4.1 Colour measurements
4.1.1 Measurements on substrates
In order to ensure the reliability of the colour measurements on the gold surface of the models, it is necessary to first check the colour quality of the substrates. Four substrate groups including yellow, red, blue-grey boles and white grounds, which were made either in 2019 or in 2020 (details presented in Table 1), were investigated through colorimetry with the MAV aperture. Note that the oil-based gold size was not measured, due to its tacky nature and tendency to easily collect dust. Measurement results show that there is no significant colour difference (for both SCE and SCI values of ∆E*ab) within the individual substrates and within the same substrate groups, indicating that the colours of the substrates are even and homogeneous. Colorimetric data of the substrates is presented in Table S1 in the Supplementary Materials: S4.1.
4.1.2 Measurements on gold surfaces with different apertures
During the colour measurements on gold surface of models, it is observed that the SCI values of ∆E*ab for almost all “nb” and “b” sub-models are below JND, which is inconsistent with our visual perception that the burnished gold surface appears darker and more saturated than the unburnished one. Indeed, according to the colorimeter manufacturer Konica Minolta, the SCE mode (i.e. diffuse reflection3) is similar to the visual perceptions by human eyes for a glossy surface . Therefore, further analysis is only focused on the SCE data.
Both MAV and SAV apertures were used for the colour measurements on gold surface. For MAV measurements, about 20 sample spots were selected on each “nb” and “b” sub-models. Minor imperfections in the gold leaf (e.g. fine scratches, small worn spots and stains), which are usually generated during the application and burnishing procedures, were included in these spots. In SAV measurements about 25 spots were selected, in which the minor defects were avoided. An example of the selection of MAV and SAV measurement spots on Model “y4_pg” is presented in Fig. S3 in the Supplementary Materials: S4.2. The comparison between the MAV and SAV measurement data is expected to show whether minor leaf defects could affect the visual appearance of the gold surface. Note that during the analysis of SAV data around 1–3 spots in a few sub-models, which show unusual ∆E*ab values compared to the averaged one, were further excluded.
Fig. 3 presents the data charts of the MAV and SAV colour differences (∆E*ab) measured in Poliergold models (a) and Doppelgold models (b). It is obvious that almost all sub-models have very close ∆E*ab values, except for “r5_pgx2_b” and “r7_dg_b”, which show higher values in MAV measurements than SAV by ca. 7 and 9 units respectively. We are not certain about the reason for such colour discrepancy. It is assumed that the MAV measurement spots in these two sub-models might contain relatively large defects, which could lead to certain levels of colour change of the gold surface. The comparison between the MAV and SAV data indicates that minor imperfections in the gold leaf do not significantly influence the colour appearance of the gold surface. Further analysis is focused on the SAV data.
4.1.3 Measurements on Poliergold models
The Poliergold models include three water gilding models “y4_pg”, “r1_pg” and “r5_pgx2”. Fig. 4 shows that the colour differences (compared to the target) of the three “nb” sub-models are not significant (∆E*ab values < JND); the same also applies for their “b” sub-models. In the latter, the ∆L* values drop dramatically (by 39–43 units compared to the corresponding “nb” sub-models), followed by the ∆b* values (drop by 8–9 units); while the ∆a* values slightly increase (by ca. 4–5 units). This observation indicates that after surface burnishing the gold leaf appears much darker and its colour change is in the direction of blue and red. Compared to “y4_pg_b” and “r1_pg_b”, the sub-model “r5_dgx2_b” seems even darker and more red-blueish (indicated with lower ∆L* and ∆b*, and higher ∆a*). We believe that this effect was likely caused by a stronger surface burnishing.
Since the gold leaves on differently coloured (i.e. red and yellow) bole substrates do not show significant colour change in both unburnished and burnished states, we have evidence against the hypothesis that the substrate colour plays a role in the colour appearance of the gold leaf laid above. However, to be conclusive it is necessary to study more models, in order to better understand the correlation between the colour of the gold surface and its substrate.
4.1.4 Measurements on Doppelgold models
Fig. 5a–b presents the colour measurements of nine Doppelgold models, including one ground gilding, one oil gilding and seven water gilding models. Four subsets of models with specific features (Fig. 5c–j) were further compared and analysed.
