On the trail of the world's most beautiful blue colour

Soon they became a team, and were given permission to collect some small samples that the miners had left behind in the 1860s. These are the starting point for a complete reconstruction of dye production, to once again create the world's most beautiful blue colour.

For the weekend of 23-25 August 2024, a calciner furnace was built at Nordgruvene. The furnace was designed to be able to withstand high temperatures, but also to capture toxins that would leave the samples at 615 degrees centigrade. At that point, the arsenic would sneak out of the stone in the form of white smoke, with a faint odour of garlic. This went out through a nine-metre-long pipe, where the arsenic soot got stuck in a system that could be hermetically sealed.

The original team of Thomas Iversen and Halvor Skiftun Digernes was both strengthened and weakened at the moment, as the latter had fallen ill. The entire process is therefore being assisted by Ane Fabricius, a Danish ceramicist with a particular interest in cobalt blue.

The kiln was created from some roughly sorted stones from the mining process, which were placed in a circle. Inside the kiln, there was room for a boiler in which cylinders of stone treated in different ways could be placed. The kiln was bricked up using natural clay, interspersed with some straw. When the kiln was erected, it was raining heavily, so construction had to take place under a patchwork of tarpaulins. One challenge was whether the oven would dry sufficiently during the night.

The boiler was filled with cylinders of different types. Thomas Iversen had prepared different grades, while Halvor Skiftun Digernes had prepared the cylinders in ceramic. The amount of each gradation added was documented at gram level. An indicator of the change in the chemical composition of the ore after calcination will be weight.

The 15 cylinders in the furnace were placed and the lid of the furnace screwed on. There was a pipe from the lid, which was connected to the long chimney to ensure that any release of toxins did not affect the people working here or the surrounding nature. The boiler was placed in the furnace, and dry birch wood was placed around it on all sides. Soon, pine longwood was added to the stove to increase the temperature.

Using laser measurement, Thoms constantly monitored the temperature development. After a few hours of firing, the oven reached a core temperature of over 650 degrees. This was maintained for several hours. At 18:00, coal was added to the furnace to ensure an even higher temperature would be developed before the furnace, boiler and samples would slowly cool down overnight.

On Sunday 25 August at 11:00, the oven is still lukewarm. Dressed in full protective suits, Thomas and Ane first pack up the pipe system and ensure that it is hermetically sealed. The kettle is then lifted out of the oven. A glance down into the oven reveals that the temperatures have been high, there are hardly any soot residues, and these can only be found on the outside where the fire has passed through.

The boiler is placed on a solid base. Unfortunately, the screw mechanisms holding the lid in place have partially melted, so it was a major challenge to open the lid. Hand power had to be used, but still without shaking and damaging the containers inside the boiler. Millimetre after millimetre of the lid eventually gave way. Soon the lid was open.

For the first time since the 1800s, cobalt was calcined in Modum, but what did this look like? A visual examination of the material from before it was calcined to after shows a clear change in colour, to a warm yellow-brown colour, replacing the stone-grey. All the samples proved to be thoroughly heated, and the colour within the sample was completely uniform in all of them. It was therefore safe to say that the calcination must have been successful, according to the plan.

But then there was a small surprise. The change in weight was not as expected and could not be read with any accuracy. In order to be able to prove that the calcination had been done correctly, in relation to the 19th century tradition, a chemical analysis must be done. This will take place as soon as possible, and will be the next step before the colour from Modum can once again appear on ceramics and porcelain.

- Sverre Følstad