Extracts courtesy of the late Robert Copeland : From his book: Manufacturing Processes of Tableware during the Eighteenth & Nineteenth Centuries. Published by Northern Ceramic Society EXTRACTS from CHAPTER 16 - THE POTTERY BOTTLE OVEN AND FIRING PROCESS, Placing
“The ordeal by fire is of the greatest importance in any pottery manufacture. Pottery factories have closed because the body formula permitted such a narrow range of safety in firing that it took only two or three unsuccessfully fired ovens to cause bankruptcy. Coal-fired kilns were built inside a protective hovel in the shape of a bottle; these provided cover from the weather and enabled the fireman to control the air supplied to the fire mouths. Round ovens were used for firing pottery since early times. The basic structure is a round kiln with firemouths around the outside whose flues emerge inside the kiln to carry the flames which heat the load, or setting, of pottery. When larger kilns were built and higher temperatures were required (for firing porcelain or stoneware, for example) the control of the draught of air became more important, so a brick hovel was built around the kiln to increase the draught as in a chimney; various holes were left in the crown, that is the roof of the kiln, through which the airflow was controlled by dampers operated by means of long rods or chains from ground level. The objective in firing any intermittent kiln is to obtain as even a distribution of heat as possible throughout the setting and to maintain a controlled rise in temperature in all parts of the kiln during the firing cycle. In firing earthenware clay-ware great care has to be taken at certain stages to avoid the ware bursting due to water evaporating too quickly from the outer surfaces while water of combination (molecular water) remains inside the ware and, in turning to steam, will burst the pot; this early stage is known as the water smoking period - 80° - 120°C. The other stage is when the silica changes its crystalline structure at about 200°-220°C and again at 550°C; then the rise or fall in temperature must be quite gentle to avoid cracking. The kiln is constructed of two types of bricks: the outer shell wall is about 12 inches thick and built of common building bricks, while the inner-facing surface is built of fire-bricks. The floor of the kiln is built of fire-bricks and beneath it are flues, also built of fire-bricks, which lead from the fire-mouths to the
centre-hole; this floor rises towards the centre-hole, the flues allowing the flames to be drawn up the middle of the kiln to balance the heat rising up the side of the kiln through the vertical flues, or bags. The hottest part of the kiln is the area just above the bags, where no ware is placed for fear of overfiring it; those parts of the kiln between the bags are called the arches. Up-draught kilns might have four, six, eight or ten fire-mouths depending on the size of the kiln. The area inside the kiln heated by two fire-mouths is called a quarter. There were not always four quarters in a kiln; a small kiln might have only three quarters - six firemouths, a large kiln might have five quarters. Fire-mouths are of two types. A popular form, as seen at the Gladstone Pottery Museum,(Longton, Stoke-on-Trent) has an opening in the kiln wall with iron doors to permit baiting (fuelling) with the coal and control of the air flow over the top of the fire. The kilns at Spode had hob type fire-mouths, that is the coal was fed into them from above through a large rectangular opening in the brickwork which could be sealed with a cover-brick; this would be manoeuvred to one side with a heavy iron poker and then back into position after baiting. The fire-box was of such a size as would enable the heat to be sustained for about four hours without the need for refuelling. Resting upon the fire-bars, at the front, would be loose bricks placed there to prevent the red hot coals from falling out, whilst permitting an ample supply of air to reach the fire. This honeycomb type of brick structure was known as the gluts. Coal for firing was stacked around the inside of the hovel and was replenished once or twice at convenient times during the firing cycle. Doors in the side of the hovel might be of the single or double type. The coal needed to be of the high bituminous sort which yielded plenty of gas to give long flames. Entrance to the inside of the kiln was through an arched opening in the wall, called the clammins arch, also called the wicket; when the kiln was set in, that is, the saggars completely filled the kiln, this opening was bricked up, using wet sand and turner's clay shavings as a mortar. Just behind the clammins, about two feet from the floor level, was placed a trial saggar, one with a piece cut out of the side, in which the fireman's trial pieces and thermoscope bars were placed. There would be similar trial saggars in each of the quarters; one at about two feet from the floor and another at about six feet from the floor level. These trials would be inspected through holes in the kiln wall. The roof of the kiln is called the crown, being slightly domed for structural strength during its life of being alternately heated and cooled. In its centre is a round hole, the crown hole, while at intervals towards the outer edge were smaller holes which permitted the escape of the smoke during fuelling, or baiting, and allowed a permanent draught through the kiln. Near the edge of the crown were larger holes with iron dampers connected by rods and chains to within reach of the fireman; in certain conditions he could adjust these dampers to regulate the draught during firing. A bottle oven, that is the inner kiln, might require to be rebuilt completely every forty years or so, but would need minor repairs after every two or three firings, while a new floor or bottom might be built every third year.
