7 TECHNICAL EXAMINATION OF THE PIGMENTS

The initials “W. D.,” in pencil, had been written on the labels of all the paint tubes in the paintbox of Gerrit Willem Dijsselhof. On one of the tubes of Terre de sienne was written “Mei 21” (May 21). Because the tubes were still full and in good condition, it seems likely that they were purchased by Dijsselhof toward the end of his life. It is tempting to attribute the tubes of Van Borssum Buisman to the same years due to the similarity of design and printing on the label.

Samples of the paint were prepared for analysis of the inorganic pigments using light microscopy and energy dispersive x-ray spectroscopy (EDX). Samples were taken by Aviva Burnstock at the Courtauld Institute of Art in London in January 1996. Particle characterization of selected samples was carried out using secondary electron imaging with a scanning electron microscope (SEM).

The results of the analysis are presented in table 1. The following discussion summarizes points of interest from the analyses of the paints.

TABLE 1 RESULTS OF SEM-EDX RESEARCH ON CLAUS & FRITZ COLORS

7.1 ADDITIONS AND EXTENDERS

Calcium carbonate (probably precipitated chalk) is the most extensively used addition to the tube paints in this series. It is added to the black pigments, all the paints based on iron oxide pigments and greens including terre vert, sap groen, cobalt groen (nos. 1 and 3), and ultramarin guimet (extra).

The proportion of chalk added varies, for example, brun d'mars contains a relatively small amount of chalk, while oranje d'mars contains a relatively high proportion. In general, the lighter shades of earth colors contain proportionally more chalk than the darker colors. This finding may suggest that chalk functions not only as an extender but is added to lighten the color of the paint. Another possibility is that more chalk was added to the light colors because it would be less noticeable than if added to the dark colors.

Barium sulphate, found in some of the oker colors (in addition to chalk) fulfills a similar role. Optically, particles are characterized by large, translucent glassy shards that have low birefringence in polarized light and an intermediate refractive index. On this basis they were identified as barite, the naturally occurring form of barium sulphate (Feller 1986). Paints that include barite without chalk are cobalt groen no. 3 and smaragd groen. Barite was used as an extender for paints from the last decade of the 18th century. Because it has low oil absorption, addition of barite to pigments that are highly oil absorbing may reduce the total oil content of the paint formulation. Drying of the paint is enhanced both by increasing the pigment volume concentration and by providing a relatively large particle surface area.

More transparent colors contain neither chalk nor barite. These include the lake pigments carmin fix de garance and donkere gekristalliseerde kraplak and the more transparent earth colors violet d'mars and paarsche dodekop. Other paints without extenders are cobalt (blue), the three white pigments—zincwit (zinc oxide), loodwit (lead carbonate), and cremswit (a mixture of lead carbonate and zinc oxide), and the series of colors based on cadmium.

Silica in the form of quartz has been added to half gebrande geel oker.

Aluminum is added, probably in the form of aluminum hydroxide (Al(OH)3), to all pigments except the white pigments, cadmium rood, vert veronese, and cobalt groen no. 3.

7.2 PIGMENTS BASED ON IRON OXIDE

The range of oil colors offered by Claus & Fritz include paints made using a variety of grades of natural and artificial pigments based on oxides of iron. In general, the paints made using natural iron oxides (including the range of okers, siennes, and ombers, and paarsche dodekop) contain coarsely ground particles of variable size and shade, while the artificial iron oxide pigments used for the mars colors contain submicron-sized particles of a uniform color.

Colors based on natural earth pigments can be grouped according to proportions of trace elements revealed in semiquantitative EDX analysis: Okers contain a relatively high proportion of silica, trace amounts of potassium, and occasionally titanium, and terre de siennes contain similarly small proportions of potassium and silicon. Ombers are distinguished by the presence of manganese, and most of the mars colors contain trace amounts of silica without potassium. Terre vert was also identified as containing the characteristic silicates of aluminum, magnesium, potassium, and calcium, used together with other pigments to make a range of green colors (discussed in more detail below).

The range of shades was achieved in some cases by mixing two grades of earth color. For example, the medium brown shade of bruin oker contains a mixture of brown and yellow particles, and roode oker is made with a proportion of finely ground red (similar to the mars pigments) with coarser brown earth pigment.

