FIRING THE BISCUIT BOTTLE OVEN
An essay by John A T Duncan FFARCS
I had served my apprenticeship on a series of factories both ancient and modern, while sandwiching myself into a three year course at the North Staffs Technical College (I believe that it's a polytechnic now). People who get their qualifications by attending night schools have my enduring admiration. There is no harder way to learn, yet the encaustic tiled corridors of the old Stoke Tech were thronged each evening with work-stained men, tired but determined to get the diploma that would qualify them to become pit deputies, pottery factory managers, engineers, accountants, and a hundred other things. Being a bright grammar school boy, I found examinations very much to my taste and did well at them. I had no doubt that, when I qualified, one of the modern factories would be put in my charge. Thomas Cone Ltd was old and run down. It was unclear whether I was sent there to test what I had learnt or to teach me a lesson.
Our fireman was a red eyed, bloody-minded, unshaven, uncouth old drunk who could fire ovens as perfectly as the baker. On the night in question he lay down on 20 tons of best cobbles, freshly delivered from Florence Colliery, and died.
Everything was ready for "kindding off." Dennis, the fireman's mate, had prepared each of the 12 firemouths with four hundredweight of coals topped off with a packet of firelighters and now stood, leaning on his shovel, awaiting orders. It seemed that life's dirty tricks department couldn't wait to expose my lack of practical experience.
Firing that 100 year old bottle oven was a lasting lesson to me. For all my book learning I still had enough gumption to realise that bluffing was useless but that scattered through the work people around me there was a wealth of practical knowledge in small packets, willingly given if sought. Dennis, for example, knew how to prepare the firemouths and what the pace of the various phases of firing should be. He didn't know how to judge when these phases should start, but he knew where the Bullers' rings and Seger cones were that would give me that information. Arnold, the cod (chief) placer, had set out the bungs of saggers in the oven and these included the stacks of hollow fireclay saggers that formed the internal flues. He, above all, could tell me where the different types of clayware were placed inside this particular oven and what sort of firing he expected them to receive. Finally, Norman, the lodgeman, knew from looking back in the day book how much coal and of what grades the previous (now deceased) fireman had ordered. It looked as though he usually had 20 tons delivered at the outset and that this was usually sufficient.
The first 36 hours were quite straightforward. Black smoke poured from our biscuit bottle oven and down Uttoxeter Road into Longton as the clayware slowly warmed and dried through the "watersmoke" period. Dennis was quite happy slowly feeding the smoky fires and catnapping on the coal. By the middle of the second day the kiln was black hot at about 800C. It was time to open the dampers and send the draught roaring through the fires, down under the floor, up the flues among the wares, and out into the filthy sky that daily deposited one ton of grit per square mile of Stoke-on-Trent.
Flourishes and mumbo jumbo
At six in the evening Dennis said to me, "You'll take over from me at ten tonight, then." This was the first time that I fully understood what a fireman was paid to do. Tradition, custom, and practice demanded that the important final 24 hours be done by the fireman himself, with all the flourishes and mumbo jumbo that his exalted position warranted. Mine was a lonely responsibility, and it was quite obvious that it was to be a recurrent event. The only hope of getting a replacement for the late fireman would lie in poaching one from another factory, unless one could be conjured forth.
"Dennis," I said, "It's time you saw an oven right through."
I believe to this day that he had been waiting for this, and he certainly had a better idea of how to conduct the closing stages of firing than had been credited to him. Through that-long night I explained to him the theory of balancing the four quarters of the bottle oven so that the heat increased evenly throughout. He had never used the gauge for measuring Bullers' rings and it was necessary to explain the concept of the well fired pot as a product of time and temperature represented in these shrinking rings. We discussed the inevitable variations in temperature throughout the bottle oven and how one placed various types of pot so as to take best advantage of this (or, at least, to minimise this
disadvantage). We agreed, as the dirty grey dawn appeared, that tunnel ovens would be a great improvement.