The hovel, however, should last for many years. That of the last oven at Spode Works was built in 1789 and collapsed on the 23rd February 1972 having been out of use since 1960. This hovel was of nine inches brickwork and, although probably symmetrical when first built, it distorted in shape over 183 years. In about 1940 the height was increased; this was done to compensate for the loss of draught caused by placing an iron baffle over the top to prevent the glare of the oven from lighting up the sky at night during World War II. Unfortunately, the hovel could not support the extra weight of the new brickwork and after twelve years of absorbing the rain the structure became sodden with water and softened because there was no fire inside to evaporate it. The old bricks and mortar had perished allowing the hovel to bulge and fail. Clay ware needed to be supported during the firing process to minimise warping and distortion, as well as being protected from the incompletely burnt products of combustion. This was achieved by placing the ware in boxes or containers made of fireclay - a special type of clay having refractory qualities. These boxes are known as saggars, their shape and size varying according to the ware they are designed to hold. One other type of container is known as a setter: these are mainly used in the firing of bone china flatware when each plate or saucer needs to be placed individually on a supporting bed of flint or alumina to avoid the item warping. The hottest parts of the kiln are closest to the vertical flues, or bags, so the densest setting of flatware plates and saucers - is in this area called the first ring. Inside that, in the second and third rings, there would be more bungs (columns) of saggars or setters holding plates - usually larger sized ones than those in the first ring near the bags. Then, in the centre, round the hole, the items of hollow-ware would be placed - teapots, coffee pots, sauceboats, vegetable dishes, meat dishes and platters, and tureens. Cups could be placed here and also on top of the bungs of flatware in the second and third rings; often in green or newly made unfired saggars. The fireman made sure that he knew what ware was in the kiln and where it was placed, particularly if there were any variations from the normal setting, or if there were special pieces needing extra care.
The objective in firing is to ensure an even temperature and oxidizing atmosphere throughout the kiln. Three main stages may be noted: 1. The heat moves to the bottom and centre first, and is then drawn to the top where, striking the crown, it spreads out. 2. The heat must now build up at the bottom and top until the two masses of heat meet, one ascending, the other descending. 3. When this occurs the soaking period can begin. This is a sustained level of heat and is essential to ensure proper vitrification and satisfactory translucence in the case of bone china.
The firing of Spode’s No.4 oven, the 'Big' Oven, fired by Jim Evans in January 1951 The descriptions below refer to 'hob' type fire-mouths as used at Spode. A brief outline only of the firing programme is possible to indicate the complexity of the operation. The fires are kindled (lit) at 07.00 hrs, having been laid with paper, sticks and coal. Baiting is the word used for describing the stoking of the fires; it is a very skilled operation and is not just shovelling coal! After levelling the red fire and clearing the clinker from the flues, large pieces of coal are thrown in so as to provide a solid layer; this is called lumping. This is followed by medium sized coal and lastly shovels of small slack-coal on the top.
