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How was a glost bottle oven fired? Courtesy of British Pottery Research Assoc.

Here is a really informative article into the skills that went into firing the bottle ovens.

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TECHNICAL PAPER No. 45
Courtesy British Pottery Research Association, Penkhull, Stoke-on-Trent
Volume IV – 1941 – Extracts 

For definitions of unusual words used in the text refer to the Potbank Dictionary here> 

THE FIRING AND PERFORMANCE OF AN UPDRAUGHT OVEN FOR GLOST EARTHENWARE

By R. J. WALLER, M.Sc.Tech.

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 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 – I90 doz.
  • Saucers and sweets – 174 doz.
  • Cups – 172 doz.
  • Trays, sundries, etc. – 145 doz.
  • Total 1192 doz.

The total weight of the setting, when fired, was 28.78 tons.

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 hrs. 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 hrs. 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 hrs. 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 :

  • First Ring, Middle – 20 to 23 points
  • First Ring, Bottom – 15 to 18 points
  • Third Ring, Bottom – 10 to 11 points

22, 17 and 11 being considered by the management to be ideal.

These were found to correspond to finishing temperatures of 1115° 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 clammings knocked in.

On the next day the ashes were punched out at 5.30 am, the remainder of the clammings 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 consumptions in tests E and F were 10 tons 15 cwt. and 10 tons l7 cwt. respectively.

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