Later in life I found sitting up with a biscuit oven to be similar to being the all night casualty officer in Edinburgh Royal Infirmary. It was boring and exciting by turns; too hot or too cold; the atmosphere depressing or evocative, according to your state of mind. The hood and hovel were stark, blackened brick, lit only by the orange glow of 12 hungry firemouths. A cold moaning draught swept round the restricted space between the hood of the bottle oven and the circular beehive of the kiln itself, pulled by the throbbing, fiery furnace within. This monster, now that its fires were clear, ate coal at a prodigious rate, but we didn't dare fill a firemouth full for fear of drawing the balance of heat unevenly towards it. We sweated in front of the fires, but crept shivering back within seconds of stepping into the night air. We frowned and fumbled with the fireman's long trial rod, trying to lift Bullers' rings cleanly from the outermost saggers through a peephole, wrapping wet towels around our forearms against the yellow cone of heat blowing out of the small aperture. We worried and waited for measurements, exactly as, in later years, I would await the full dilatation of the cervix in the labour suite at Simpson's. The same responsibility lay with me at that moment, tempered by the same excitement of assisting in the final act of creation.
As the unwashed dawn began to shroud the surrounding buildings in a decent curtain of mist the picket gate across the works yard banged open. Eli, the boilerman, always first in, came across to greet us with a small parcel wrapped in an old Evening Sentinel. Inside lay half a pound of bacon, half a pound of cheese, and a dozen oatmeal pancakes.
"Potters' breakfast," said Dennis, polishing his shovel, already shiny from scraping its way through 20 tons of coal. "It don't taste reet from a pan," said Eli, and he cooked our breakfast on Dennis's shovel, laid on the coals. He was right. Potters' breakfast never tasted so good.
At eight we shut the dampers and closed the firedoors on each of the 12 firemouths. Dennis went home to bed while I settled into another busy day on the potbank. By six in the evening the last Seger cones had drooped, finally admitting that they had had enough heat. At eight the following morning I gingerly removed the first two bricks from the clammings, the temporary wall across the entrance to the bottle oven.
A cold wind whistled into the hot kiln, causing a thousand small, sharp pingings to take up a thousand variable rhythms. To their staccato notes Dennis added a duller slower bass as he "punched' the oven, clearing the clinker from the firebars with a heavy steel rod that resembled the tillerpost of a trawler. By now the pottery inside the oven would be too cool for the fly ash to cling and stain.
"Never mind the credit; take the profit"
We pinned a cardboard notice to the factory gates: "Biscuit oven emptying [sic] on Thursday." One of the wittier graffiti artists corrected this, adding, "The P is silent, as in bath."
At seven thirty on a cold, foggy morning our gang of casual labourers materialised. They were all freshly showered, straight from the night shift at Florence Colliery, but in their working gear they looked like a gang of pirates. They knew all about the irregular circumstances of this oven and shamelessly turned it to account by declaring it to be a 'hot oven.' There was nothing for it but to be first up the ladder inside the kiln and to place a thermometer atop the highest bung of saggars, 20 feet above the floor.
They were right. I hadn't cooled the thing properly. The alternatives were bullying (not this bunch of toughs), persuasion (no, they'd got me and they knew it), or bribery, and the latter course was obvious.
Twelve men drank 12 gallons of draught ale in the course of the forenoon and they had everything laid on the biscuit warehouse floor by one o'clock. They called me "gaffer" and a "gent," so I knew they thought that I was an easy touch. Why should that matter? In securing my objective I had demonstrated one of Carnegie's aphorisms: "Never mind the credit; take the profit."
Seven years after entering the pottery industry I realised that the senior jobs in our business were passing to members of one family and that this process would continue until the cuckoos had displaced my father, my brother, and me. The difficult trading times that followed the Suez crisis destroyed my father's determination to risk his capital on an independent venture and nearly destroyed my own self confidence. Success in industry comes easily when your father is a senior director of the firm, but it is not clean won and lays no foundations. For the sake of my self esteem I needed to succeed in something desperately difficult.
Medicine offered the most daunting challenge that I could imagine, but beyond the struggle lay the greatest prize that life could offer - independence.