Day 1 07.00 hrs
Fires kindled
12.00 hrs
1st Baiting
16.00 hrs 2nd Baiting The kiln smokes more on the first day because it is not hot enough to burn the smoke before it escapes. Also, water in the clay-ware forms steam which increases the density of smoke. This is called the water smoking period. 20.00 hrs
3rd Baiting
Day 2 00.30 hrs
4th Baiting
05.00 hrs 5th Baiting The firemouths must not be filled so full that air is prevented from passing over the top of the coal; this could cause a reducing atmosphere (lack of oxygen). 09.30 hrs
6th Baiting
10.20 hrs Re-coaling Cover bricks (or fire doors) are closed up while the outside hovel doors are open thus allowing cold air to enter. Coal is brought in a wheel barrow and stacked all around inside the hovel. 12.00 hrs
7th Baiting
13.30 hrs
Complete the re-coaling; close up bricks, etc.
15.15 hrs
Top up fires to last until...
16.00 hrs 8th Baiting between brick and wall
Cover bricks (fire doors) now used to control the draught; a 3 inch gap
17.00 hrs
Bricks closed to I.5 inch gap.
20.00 hrs 9th Baiting If the fireman is really good the oven should only smoke for a maximum of twenty minutes after each baiting.
Day 3 00.30 hrs
10th Baiting
01.20 hrs
Begin to adjust the regulating bricks at 20 minute intervals
05.00 hrs
11th Baiting
06.30 hrs First Holdcroft thermoscope bar trials begin to melt. Small occasional baitings to individual firemouths to ensure evenness of firing in all quarters. 0930 hrs Further control by adding small bricks to close the narrow gaps between the cover and regulating bricks and the lintel over the fire mouth. 1030 hrs
Bone china firing trials are withdrawn to check on the effect of firing on the ware.
10.40 hrs
12th Baiting
11.15 hrs
All regulating bricks replaced after the fires have settled after baiting. Trials inspected.
11.50 hrs
Heat now being collected (built up).
12.30 hrs
Constant checks carried out from now on.
14.30 hrs Occasional baiting to prevent the chance of cold air entering. If cold air is admitted for ten minutes, the damage cannot be repaired in less than sixty minutes. 15.00 hrs
Trials drawn
15.10 hrs 13th Baiting Different fire mouths will require different amounts of coal in order to balance the heat in various parts of the kiln. Bone china trials withdrawn and ring with a tinkling sound when put down off the long rod used to withdraw them. 19.15 hrs
14th Baiting
More trials drawn. Soaking period in progress.
20.40 hrs
Another check
22.10 hrs yet.
Trials drawn, 2nd and 3rd quarters. Bottom trials still dull and 4th quarter not fully fired
22.50 hrs
Trials of 2nd and 3rd quarters better.
23.00 hrs 15th Baiting Three quarters of a baiting on five mouths in the 2nd and 3rd quarters, and about one third of this on the 1st and 4th quarters; this is to prevent the fires in the 2nd and 3rd quarters being pulled up the bags only, instead of being divided between the bags and the centre. When all the fires are baited the heat is drawn up evenly to the centre 23.35 hrs
All bricks closed up.
Day 4 02.00 hrs Bottom trials drawn; all much better and up to standard. Doors of the hovel shut tight and the kiln left to cool down naturally.
67 hours from kindling to closing up, and a kiln temperature of 1350 - 1400°C to give a temperature of about 1260°C inside the saggars and setters. The clammins are taken down in the evening of Day 7 and the contents drawn (withdrawn) on Day 8. On small manufactories which have only one biscuit oven, the ovens are drawn very much sooner and much hotter than this to shorten the delay.
From this very brief outline, the importance of the fireman can be realised. In 1866, when the potters refused to renew their engagements, the employers locked them out. After several weeks most of the potters had given in and the employers regarded themselves as victorious. Only one branch - the Union of Oven-men - refused to submit. Every other branch of potters might return to work but production was impossible without the firing process to complete it. The employers were powerless. Again, a careless or not entirely sober fireman could cause the whole contents of an oven or two to be spoiled which might mean the ruin of the enterprise. So, when admiring a piece of fine pottery, spare a grateful thought for the fireman.”
Extracts courtesy of the late Robert Copeland : From his book Manufacturing Processes of Tableware during the Eighteenth & Nineteenth Centuries. Published by Northern Ceramic Society 2009 ISBN 978-0-9563159-0-8
