Bisque firing
When goods were sufficiently dry, they were ready to be fired. The firing was done in circular kilns. These were made of refractory bricks and shaped like large bottles, releasing smoke from the top. The kilns were coal-fired. Each kiln had its own name. There was Samson (the largest), Møllbach (after an Eidsvold-representative from Egersund) and Harald (unknown origin). To make sure the items were not harmed by smoke, they were placed in saggars, which were crates made of refractory clay. The saggars were made at a designated saggar workshop, which employed six workers, all men.
Work with the kilns was divided into five different operations. First, the items were placed in saggars. Secondly, the saggars were stowed in the kiln. Then, the kiln was fired. After firing, the kiln was emptied. Lastly, the items were transported to be removed from the saggars, brushed clean from dust and sorted into three different qualities of goods.
Work with the kilns was done as piecework by work teams. In the bisque kilns, the kilns used for the first firing, 19 men worked under a leader who took orders from the factory manager. The work was divided so that each man had a dedicated task. The various tasks rotated within the work team, to avoid one person being left with the same assignment over a longer period.
Until the end of the 1920s, two workers were permanently tasked with stowing goods in the kilns. This was considered a specialized type of work. It was important to place the items correctly, to utilize the space efficiently and make sure items were fired at the correct temperature. Temperature could vary according to where items were placed in the kiln. Each kiln had its distinctive characteristics.
When the two placers grew older, they asked to be transferred to easier assignments. Instead of just teaching two younger men the art of stowing, the young workers on the team all agreed on wanting to learn. This meant they could share the heavy work.
Items were placed at the bottom of saggars. The walls of the saggars were glued to the bottom using a paste of refractory clay. The saggars had to be completely sealed to prevent smoke from seeping in during firing. The saggars were shaped based on the objects they were meant to contain. The different types of saggars had different names. The names were the same as those used in English potteries. There were names such as Oval, Round, Shilling, Six Pence and Eights.
A work team stowing items in a coal-fired kiln, cirka 1946
During stowing, the two placers worked inside the kiln. The rest of the team did various tasks outside - transporting objects, placing objects in saggars, and bringing saggars to the workers in the kiln. The placers put the first saggars at the back of the stove, then kept stacking them on top of each other, and towards the mouth of the kiln. As the saggars were placed on top of each other, they served as lids. The placers used ladders to keep on stacking saggars all the way to the top of the kiln. As the kiln started to fill up, the placers stacked saggars downwards, towards the opening. This made it possible to stack items all the way to the top, at the neck of the oven. The kiln could encompass as much as 40 000 to 60 000 items, depending on size. When the kiln was fully stacked, it was sealed with refractory bricks and clay. Then, ten tons of coal was transported to the oven, to be used as fuel. The coal was placed in heaps around the oven, by each of its firemouths. From then on, a dedicated worker was tasked with the firing.
Firing was one of the most specialized assignments at the factory. Four men were employed to monitor firings - two working with the bisque kilns and two with the glaze kilns. They needed in-depth knowledge of how temperature would distribute in the kiln as the firing progressed. A small mistake could lead to an uneven firing and destroyed goods.
The kilns were built with several canals in the walls and ten firemouths around the bottom periphery. It was necessary to have this many firemouths to keep an even temperature in the kiln during firing. The kiln was sealed off at the top, except for small holes to let out smoke.
When the kiln was fired, flames rose towards the top. When they hit the curved roof, the flames were pushed down towards the bottom again and out towards the walls, which had several canals leading the heat up towards the roof again and through the chimney. This is a counterpressure principle where the flames are forced to take a longer path than they would normally do. The smoke holes were small, emitting little heat. Because of this, the flames were forced back towards the bottom and out into the side canals. The kilns had to be built based on this principle to ensure that items in the middle of the kiln were fired at a high enough temperature. There were small holes along the walls of the kiln, to control how the firing was progressing. The supervisor could climb a ladder to study the flames. Judging on the colour and shape of the flames he could decide if the firing was progressing correctly. Pyrometric cones were used to control temperature. The cones were approximately ten cm long and placed in groups of three. They stood upright, points upwards. The cones were made of a special kind of clay that would start to bend over at different temperatures. The first cone would bend at 1000 degrees Celsius, the second at approximately 1100 degrees Celsius, and the last at approximately 1200 degrees Celsius. When the last cone started to bend over, the firing was finished.
The four men who fired the kilns were the only ones who worked shifts at the factory. Each had an assistant who lit the kiln for him. For the first 12 hours, the kiln burned at low heat. This was done to remove any residual moisture from the items inside. After this, the dedicated “burner” took over. He heated the oven to 1200 degrees Celsius. During firing, he worked alone. It usually took 36 hours from the burner taking over the firing to the firing being done. After a shift like this, the other burner took over for the next firing. The two men worked shifts on the kilns.
To become a «burner», an apprentice worked alongside an experienced workman to master the technique of spreading the heat evenly towards the top. The burner also had to make sure that the kiln was evenly warm.
When the firing was done, the kiln had to cool down for three days. It was important to avoid emptying the kiln too early. If opened early, the goods inside could crack as a result of the change in temperature.
Saggars were extracted from the kiln in reverse order of the stowing.