More about Thomas Cone Ltd here>
John A T Duncan, FFARcs, consultant anaesthetist
Department of Anaesthetics, Dunfermline and West Fife Hospital
Courtesy : BMJ VOLUME 297, 1988
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1941 - British Pottery Research Association - TECHNICAL PAPER No.45
THE FIRING AND PERFORMANCE OF AN UPDRAUGHT OVEN FOR GLOST EARTHENWARE By R. J. WALLER, M.Sc.Tech.Courtesy British Pottery Research Association, Penkhull, Stoke-on-Trent. Extracts from Volume IV
As soon as the oven was empty, placing began, usually late in the morning. Ware from the drying frames and mangles was fixed in position in the saggars by means of thimbles, spurs, saddles or other accessories. Bitstone was used as a foundation for hollow ware, spurs and saddles also being employed for this purpose, and the ware being covered by pitcher or sherd bats.
The rings of bungs were continued until each had been completed except for a narrow passage-way opposite the wicket, four rings in all being placed. A column of refractory rings formed a “pipe bung” or chimney, above the well-hole, 17.5 ft. high. The remaining space was then built up with saggars as the placers withdrew from the oven.
The bungs immediately opposite the doorway were not carried right up because of the physical difficulty of placing the higher saggars. A free space was left where the arch bung should have been because of the difficulty of heating this region owing to the rapid radiation from the clammings, which were built up of 4.5in of firebrick and daubed over with refractory clay.
The ware placed in the oven on the occasion of the test consisted of:
The count of hotel and earthenware, 12 to the dozen, was :
In each test the firemouths were prepared for lighting off (kindling) by placing sticks and about four shovelfuls of beans on pieces of saggar provided to protect the firebars and simplify punching of clinker. All the firemouths were lit of in succession at 11 a.m., the firedoors being closed immediately. The fires were baited with 5 to 6 shovels per mouth after 50 mins., and then at intervals of about an hour, the fires being built up gradually. The doors were closed soon after baiting, and no further attention was paid to the oven until the next baiting.
At 2 hour 50 mins. from lighting off, the central annular damper was lowered, reducing the diameter of the opening from 3 ft. 3 in. to 1 ft. 3 in.
3 hrs. 50 mins. later, baitings having been continued in the interim at intervals of 1—14; hrs., the three second ring dampers were closed, and after a further 1 hr. 20 mins. to 1 hr. 35 mins. the four first ring dampers were closed.
The sitter-up took charge of the oven before the ring dampers were closed, and from this stage greater attention was paid to the control of the oven. The first ring dampers were opened to increase the draught through the oven during each baiting period, when fuel was being volatilized rapidly. This was termed “clearing the oven."
From this stage there was never more than light-medium smoke for very short periods at baiting. As the successive mouths were baited the firedoors were left open. They remained open to some extent, though adjusted from time to time, for from a quarter to three-quarters of an hour, allowing secondary air to be drawn between the fuel and the drop arch of the firemouth. This not only supplied air for the extra material being burnt but appeared to reduce the temperature in the firemouth by lessening the amount of air drawn through the fire bed itself. The doors were adjusted so that the flickering flame in the bags just reached to the level of the spy-hole.
After the doors were fully closed, the effective area of the regulator hole at each mouth was restricted either by means of an iron slide or by putting bricks across the opening. This required constant attention, particularly in the hour following baiting.
Mouths had been baited alternately all through the firing, and at this stage greater care was taken in controlling both the amount of fuel placed in the mouth and the space left below the drop arch. Each mouth was given individual treatment, which was adjusted to achieve definite effects and not merely applied according to a fixed schedule. Certain of the mouths were allowed consistently larger or smaller amounts of fuel, or air spaces oven-the fuel, according to the fireman’s knowledge of their condition and behaviour.
On the next day the ashes were punched out at 5.30 am, the remainder of the clammins knocked down at 7 a.m., and the crown damper raised at 9.30 am.
Drawing of the oven was begun at 7 am. on the following day, 68 hours after the start of firing.
The fuel consumption in tests E and F were 10 tons 15 cwt. and 10 tons 17 cwt. respectively.
As soon as the oven was empty, placing began, usually late in the morning. Ware from the drying frames and mangles was fixed in position in the saggars by means of thimbles, spurs, saddles or other accessories. Bitstone was used as a foundation for hollow ware, spurs and saddles also being employed for this purpose, and the ware being covered by pitcher or sherd bats.
Each saggar was provided with a strip of wad clay round its rim, which was intended to form a seal with the saggar above. This wad clay also facilitated the building up of stable bungs of saggars in the oven.
