Laboratory

Before production starts each separate paint has a unique formula developed for it in the laboratory. During this development stage the paint's quality aspects are comprehensively tested. The laboratory is also responsible for inspecting the raw materials before they go into the product and monitoring the quality during the production process. In addition to the above activities, particular attention is also given to research into new raw materials and paint systems.

Weighing the raw materials

The central warehouse stores all the dry raw materials. This includes pigments and extenders, as well as binders such as dextrine and Gum arabic which is later made liquid through dilution. At the start of the production process the dry raw materials are weighed here accurately to the gram and prepared for transport to the next department, the pre-mixing department. The pre-mixing department stores the liquid binders and additives. The correct amount is tapped off, after which the weighed dry raw materials are added. As small amounts of dry raw materials are left behind in the sacks during this process, the paint has to be checked at each subsequent production step against standards for the relevant product according to colour and viscosity, and is adjusted if necessary.

Dispersion

Pigments are insoluble in the binder. They therefore have to be regularly dispersed in the binder, that is to say finely distributed. This dispersal occurs in two steps: first through pre-mixing with a mixer and then the final fineness is determined with a triple roll mill.

Pre-mixing

The pigments are supplied in dry powder form, with the pigment components held together in clusters. During the pre-mixing these are turned to smaller clusters and homogenously distributed throughout the binder. The tinting strength of the paint is now relatively low, but this will increase as the pigment is ground finer in the next production phase.

Grinding the pigments

In the triple roll mill the pigment clusters are ground further to the desired fineness. During this process the pigment particles must not get damaged as this may result in a change of colour. The three rollers rotate in opposite directions with different speeds. The first roller turns relatively slowly, the second 3 times as fast and the third 9 or 12 times as fast, depending on the type of mill. The difference in speed helps to grind the pigment lumps and the paint goes from one roller to the other. The paint is finally removed using a scraper.

Fineness inspection

The distance between the rollers determines the fineness of the grind. This process is run through for a maximum of five times. The finer the grind, the greater the tinting strength of the paint. The rollers are hollow and are cooled on the inside with water, as the pigments would otherwise burn through the heat of the friction. Depending on the nature of the pigment and the purpose of the paint or ink, the pigments are ground to between 2 and 55 microns (1 micron = 1/1000 mm). After the last grinding the paint finally has the required viscosity.

After the grinding the fineness is checked using the Hegman bar. The depth of the groove in the bar gradually increases, indicated in microns. The paint is thinned to a certain viscosity and then applied to the groove and levelled off. At the point where the pigment particles are larger than the thickness of the paint layer they stick up above the surface of the paint film. At this point, visible to the human eye under skimming light, the fineness is read off.

Colour inspection

The new colour is now compared against the standard colour. Both are applied thick as well as thin so that both the mass tone and the undertone of the colour can be seen. There must not be a difference between the new colour and the standard colour.

Stability inspection

In order to ensure that only quality products are sold, each batch is left to stand for a certain time in order to check the stability. After this period another laboratory check is carried out, and after approval the paint is filled into containers.

Filling

Tubes are filled from the underside, with the top already in place. Plastic tubes are closed by melting the material, while the aluminium tubes are folded step-by-step and squeezed shut. Bottles are filled, closed and then labelled. The final step is to check that the packaging contains the right amount of paint.

Rembrandt soft pastels

Rembrandt Soft pastels are made on the basis of the finest kaolin, also known as china clay or pipe clay. In contrast to the binders of the paints, kaolin does not help in the adherence to the ground but holds the pigment in the pastel together. The ratio of pigment-kaolin helps to determine the consistency of the pastels.

Mixing the ingredients

Using a mixer the dry raw materials are mixed with water to a semi-wet mass from which the pastels can be pressed. This mixing should be carried out with the greatest of care. If the mixing is done for too long, too short, too quickly or insufficiently, then the final colour will be different from the standard colour, or hard pieces may get into the pastel which may cause scratches in the painting. As the water absorption for each batch cannot be predicted accurately, and because the right amount of moisture is crucial, the pastel maker determines the consistency of the mass for each colour by hand. It is only after years of experience that such a skill is perfected.

Colour inspection

In order to determine whether the colour is good, the pastel maker produces a pastel by hand. This is dried (the colour of the wet mass is not the final colour) and applied on paper alongside the standard colour. Even with the slightest colour deviation the pastel ingredients would have to be adjusted.

Drying and labelling

Once the pastels have been made from the ingredients, they are dried and labelled. The label acts as protection during transport and provides information on the colour, the pigment used and the lightfastness.