There is a considerable amount of variation in the analysed dataset, both regarding the number of ground layers and their composition. In several paintings, layers were found consisting of only one pigment, while in other paintings 14 pigments were combined to form the ground layer. At most Steen used three ground layers, while there are also instances of using only one ground layer. PCA was used to explore the datasets in a multivariate way.
PCA of elemental and pigment data
PCA on the quantitative elemental data of the bottom ground layers roughly shows four clusters in the score plot of the first two PCs (see Fig. 2a). This initial clustering separates the canvas paintings from the panel paintings. All 15 panel paintings (cluster 3) contain a bottom ground layer made of chalk, a calcium carbonate, as indicated by the large loading of calcium (CaO) in this direction in the corresponding loading plot (see Fig. 2b). Apart from the panel paintings, three other clusters can be discerned in Fig. 2a: bottom ground layers (cluster 4, contains 4 paintings) consisting mainly of iron-containing earth pigments, as indicated by the loadings of silicon (SiO2), aluminium (Al2O3), iron (Fe2O3) and magnesium (MgO); bottom ground layers consisting mainly of lead white, a basic lead carbonate (cluster 1, with 6 paintings), as indicated by the large loading of lead (PbO) in this direction; and bottom ground layers consisting of a mix of lead white and chalk (cluster 2 with 8 paintings).
The binary pigment data was analysed with nonlinear PCA using optimal scaling. Since the input data consists of zeroes and ones, objects (paintings) and variables (pigments) may coincide in score and loading plots. Indeed, all paintings on panel with only chalk in the bottom ground layer coincide in Fig. 3a (indicated by the arrow). The first dimensions are dominated by the number of pigments present in the bottom ground layer, as shown in Fig. 3b. Since the presence of a pigment provides neither information on its (relative) amount, nor on its chemical composition, only limited information can be obtained for dating purposes. This also explains why the matrices with pigment and elemental data are only weakly correlated. For the first ground layer this correlation was found to be 0.43 using Indahl’s “similarity of matrices index” [20]. Nevertheless, the two data blocks can be fused after optimal scaling followed by block scaling and analysed by means of Consensus PCA. The multi-block data analysis results are illustrated in Fig. 4, where three paintings immediately pop up as exceptional in the PC3/PC4 score plot (see Fig. 4a). The Merry Family (Vh) is an outlier on PC3, because it has by far the highest concentration of copper (Cu2O) in the elemental dataset, and it is the only one with vivianite (vi), an iron phosphate, and copper-containing particles (ko) in the pigment dataset (see Fig. 4b). The Alchemist (Al) is an outlier on PC4 due to its highest concentration of phosphorus (P2O5), caused by the ample use of bone black, mainly composed of calcium phosphate and carbon, in this layer. The presence of green earth (ga), a complex silicate of Fe, Mg, Al and K (mainly minerals as celadonite and glauconite) is distinct as well, causing the high concentration of magnesium (MgO) and adding to the distinctiveness of this layer. The Oyster Meal (Om) is an extreme data point in the lower left corner of the score plot due to its highest concentration of aluminium (Al2O3) and titanium (TiO2), caused by the iron-containing earth pigments that make up this layer.
Panels
All paintings on panel contain two ground layers, of which the bottom layer consists of only chalk with the exception of Cardplayers in a Brothel (ca. 1663–65). In this painting the bottom ground layer contains some gypsum as well. In the PCA score plot this painting (Kb) is placed outside of the cluster of panel paintings, showing the abilities of this method to find outliers (see Fig. 2a). The second ground layers on panels do show variation. They are all light-coloured, varying in tone from cream to light yellow to light grey. PCA of the second ground layers of all paintings was done to further analyse the similarities between panel paintings.
A large cluster can be seen in the score plot for the second ground layer (see Fig. 5a). This group of panel paintings contains a second ground layer based on lead white, mixed with red and yellow earth, umber and chalk. The composition of this layer is indicated by the loadings of calcium (CaO) and lead (PbO) on either side of the cluster in the corresponding loading plot (Fig. 5b). The clustering of these paintings can be seen in the first, second, third and fourth dimension, meaning there’s little variance between the paintings. Only one painting dated by Steen himself (A Lady at her Toilet, Mt) falls in this cluster, dated in 1663 when Steen was working in Haarlem. A Twelfth Night Feast (Kd, ca. 1661), Lady Playing the Cittern (Sv, ca. 1662) and The Caudle Makers (Km, ca. 1665–70) are all dated in the Haarlem period and seem to fit in well with this clustering. On Lady Playing the Cittern, the second ground layer has a slightly different composition, it also contains red lead. With the elemental analysis done with SEM no distinction is made between the types of lead compound present and therefore this does not show up as an outlier in PCA. It’s worth noting that these panel paintings are all painted on standard-sized panels (see Table 1). It seems likely that standard-sized panels were sold to artists ready to go, with both ground layers already applied to them.
