The Elemental Message from the Land of the Wallaby

To get into the Christmas spirit Portable Spectral Services (PSS) scanned a vintage Australian card depicting a native wallaby with the inscription “A Message from the Land of the Wallaby” (Image 1a). The Wallaby, standing on a patch of grass is framed in an ovoid surrounded by eight “X”s. When examining the inside of the card, it is clear the wallaby, crosses and matching message have been embossed and have likely been painted over on the embossed areas afterwards (Image 1b). The embossing creates a three-dimensional effect, where there is particular emphasis on the wallaby as a result. The embossed features of the card are painted.

The card is not dated, however there are clues to suggest it can be considered ‘vintage’. Firstly, the style of card is uncommon when compared to contemporary Christmas card styles available. There is the main card (Image 1d), and a separate removable message card that is attached by some string by the hole punched area (see image 1c for reference).  It also utilises embossing, whilst not uncommon for contemporary Christmas cards, it is uncommon for everything to be painted and not printed. Secondly, the paper has yellowed in certain areas. Areas such as the margins demonstrate the age as it has yellowed more rapidly than other areas such as inside the card. Such an effect takes place over significant time and would not occur quickly in a contemporary Christmas card. Thirdly, the elemental analysis completed by the micro-XRF suggests paints which are no longer in production were used. This was determined by the detection of high abundances of lead.

Using the parameters shown in table 1, the entire card was analysed using the micro-XRF (see microXRF.com.au). Including the painted embossed regions to identify the elemental composition of paint pigments and to map the compositional variation associated with specific colours.

Image 1: (a) Front of Christmas card, featuring a wallaby “a Message from the Land of the Wallaby”, (b) detail of inside of card featuring embossed page “To dear Millie and Will with best love from mum and dad”, (c) detail of inside of card featuring main card and removable message card. (d) detail of the inside of the removable message card “My loving wish, to greet you this happy Christmas day. May all good fortune meet you upon your onward way; And be what may the weather, grey sky above or blue, though we are not together, my thoughts are still with you.”

          Table 1: Scan parameters applied to acquire the elemental maps.

The chemistry and techniques used in producing paint pigments have varied greatly over the centuries. A micro-XRF instrument was used to scan these Christmas cards to identify the elemental composition of paint pigments and to map the compositional variation associated with specific colours. Additionally, we are curious as to whether the paint pigments identified in the Christmas card have the same compositions as those used in modern times. The entire mapped area (Fig. 1) of the card was analysed at resolution of 100 µm (approximately the thickness of a strand of hair), with the elemental composition of the card verified and mapped. Seven main pigment zones were identified in the card and the elements associated with these zones are outlined in Table 2.

Figure 1: Mapped area of card scanned with components from table 1.

Table 2: Confirmed elements associated to each component of the front of the Christmas card.

 

Kaolin clay is commonly used to fill and coat paper. The clay is obtained via mining crude kaolin which will provide a unique combination of minerals such as, muscovite, quartz, feldspar and anatase (6). This range in mineral composition will lead to desirable or undesirable properties for paper filling and coating. Looking at table 2 and the element maps (Fig. 2), the presence of aluminium, potassium, silica, titanium, and iron can be confirmed in the paper. These confirmed elements are the main constituents of the minerals found in kaolin as listed earlier, muscovite (Al), quartz (Si), feldspar (K).  The main impurities of Kaolin come from ferruginous or titaniferous minerals (5).  Anatase, a titanium oxide effects the brightness of the paper, along with crude kaolin, which will stain paper yellow from the fine ferruginous particles.

                                           Figure 2: Elemental maps of silica (Si), aluminium (Al), potassium (K), sulfer (S) and combined map overlay.

Towards the end of the 19th century and throughout the 20th century the preferred method of producing wood pulp for use in the production of paper was using sulfur dioxide (7). A solution using a base, typically magnesium or calcium with sulfur dioxide would be used to treat wood chips, producing a wood pulp (7). The concentrations of calcium seen in the element distribution maps (Fig. 4) indicate either the use of a calcium base during the processing of the wood chips, or the appearance of anorthite or smectite in the mineral assemblage of the kaolin clay pigments.