The arches, or spaces between the bags, in the oven were first placed with saggars, and then a ring of bungs was started nearer to the central well-hole. The lowest saggars were placed solidly on firebricks, the bungs being supported by scotches placed between the saggars and the wall or neighbouring saggars.
The fired saggars were not built up to the crown of the oven, some space being left for empty green saggars to be placed for their first fire, and a clear space for the passage of the products of combustion below the crown. Space was also left above each bag for the passage of gases. (In calculating the available placing volume of the oven allowance was made for the bags and the space above them, but not for the free space at the crown, by taking the volume projected above the free area of the floor, i.e. less the bags and the well-hole.)
The fired saggars were not built up to the crown of the oven, some space being left for empty green saggars to be placed for their first fire, and a clear space for the passage of the products of combustion below the crown. Space was also left above each bag for the passage of gases. (In calculating the available placing volume of the oven allowance was made for the bags and the space above them, but not for the free space at the crown, by taking the volume projected above the free area of the floor, i.e. less the bags and the well-hole.)
The arch bungs were “tied” to the crown of the oven by wedging a fired saggar between the top of the bung and the crown.
The rings of bungs were continued until each had been completed except for a narrow passage-way opposite the wicket, four rings in all being placed. A column of refractory rings formed a “pipe bung” or chimney, above the well-hole, 17.5 ft. high. The remaining space was then built up with saggars as the placers withdrew from the oven.
The bungs immediately opposite the doorway were not carried right up because of the physical difficulty of placing the higher saggars. A free space was left where the arch bung should have been because of the difficulty of heating this region owing to the rapid radiation from the clammings, which were built up of 4.5in of firebrick and daubed over with refractory clay.
The ware placed in the oven on the occasion of the test consisted of:
- 763 lb. of insulators
- 1785 lb. of whiteware
- 8181 lb. of ivory earthenware
The count of hotel and earthenware, 12 to the dozen, was :
- Plates up to 7 in. - 258 doz.
- Large plates and dishes - 252.75 doz.
- Hollow-ware - 190 doz.
- Saucers and sweets - 174 doz.
- Cups - 172 doz.
- Trays, sundries, etc. - 145 doz.
- Total 1192 doz.
In each test the firemouths were prepared for lighting off (kindling) by placing sticks and about four shovelfuls of beans on pieces of saggar provided to protect the firebars and simplify punching of clinker. All the firemouths were lit of in succession at 11 a.m., the firedoors being closed immediately. The fires were baited with 5 to 6 shovels per mouth after 50 mins., and then at intervals of about an hour, the fires being built up gradually. The doors were closed soon after baiting, and no further attention was paid to the oven until the next baiting.
At 2 hour 50 mins. from lighting off, the central annular damper was lowered, reducing the diameter of the opening from 3 ft. 3 in. to 1 ft. 3 in.
3 hrs. 50 mins. later, baitings having been continued in the interim at intervals of 1—14; hrs., the three second ring dampers were closed, and after a further 1 hr. 20 mins. to 1 hr. 35 mins. the four first ring dampers were closed.
The sitter-up took charge of the oven before the ring dampers were closed, and from this stage greater attention was paid to the control of the oven. The first ring dampers were opened to increase the draught through the oven during each baiting period, when fuel was being volatilized rapidly. This was termed “clearing the oven."
From this stage there was never more than light-medium smoke for very short periods at baiting. As the successive mouths were baited the firedoors were left open. They remained open to some extent, though adjusted from time to time, for from a quarter to three-quarters of an hour, allowing secondary air to be drawn between the fuel and the drop arch of the firemouth. This not only supplied air for the extra material being burnt but appeared to reduce the temperature in the firemouth by lessening the amount of air drawn through the fire bed itself. The doors were adjusted so that the flickering flame in the bags just reached to the level of the spy-hole.
After the doors were fully closed, the effective area of the regulator hole at each mouth was restricted either by means of an iron slide or by putting bricks across the opening. This required constant attention, particularly in the hour following baiting.