· Subset 1: Models with old bole substrates
The first colour comparison was implemented between “y7_dg” and “r7_dg” (Fig. 5c–d), in which the bole substrates “y7” and “r7” were made in 2019. It is obvious that the colorimetric data for both “nb” and “b” sub-models of these two models are very similar, indicating that there is no significant colour difference between the gold surfaces on the differently coloured bole substrates. Such observation further confirms the measurement output from the Poliergold models.
· Subset 2: Models with new bole substrates
Five models (“b1_dg”, “b4_dg”, “b2_dgx2”, “r9_dg” and “r10_dgx2”) were built on new bole substrates made in 2020 (Fig. 5e–f). Sub-models “b4_dg_nb” and “b2_dgx2_nb” show higher L* values (76.43 and 72.74) than that of “b1_dg_nb” (68.28), indicating relatively higher diffuse light reflection and also leading to their slightly different ∆E*ab values (8.66, 5.16 and 2.67). Indeed, digital microscopy images of these three “nb” sub-models (Fig. 7a–c) show that the latter two sub-models appear rougher than the former. Here, it is worth noting that the blue-grey bole materials seem to contain larger pigment or filler particles than the yellow and red bole materials; and the substrate “b1” was slightly sanded to obtain a relatively smoother surface than “b2” and “b4”, for the purpose of comparison. Since “b1”, “b2” and “b4” exhibit different levels of surface roughness but very similar colorimetric values (Table S1 in the Supplementary Materials: S4.1), the colour discrepancy in their “nb” sub-models indicates that the surface roughness of the substrate could play a role in the visual appearance of an unburnished gold leaf laid above.
Models “r9_dg” and “r10_dgx2” show almost the same colorimetric values in both their “nb” and “b” sub-models, indicating that there is no significant colour difference between the single- and double-layered gold leaf. This is also an evidence to prove that the slight colour difference between the Poliergold models “r1_pg_b” and “r5_pgx2_b” was likely caused by different magnitudes of surface burnishing.
It is interesting to observe that the ∆E*ab values of “r9_dg_nb” (2.67) and “r10_dgx2_nb” (3.00) are very close to that of “b1_dg_nb” (3.10), which could be attributed to the fact that the substrate “b1” was sanded to obtain a smoother surface and its roughness could be similar to the new red boles.
As for “b” sub-models, except for “b4_dg_b”, the other four sub-models show close colorimetric values. The ∆E*ab value of “b4_dg_b” is slightly higher than the others. Again, this could be attributed to a stronger surface burnishing.
· Subset 3: Models with red bole substrates
The “nb” sub-models of three models with red bole substrates (“r7_dg”, “r9_dg” and “r10_dgx2) show very close colorimetric values; while “r7_dg_b” exhibits higher ∆E*ab values than the other two “b” sub-models (Fig. 5g–h). Note that the substrates “r7”, “r9” and “r10” do not show significant differences in their colour measurements (Table S1 in the Supplementary Materials: S4.1).
· Subset 4: Models made with different gilding techniques
This comparison is performed between the water gilding model “b1_dg”, ground gilding model “w1_dg” and oil gilding model “w4_dg_oil” (Fig. 5i–j). Note that the comparison for “b” sub-models (j) is only performed between “b1_dg_b” and “w1_dg_b”, since the gold surface in oil gilding is unburnishable. It is not surprising to observe that “w1_dg_b” shows much lower ∆E*ab value (16.23) compared to “b1_dg_b” (32.62), which is mainly attributed to the fact that the L* value of “w1_dg_b” (53.10) is much higher than that of “b1_dg_b” (36.76) by ca. 16 units. This observation indicates that a poor-quality surface burnishing has been performed on this ground gilding model. Indeed, historical literature states that ground gilding can be only slightly burnished .