Two other paintings dated earlier are clustered with this group of Haarlem paintings based on the composition of their second ground layers. Girl Eating Oysters (Oe) is dated slightly earlier, around 1658–1660. Could it be that this painting needs to be dated later and it was painted in Haarlem?
Village Fair (Dk), dating 1650–1651 also falls in the cluster of panel paintings from Haarlem on PC1-4. However, this painting is probably not painted in Haarlem since it has an underdrawing which is closely related to Steen’s teacher in Den Haag, Jan van Goyen, and has all the characteristics of a painting from Jan Steen’s early career. Indeed, on PC5 (which explains 8.9% of the variation in the data) it is not in the cluster.
Apart from this large cluster, no further trends can be observed in Steen’s ground use for panel paintings. Several other paintings are clustered together or overlap in the score plot in the first two dimensions, but are placed far apart in dimensions 3–6. This is the case for The Quack (Kz) and Doctor’s Visit (Db), and Dancing Peasants at an Inn (Bh) and The Sick Girl (Zm). Perhaps a separate PCA of the data from the 15 panel paintings could reveal more similarities between the second ground layers found on Steen’s panels, although the precision of the data limits the usefulness of the results for small datasets.
Canvas
The Hague, Delft (1649–1656)
All the analysed canvas paintings made in The Hague and Delft show a similar composition of the ground layers. The ground layers on these paintings are light in colour and consist mainly of chalk, lead white with some umber and/or fine carbon black mixed in. Three of the four analysed paintings from this period contain only one ground layer. The fourth painting, The Tooth-Puller (Tt) from the Mauritshuis, is dated 1651 by the artist and contains three ground layers (see Fig. 6). Remarkably, these three ground layers all contain the same pigments, just mixed in different quantities. From X-radiographs it has become clear that the canvas for this painting was cut out of a larger part of ready-primed canvas: cusping, the wavy deformations of the weave that occur along the edges of a canvas when it is first stretched onto a strainer, can only be seen on one side of the canvas and it does not correspond to the format of the painting. It’s unclear whether Steen did this more often, so far, no other examples have been found. This ground layer seems typical for Den Haag and Delft. Similar light ground layers, mixtures of lead white and chalk tinted with umber, have been found in paintings by other Delft artists like Vermeer and Pieter de Hooch [21, 22].
Warmond (1656–1660), Leiden (1670–79)
From Steen’s brief period in Warmond, a village close to Leiden, only one canvas painting was analysed. This is Portrait of Jacoba Maria van Wassenaer (also known as ‘The Poultry Yard’, Hh). The canvas contains three ground layers, consisting mainly of lead white tinted with yellow and red earth pigments. X-radiographs show that the canvas was individually primed, since cusping was seen on all four sides of the painting. Several other paintings on canvas from Leiden, where Steen returned in 1670 and lived until his death in 1679, have been analysed. Some of these paintings are clustered with ‘The Poultry Yard’ based on the composition of the bottom ground layer. In Christ Expelling the Traders from the Temple (Cw), dated by Steen in 1675, one ground layer is present consisting mainly of lead white tinted with umber, earth pigments and bone black. Both The Mocking of Samson (Si) and ‘A Pig Belongs in the Sty’ (Wv) contain a comparable light-coloured ground layer, made predominantly of lead white tinted with some iron-containing earth pigments, umber and bone black. It seems this type of ground was common for Steen in Leiden. In the score plots for PCA on the bottom ground layers these paintings are placed close together, in the first 4 dimensions.
The Mocking of Samson is not lined in later conservation treatments, so the support provides detailed information on its preparation. The canvas is still stretched on its original strainer. The tacking margins are completely intact and show remnants of thread from when the canvas was laced into a larger strainer during painting. On the top and bottom of the canvas the selvedge was present. This all shows that this canvas was individually prepared. The composition of the ground layer, however, closely resembles other paintings from Leiden. This could indicate that the canvas was prepared individually, but not at the artist’s studio and was probably commercially available.
The large canvas painting A Village Revel (Hm) contains two ground layers, of which the bottom one consists of chalk and some red earth. The second, top ground layer is dark red in colour and contains predominantly red earth. The dark red ground layer plays a role in the composition, shimmering through thin paint layers in several areas. In other paintings with a medium or dark-coloured ground layer Steen obscured the darker ground by covering it with one or two more ground layers, for instance in Life of Man (c. 1665), where a light gray layer containing lead white, yellow earth, umber, fine black and chalk has been applied over the red ground. In The Alchemist (1668), a dark gray bottom ground layer was covered with lighter ground layers before painting. The composition of the second ground layer in A Village Revel with predominantly red earth seems to correspond with layers used more frequently in Haarlem, even though this painting is dated by Steen himself in 1673, his late period in Leiden. However, to have this darker red ground as a second/top ground layer, where it also plays a role in the composition, is exceptional in Steen’s oeuvre.