A variety of shades of the same combination of pigments has been achieved by adding different proportions of the various extenders. For example, geel oker, the palest shade of iron oxide yellow, contains additions of both chalk and barite, the darker shade gebrande geel oker contains chalk, with only traces of barium and sulphur, while the middle color (half gebrande geel oker) contains chalk and silica. Similarly, omber contains chalk and barite, while the darker gebrande omber is extended with a little chalk only.

Zinc white has been added to two of the iron oxide colors, omber and paarsche dodekop, presumably to modify the shade. While omber contains both extenders as well as a significant amount of zinc white, the more transparent paarsche dodekop is made from an iron oxide pigment similar in color to that found in violet d'mars, with little zinc white and no other extenders.

7.3 PIGMENT MIXTURES

A few paints were made using combinations of pigments with different qualities in order to extend the range. Engelsch rood contains a combination of an artificial red iron oxide with a red lake pigment to give a red paint with transparency between a pure lake and the relatively opaque red earth colors. Chineesche vermillioen contains a proportion of cadmium red in addition to red mercuric sulphide, perhaps designed by the manufacturer to create an attractive darker shade of vermilion that could be marketed in addition to a more orange shade of pure artificial red mercuric sulphide.

All the green paints contain mixtures of pigments. Terre vert contains a small proportion of viridian, and the darker shade, gebrande terre vert, contains umber. Sap groen contains a mixture of terre vert (green earth), bone black, and yellow ochre. Smaragd groen contains viridian modified with some red earth pigment, while cobalt groen no. 3 is differentiated from cobalt groen no. 1 by the addition of viridian to the main component, cobalt green, a combination of cobalt and zinc oxides (CoO.ZnO).

The blue paint, ultramarin guimet (extra) contains not only French ultramarine but additions of cobalt blue and red iron oxide.

7.4 PIGMENTS BASED ON CADMIUM, CHROMIUM, COPPER, COBALT, AND ANTIMONY

In addition to the iron-containing yellow pigments described above, the Claus & Fritz range included six pigments based on cadmium ranging from pale yellow (citroen cadmium) to red (cadmium rood). Shades of yellow are made from cadmium sulphide with varying particle size and hue. Cadmium rood contains a selenium in addition to cadmium and sulphur and, unlike the yellow paints, contains no added aluminum.

Pigments based on chromium include smaragd groen (chromium oxide viridian, discussed above) and a barium chromate yellow, paryt geel.

Vert Veronese was found to consist of emerald green (copper aceto-arsenite) without added aluminum or extenders.

Donker Napels geel contains lead and antimony, identified as Naples yellow and lead antimonate. The blue paint cobalt was found to be cobalt aluminate, and cobalt groen (nos. 1 and 3) consists of cobalt green, made by calcination of a cobalt salt with zinc oxide (McCrone et al. 1979, entry 935).

7.5 LAKE PIGMENTS

Carmin fix de garance was identified as carmine lake. Based on the identification of the combination of elements calcium, aluminum, and sulfur, the lake base was deduced to be aluminum and calcium hydrous oxide (Ca.Al2O3. H2O.CaSO4) as described by McCrone et al. (1979, entry 931).

Donkere gekristalliseerde kraplak contains a pink lake pigment that produced very low counts in elemental analysis. The proportions of the peaks suggest that the lake base is similar to that used for carmin fix de garance. This pigment was mixed with a small proportion of a red iron oxide pigment. Engelsch rood (discussed previously) is a mixture of a red lake with red iron oxide.

7.6 BLACK PIGMENTS

Two black pigments were included in the Claus & Fritz range. One was a carbon black, wijngaard zwart, which contain finely ground aggregates of carbon with a proportion of a black earth pigment. Ivoor zwart is brownish and contains calcium phosphate, bone black.

7.7 ANOMALOUS TRACE ELEMENTS: GERMANIUM, TIN, AND COPPER

Vleesch oker, paarsche dodekop, cadmium, lichte cadmium no. 1, and cobalt contain traces of the element germanium. Gebrande omber, oranje d'mars, jaune d'mars, paarsche dodekop, donkere gekristalliseerde kraplak, and ivoor zwart contain traces of copper, and several other colors contained traces of tin. Analysis of the metal tubes suggest that they are predominantly tin or (larger tubes) tin with lead. It is possible that traces of copper and possibly germanium may be associated with the metal tubes, although there may be a relationship between trace elements and the pigments or the process of manufacture of certain paints, which was not examined in detail.