Mouths had been baited alternately all through the firing, and at this stage greater care was taken in controlling both the amount of fuel placed in the mouth and the space left below the drop arch. Each mouth was given individual treatment, which was adjusted to achieve definite effects and not merely applied according to a fixed schedule. Certain of the mouths were allowed consistently larger or smaller amounts of fuel, or air spaces oven-the fuel, according to the fireman’s knowledge of their condition and behaviour.
The sitter-up withdrew red glaze trial rings as guides to the conditions in the various quarters. From 19 hours to the end of the firing, contraction ring trials were employed. The first set was taken from the first ring middle in all four quarters.
The fireman took charge of the oven 20 hours after the start of firing, and continued in a similar manner to the sitter-up but with greater skill. He drew his trials before each baiting, and, in addition to the first ring middle, drew them from the first and third rings bottom—the latter in the first and third quarters only at successive withdrawal intervals, as the heat effects became apparent. These rings constituted his only indication of what he had to accomplish in the various quarters, and thereafter he baited his fires, closed his dampers and doors, and controlled his regulator air supplies in order to attain the conditions which the rings indicated to be desirable.
The control consisted of two main features; first, of bringing on one or more quarters relative to the others, and, secondly, of forcing a greater proportion of the products of combustion through the flues under the floor, in order to diminish the differences in temperature between the central parts of the oven (third and fourth rings) and the circumferential parts (arches and first ring). The latter was achieved by admitting secondary air through the regulator holes, thereby reducing the draught on the bags. Further control was obtained by leaving the oven “soaking” longer before baiting.
The fireman took charge of the oven 20 hours after the start of firing, and continued in a similar manner to the sitter-up but with greater skill. He drew his trials before each baiting, and, in addition to the first ring middle, drew them from the first and third rings bottom—the latter in the first and third quarters only at successive withdrawal intervals, as the heat effects became apparent. These rings constituted his only indication of what he had to accomplish in the various quarters, and thereafter he baited his fires, closed his dampers and doors, and controlled his regulator air supplies in order to attain the conditions which the rings indicated to be desirable.
The control consisted of two main features; first, of bringing on one or more quarters relative to the others, and, secondly, of forcing a greater proportion of the products of combustion through the flues under the floor, in order to diminish the differences in temperature between the central parts of the oven (third and fourth rings) and the circumferential parts (arches and first ring). The latter was achieved by admitting secondary air through the regulator holes, thereby reducing the draught on the bags. Further control was obtained by leaving the oven “soaking” longer before baiting.
The oven was considered to be finished when the trial rings were of the following sizes :
These were found to correspond to finishing temperatures of 1115ºC to 1030°C. in these positions, the latter appearing to be the minimum necessary for the requirements of the glazes.
The oven was finished 28.5 hrs. after the commencement of firing, the firing period usually being 28 to 30 hrs.
Immediately on finishing the ring dampers were raised, the firedoors opened, and four courses at the top of the clammins knocked in.
- First Ring, Middle - 20 to 23 points
- First Ring, Bottom - 15 to 18 points
- Third Ring, Bottom - 10 to 11 points
These were found to correspond to finishing temperatures of 1115ºC to 1030°C. in these positions, the latter appearing to be the minimum necessary for the requirements of the glazes.
The oven was finished 28.5 hrs. after the commencement of firing, the firing period usually being 28 to 30 hrs.
Immediately on finishing the ring dampers were raised, the firedoors opened, and four courses at the top of the clammins knocked in.
Drawing of the oven was begun at 7 am. on the following day, 68 hours after the start of firing.
The fuel consumption in tests E and F were 10 tons 15 cwt. and 10 tons 17 cwt. respectively.
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| Smoking bottle ovens in The Potteries photo: taken at Earnshaw's potbank, Stoke-on-Trent From the Gladstone Pottery Museum Photographic Collection. Photo: courtesy Staffordshire Past Track here> Date: 1950/51 |
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The roles of FIREMAN and COD PLACER
"It was the fireman who knew how his oven worked and he, together with the cod placer, decided what went where in the oven. The fireman had to know where things were because prevailing wind and atmospheric changes played havoc with various firemouths and draughts which came in from all directions upset the controls of the oven in all sorts of ways. The fireman had the ultimate responsibility of anything up to a whole weeks output. [He stayed with the oven.] The cod placer went home each night! Many thanks to Alan Hopwood of Stoke-on-Trent for this insight. March 2020.