Haarlem (1660–1670)
Steen’s Haarlem period was the most productive period in his career. In this study 18 paintings from Haarlem were analysed. In Haarlem Steen started using strongly coloured ground layers. A cluster of four paintings containing such a ground can be seen when analyzing the bottom ground layers with PCA (see cluster 4, Fig. 2a). This cluster contains The Alchemist (Al), Interior of a Tavern with Card Players and a Violin Player (Hv), The Oyster Meal (Om) and The Merry Family (Vh). In three of these four paintings just one ground layer is used, consisting of mainly earth pigments and with a red colour. The Oyster Meal is dated 1660 by the artist, the year Steen moved from Warmond to Haarlem. The similarities in colour and composition of the ground layer with other Haarlem paintings suggest that this painting was made after the artist moved. The Merry Family is dated 1668 by Steen himself (see Fig. 7), making a date around 1668 likely for Interior of a Tavern with Card Players and a Violin Player as well.
The Alchemist, another painting dated by Steen in 1668, is clustered with the aforementioned paintings as well. The bottom ground layer has a similar elemental composition, but is very dark gray in colour. On top of this dark gray layer, two more layers were added, one being a warm beige colour and the top layer being lighter and cool gray. Apart from its unusual colour (this is the only dark gray ground layer encountered in this project), the bottom layer also contains some atypical pigments for ground layers, such as smalt and green earth. The same goes for the ground layer of The Merry Family, which contains smalt, vivianite, vermillion and lead–tin yellow next to earth pigments and chalk. Most likely both these layers were applied in the studio, using palette scrapings mixed with earths and chalk.
Since the same colour can be achieved by mixing different pigments in different ratios, not all coloured grounds are clustered using PCA. The Life of Man (Lm), a painting on canvas dating around 1665, contains a bright red bottom ground layer. However, since this layer contains more chalk than the other red grounds it is not clustered together. The same goes for The Marriage of Tobias and Sara (ca. 1667–68, Hu), from the collection of the Herzog Anton Ulrich Museum in Braunschweig. Here the red ground layer is a mixture of lead white, coloured with red earth, charcoal black and black shale (a type of black earth pigment consisting mainly of aluminum and silicon). Because of the higher quantity of lead present in this layer the painting is placed in cluster 1 instead of cluster 4 with the other coloured ground layers. Even though the paintings are not clustered together based on their elemental composition, dating them in Haarlem seems highly likely because of the red colour of the ground layer.
The cluster of paintings containing a bottom ground layer with only chalk also contains four paintings on canvas, besides the panel paintings: ‘As the Old Sing, So Pipe the Young’ (Zo), Merry-making in a Tavern with a Couple Dancing (Hd), The Village Revel (Mh) and The Sick Woman (Zv) These paintings vary in date, with two paintings dating around 1663–65, one around 1670 and one being dated by Steen himself in 1672. With PCA, Merry-making in a Tavern with a Couple Dancing (ca. 1670, Hd) and The Sick Woman (ca. 1663–65, Zv) are also clustered in a separate analysis of only the top ground layers. Both these paintings contain only one ground layer, consisting of chalk mixed with some earth pigments. It seems that these paintings are closely related in composition of the ground layer and should therefore be dated closer together. Based on the results of other paintings analysed from Steen’s Haarlem period it seems more likely that Merry-making in a Tavern with a Couple Dancing should be dated some years earlier and both paintings were made in Haarlem.
Ground layers applied in the studio
The other ‘As the old sing, so pipe the young’ (Ss) is a large painting on canvas stylistically dated ca. 1668–70, containing one ground layer made up of many pigments: large and fine lead white, lead tin yellow, umber, red earth, chalk, bone black, red lake, yellow lake, vermillion, smalt, feldspar and quartz (see Fig. 8). The ground has been applied with a palette knife and is irregular in thickness and colour. The original tacking margins of the painting have been preserved and the ground is present there as well, indicating that this canvas was individually prepared and that the current size is the original size. The unusual pigments found in this ground layer suggest that it was applied in the studio of the artist himself. But could the fact that Steen opted for a lighter, tinted ground suggest that this painting should be dated in Leiden instead of Haarlem? The ground layer shows more similarities (both in terms of colour and in terms of materials) with Leiden paintings than with Haarlem paintings, even though it has been applied by Steen himself.
As mentioned above, the bottom ground layers of The Alchemist and The Merry Family also contain unusual pigments. The addition of these pigments, such as smalt, green earth, vivianite, lead tin yellow and vermillion, indicate the use of palette scrapings and thus make it likely the grounds were applied in the studio. Even though Steen seems to have applied these ground layers himself, he does seem to follow local customs by using dark, strongly coloured ground layers consisting mainly of earth pigments in his Haarlem paintings and lighter grounds that are mixtures of lead white, chalk and earth pigments in his Leiden paintings [23].