Bottle Ovens

From the 18th century until the 1960s, bottle ovens were the dominant feature of the Staffordshire Potteries. There were over two thousand of them and they could be seen everywhere you looked. Some small factories had only one bottle oven, while other large potbanks had as many as 25.

Within a factory, ovens were not situated according to any set plan. They might be grouped around a cobbled yard or placed in a row. Sometimes they were built into the workshops with the upper part of the chimney protruding through the roof.

No two bottle ovens were exactly alike. They were all built according to the whim of the builder or of the potbank owner.

The outer part, which is bottle shaped is known as the hovel. A hovel can be up to seventy feet tall. The hovel acts as a chimney; taking away the smoke, creating draught and protecting the oven inside from the weather and uneven draughts.

The inner part is the kiln proper. It is a round structure with a domed roof, the crown, and its wall are approximately one foot thick. Iron bands known as bonts, set about twelve inches apart, run right round the circular oven to strengthen it as it expands and contracts during the firing. A doorway, the clammins or wicket, surrounded by a stout iron frame and just large enough for a man with a saggar on his head to pass through , is built into the kiln. A saggar is a fireclay box for holding ware during firing.

Around the base of the oven are firemouths - the exact number depends on the size of the oven, - in which fires are lit for the firing. Inside the oven directly above each firemouths is a bag. This is a small firebrick chimney, the purpose of which is to direct the flames from the fires below into the oven and protect the saggars nearby.

Underneath the floor of the kiln, flues, which lead from each firemouth, distribute heat throughout the interior.

In the centre of the kiln floor is the well hole over which saggars, with their bottoms knocked out, are placed. This forms a chimney to allow the smoke to escape. This is the pipe bung.

Placing an oven 

The ware to be fired was first arranged in fireclay boxes called saggars which were then placed inside the oven. The saggars were arranged in vertical stacks called bungs which extended from the floor of the oven to the ceiling.

When they were full, the saggars were carried into the oven by placers, who balanced them on their shoulders and heads. The weight of a full saggar was approximately half a hundred weight. To protect their heads and to keep the saggars in place, the placers wore rolls made from old stockings which were wedged into the front of each man's cap.

Ware was arranged differently in the saggars, according to whether it was biscuitware (all pottery after its first firing) or glostware (ware in the process of being glazed).

Biscuitware

For holloware (for example, cups bowls, jugs and vases), the bottom of each saggar was covered with a thin layer of powdered flint or silica sand, and as many pieces as possible were then packed in.

Flatware ( for example, dishes, plates and saucers) was bedded. A layer of ware was placed on top of a layer of silica sand or powdered flint and was covered with another layer of sand and flint. on top of this was placed yet another layer of ware which in its turn was covered with a layer of sand or flint. This continued until the saggar had been filled with alternate layers of sand or flint and ware. The purpose of this method was to reduce warping.

Glostware

It was necessary to separate all the pieces so that during the firing when the glaze melted, they would not fuse together. To separate the ware in the saggars, different sorts of kiln furniture were used. This included pins, saddles, thimbles and spurs. Inside the oven, the saggars were stacked from floor to ceiling, in bungs. Starting at the rear of the oven, the bungs were set-in from the walls towards the centre. The placers gradually worked their way around the ovens towards the clammins (door) and in the last section the bungs were set from the centre towards the door until the oven was full. Because the floor of the oven was sloping, the heavy bungs had to be wedged with pieces of broken saggar, to enable them to stand upright. Because the temperatures in the oven varied widely, it was necessary to place the different types of ware in carefully selected locations.

"Terribly skilled jobs, you see. If the smoke got in it, it was all up with the ware inside, you see. Sulphur and all sorts of stuff out of the coal would disfigure that"

Firing a bottle oven

On average, the bottle ovens were fired once a week. A biscuit (first) firing took three days and a glost (second) firing took two days. It needed about fifteen tons of coal to fire one bottle oven once, and almost half the heat generated would go up the bottle shaped chimney as smoke. The smoke, emerging sixty feet up, would eddy and curl down onto the buildings and street, even entering workshops and houses through ill-fitting windows and half open doors, so that the air became terribly polluted.

In Longton the town with the greatest number of bottle ovens, it used to be said, "It's a fine day if you can see the other side of the road", and when the bottle ovens were firing it was almost impossible to see your hand held in front of your face.

After placing the clammins (the entrance to the oven) was blocked up with bricks and sand and the oven was then ready to fire. Fires were let in the firemouths and baited - that is, coal was loaded onto the fires - at intervals of about four hours. In the early stages of firing the temperature was kept low while the moisture in the ware was driven out. This was known as smoking. After about 48 hours, the maximum temperature (between 1000 C and 1250 C) was achieved and this was maintained for approximately two to three hours. The fires were then left to go out.

Fine control of the draught was achieved by altering the position of the dampers in the crown. Dampers are flaps made from iron and firebrick, which the fireman could operate from ground level by means of a pulley system. By opening selected dampers, the draught in different sections of the oven could be increased, causing the fires to burn more fiercely and raising the temperature. By closing the dampers the temperature could be kept steady or lowered. When the firing was over, the clammins was broken down and the oven left to cool. As soon as it was sufficiently cool for a man to enter without being harmed by the heat, the oven was emptied or drawn.

There is evidence that men often had to enter ovens which were too hot, because the factory owner needed the ware urgently. In such cases, to protect themselves to some extent, the men wore wet rags over their hands and faces.

"You'd let your fire out on your ovens and you were supposed to wait forty eight hours until it'd go cool - ours used to be opened after twenty four hours and it would still be red hot inside it. Then men would climb on inside and they used to have five overcoats on and about three jackets wrapped around their wrists, and he'd have to lift the saggars down with his padded arms".

Types of bottle oven

There are four main types:

• Updraught ovens;
• Downdraught ovens;
• Muffle kilns;
• Calcining kilns.

The Updraught oven

This is the basic type of oven. It consists of an inner chamber with a domed roof - in which the ware is placed- enclosed in a hovel. Flues and bags lead from the firemouths to the centre of the oven, and conduct heat to the wares inside. The heat rises up through the contents of the oven (the setting) and out through the top. This type of oven is used for firing both biscuit and glost ware.

The Downdraught Oven

This type of oven which uses heat more efficiently than the updraught oven was developed in the early 20th century. It is similar in shape to the updraught oven, but the draught is controlled in such a way that the heat first rises and then is forced downwards again through the setting and out through holes in the base of the oven. The hot smoke is then sucked up through a straight chimney nearby which serves one or more neighbouring ovens. This type of oven is used for both biscuit and glost ware.

The Muffle or Enamel Kiln

A muffle kiln is very much smaller than the other types of oven and is used to fire decorated ware. Decorated wares are fired in order to make their colours permanent as, without firing, these can be washed or rubbed off. A muffle kiln does not need a temperature as high as that used for biscuit or glost ware. The flames do not enter the firing chamber, instead the oven is heated by means of encircling flues. In this way the delicate colours are protected.

The Calcining Kiln

There is no particular reason a calcining kiln should be bottle shaped, since it is used not to fire pottery, but to prepare the flint and animal bones which are added to clay to make up the material from which pottery is made. This material is called the body. Before the flint or animal bones can be added to the body, they have to be crushed to a fine powder. If they are burnt or calcined, they become brittle and can be powdered with ease.