no details are given.

From Manchester there are three accounts of the McLardy works quoted above which give details of four kilns and an enamelling stove. It appears that two of the kilns were of open chamber type and two of conventional muffle design. The kilns, linked by underground flues, were fired in pairs with an estimated 30% saving of fuel. Unfortunately the accounts do not give exact details of the manner in which the kilns were linked. The open chamber kilns were clearly of downdraught design with the heat reflected from the crown down through the array of seggers. The most practical method of linking the kilns would have been a tandem arrangement utilising the waste heat from an open chambered downdraught kiln to preheat a muffle type updraught. In this arrangement the second kiln would act in part as the chimney required to pull the hot gases down through the setting of the first. Whilst in theory a second downdraught chamber appears desirable, in reducing heat loss from the chimney, the added complexities and difficulties of temperature control rule this out. In the crowns of McLardy's kilns there were, however, adjustable dampers which conceivably could have allowed the second downdraught chamber to utilise an updraught for a period of preheating prior to its return to downdraught under the influence of its own fires. It is difficult to visualise the two muffle kilns, described as having the fire underneath, as anything other than conventional updraught kilns discharging waste heat into the atmosphere. The enamelling kiln for burning French clays, of the first account, and the peculiar stove arrangement, for vitrifying the coloured enamel on the ends of certain pipes, of the last, are almost certainly different structures. The first is clearly a muffle kiln, the fire being wholly confined in flues, which extend all round the kiln. The second may be a similar arrangement to that described by George Spencer Watkinson - a coke fire in a confined space used to melt the glaze.

From Nantgarw there is evidence of two distinct kilns used for the manufacture of clay pipes. A multiple kiln of updraught design was standing until 1932 and is illustrated by Williams in his paper on the Nantgarw pottery and pipe works (Williams 1932, 110-111). A section of muffle wall with flue bar and peripheral shelf was recovered in 1971 by the author from a collapsed wall on the site. This section is from the type of highly developed muffle in common use in the early part of the nineteenth century. Manufacture of clay pipes on the site began in 1833 and the muffle kiln probably dates from that period. When the multiple kiln was built is uncertain, though evidence from Boston and Gloucester suggests that the open chamber kilns were adopted at those places in the 1860s and 1870s respectively. The two kilns at Nantgarw need not have been exclusive. There could have been a period of complementary coexistence as seems to have been the case at Manchester and Rainford.

From Taunton there is an agreement between members of the Pratt family dated February 10th, 1814 which records:

2nd The stock of Wood and Clay to be paid for as it is used.
4th ......his parents share of the stock of Wood and Clay shall be at their future disposal.
10th The following are to be the prices of the Various sorts of work given to each one of the family as long as the present price of Goods can be maintained - viz - for making the best pipes 1s-6d per groce for Trimming the same 6 pence for making eighteens 1 shilling for trimming the same 4 pence for burning large Kiln 6 shillings for drawing the same 1s-3d for small Kiln 5s for drawing the same 1 shilling for going one days Journey such as Bridgewater &c 2s-6d and expenses paid. for two days Journey 5s-0d and Expenses paid. for packing of pipes at the rate of one shilling for 18 Groce: for serving Town & home customers 5 shillings per week. (Lewcun 1991, 9)

This is an interesting early record of plural kilns. Unfortunately it provides no clue regarding their function. The kilns may have differed in size only; the smaller kiln allowing greater flexibility for urgent orders. The alternative is that they were constructed along different lines to cope with specific requirements. The account bears witness to the fact that the labour cost of firing the large kiln was 20% greater than for the small one. This figure can not be used to calculate the respective proportions of these kilns. A number of factors dictate firing times. The lower limit is set by the properties of the material to be fired. Longer schedules would be required for: any increase of chamber size; any increase in volume of material within the chamber; any increase in volume of structure; all requiring additional calorific input. The two kilns may even have been of different design. It is, however, interesting to note that by comparison with the rate of 2s-6d for one days journey, burning the small kiln is valued as two days and the large kiln slightly more. For drawing the large kiln half a day and for the small kiln 2/5ths of a day. Unfortunately there are no rates for packing the kilns.

From Colchester, a newspaper account from 1906 records the change from one type of kiln to another.

A visit the other day to the Colchester pipe factory to witness the process of manufacture was full of interest, Mr. Jennings, of 21 George Street, the monopolist of this unique trade in the borough, kindly offering every facility. .....The churchwarden pipe is becoming almost a thing of the past. Thousands have been made in the town of Colchester, but at the only surviving tobacco-pipe factory in the Borough they are no longer made. Trays of them ready for firing are indeed to be seen there, but these are never likely to be finished, for the furnace more recently introduced is not constructed of dimensions suitable for these graceful long-stemmed, yet almost obsolete, pipes, with their inviting red-tipped mouth-pieces, and the shorter clays are now, in obedience to the dictates of fashion, the staple article of manufacture.

All that remains to be done is to draw out the perforating wire, trim off the "seam" caused by the mould joint, curve the stem to a line of beauty, and the pipe is ready to be fired in the muffle, to emerge from the kiln in due course the firm, clean, white, porous tobacco pipe familiar to everyone.

The firing is performed in a closed coke-heated muffle, and it is some five hours before the drab-coloured clay assumes its final stage - firm, snow-white, unalterable. The temperature must be raised to an absolute white heat in this furnace, and nothing short of this perfect incandescence suffices to bring the pipes to their required condition of immaculate whiteness and absolute immunity from all further vicissitudes of form (until they get broken).(Benham 1906)

This account records the recent introduction of an open chambered kiln to replace one more suited to the production of long stemmed pipes. As can be seen from records of Southorn's and Pollock's long stemmed pipes could be fired in special saggars in open flame kilns. Perhaps by this late date there was insufficient demand, for the longer pipe, to justify investment in special saggars. In the second quoted paragraph the term muffle may harp back to the production of longer stems with the requisite elegant curve. It is unlikely that the new kiln using saggars was of true muffle type.


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