Figure 3 shows the overlapping of zinc with the light brown of the wallaby where a white pigment has been used. Titanium white and lead white have both been ruled out through spectral analysis as the key elemental components of these pigments (titanium and lead) were not apparent in these areas, confirming the use of zinc oxide.  As zinc oxide has been used as a white pigment since the 1840s, it is likely these lighter shades are made by the blending this with a brown paint (2).

Figure 3: Zinc (Zn) oxide bending the lighter tones of the wallaby. 

Figure 4: Elemental maps of calcium (Ca), iron (Fe), chlorine (Cl), zinc (Zn).

The wallaby exhibits few distinct changes in colours, alternating between light and dark shades of brown, fading to what appears to be a grey undercoat. Figure 4 shows the iron, calcium, zinc and chlorine element distribution maps of the wallaby, allowing interpretation of pigment composition. Chlorine is seen to be in high concentrations around the zinc and calcium, indicating its relationship with the lighter colouring (Fig. 3). The Fe seen throughout the wallaby is indicative of an iron oxide as a main component of the brown pigment (Fig. 4). Calcium and chlorine are seen to be associated with a grey pigment, seen on the right arm of the wallaby. This grey paint looks to have been used as a base layer for the wallaby with a chlorine and calcium-based pigment to produce the colour (Fig. 4).

 

 

Figure 5 features the element maps that are associated with the metallic paint, seen in the text, the “X”s and the outline of the wallaby. The resulting maps (Fig. 5) confirmed that barium, chromium, copper, zinc, and lead were associated to the same pigments based on their overlapping distributions.

Figure 5: Elemental maps of chromium (Cr), lead (Pb), copper (Cu), barium (Ba), zinc (Zn) and combined map overlay.

The zinc element map shows what happens when you have a large contrast in distribution. Element maps show the distribution of an element, so the intensity of the colour does not reflect overall concentration within a sample. Figure 6 shows that object 1 shows more prominently as this is the zinc used in zinc white (Fig. 3), which the composition would predominantly zinc. Although object 2 shows copper and zinc displaying the highest concentrations in relation to the other elements seen in figure 5, which is seen in the spectrum for the grass, wallaby outlines and the component of the “X”s. It is likely the metallic paint was formed with a pigment of copper-zinc powder, which is referred to as a gold powder (4). 

                                                                      Figure 6: Object 1 & 2 element spectrum on zinc (Zn) element map.

Figure 7: Elemental maps of chromium (Cr), strontium (Sr) and arsenic (As)..

All the green paints used on the card were in the patch of grass where the wallaby stands (Fig. 7). The pigment used to form these greens have been confirmed as chromium, arsenic and strontium. The arsenic is in the highest concentration, chromium and strontium would have been added to the pigment to create a lighter green.

Lead is well known to be an element present in paints used in Australia up until the mid-1970’s. Prior to this, red, yellows, and oranges along with derivatives such as some greens had high lead-chromate content (1). Figure 8 shows the lead and chromium overlap in the blades of the grass, the wallaby outlines and the component of the “X”s. Arsenic was first used to create a paint in the late 18th century, when it was used to make a vivid green colour. This colour has been found to be in production late into the 19th century (8). Arsenic mapped onto chromium shows that they overlap on the green patch below the wallaby.

Many common pigments used up until the late 20th century have contained heavy metals, some still actively being used as seen in (Table. 2). Heavy metals have been labelled as toxic, with carcinogenic properties (3).  Using micro-XRF technology we have been able to map determine which heavy metals are present (arsenic, barium, chromium, copper, zinc, lead and strontium), their relation to pigment colour and their abundance.

 

For more information on micro-XRF spectroscopy visit www.microxrf.com.au/. For more information on the Bruker M4 Tornado micro-XRF visit https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/micro-xrf-and-txrf/m4-tornado/overview.html

Figure 8: Cr-Pb & Cr-As element map overlay. By using complementary colours such as green and red when overlaying two element distribution maps, you can easily distinguish areas which overlap, and elements that associate with each-other.  (Pb and As X-ray energy count distribution have been subtracted from each other)

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