Fig. 5i show that the ∆E*ab values of the three “nb” sub-models vary on a small scale, of which “w1_dg_nb” shows the highest value of 6.08, while the values of the other two are very close (3.10 for “b1_dg_nb”; 4.41 for “w4_dg_oil_nb”). This discrepancy also results from their different L* values: “w1_dg_nb” shows higher lightness (74.09) than the other two “nb” sub-models (68.28 and 67.36), indicating a slightly more diffuse light reflection. Since the chalk ground was polished by fine sandpaper and thus appears very flat and smooth, it is worth to further investigate why more light could be diffusely reflected from the gold leaf laid atop such a highly polished and flat substrate surface.
4.1.5 Colour comparison between Poliergold models and Doppelgold models
The analysis in the previous section exhibits some examples about the possible correlation between the surface roughness of the substrate and the colour change of the gold leaf. Therefore, the colour comparison between different gold leaves must be performed on the models with substrates of similar roughness, likely the substrates made at the same time.
The comparison was implemented between two Poliergold and two Doppelgold models with yellow and red bole substrates. Fig. 6 show that these four models exhibit very close colorimetric values in both their “nb” and “b” sub-models, indicating that the small discrepancies in the gold content and leaf thickness of the gold leaf do not cause significant colour change to its surface.
From the colour measurements on the models with different types of gilding techniques, gold leaves and substrates, we see strong evidence that the substrate colour does not play an essential role in the visual appearance of the gold leaf laid above. Instead, the surface burnishing can strongly alter the colour appearance of the gold leaf and its quality is dependent on the substrate materials. Within the three common gilding substrates, the coloured bole provides the best surface burnishing due to the presence of its elastic clay ingredients, which correspondingly leads to more depth effects to the gold leaf laid above; while the gold surface above the ground substrate can be only slightly burnished due to the high hardness of the chalk ground. However, it is worth pointing out that the quality of surface burnishing is not only dependant on the substrate materials but also due to the implementers and preparation processing. Details are presented in Supplementary Materials: S3.1.
Surface roughness of the substrate seems to be another critical factor to influence the colour of the gold leaf. The correlation between the surface roughness of the substrate and the visual appearance of the gold leaf are further studied through digital microscopy and interferometric microscopy in the following sections.
4.2 Digital microscopy imaging on models
Fig. 7 exhibits the digital microscopy (DM) images of three models with the blue-grey bole substrates (“b1_dg”, “b4_dg”, “b2_dgx2”) through the 3D-stitching mode. Although it is known that the substrate “b1” was sanded to obtain a relatively smoother surface than “b2” and ‘b4”, these three bare substrates cannot not be well differentiated in their DM images (Fig. 7g–i). Instead, the substrate roughness can be reflected through the DM image of an unburnished gold leaf laid above, especially with the MIX lighting and the HDR image quality. The sub-model “b1_dg_nb” clearly exhibits a less rough gold surface than “b4_dg_nb” and “b2_dgx2_nb” (Fig. 7a–c); and it is not surprising to observe that after burnishing, all the “b” sub-models show smoother surfaces in a similar level (Fig. 7d–f) compared to their “nb” sub-models, indicating a greater specular/diffuse proportion ratio in the light reflection. The DM observations on these three models is consistent with their colorimetry measurements.
It is also interesting to compare the DM images of models with different gilding techniques. Fig. 8 shows DM observations on the gold surfaces of “b1_dg”, “w1_dg” and “w4_dg_oil”. These three models look similarly rough but with different textures in their “nb” and “b” sub-models respectively. For example, “b1_dg” exhibits a relatively even and homogenous texture (Fig. 8a, d); while “w1_dg” shows many fine horizontal lines in both its “nb” and “b” sub-models (Fig. 8b, e), although the surface burnishing was performed on the latter in the vertical direction. The fine horizontal lines present in “w1_dg” were likely caused by the sanding marks on its ground substrate, since the sanding and polishing of all chalk grounds were performed in this direction. Different from “w1_dg”, the model “w4_dg_oil” shows a few larger-scaled vertical lines in its “nb” sub-model (Fig. 8c). Such vertical lines could be possibly attributed to the presence of leaf folds. Since the gold leaf in oil gilding is unburnishable, leaf folds created during the manual pressing with cotton balls cannot be flattened through a surface burnishing.
4.3 Interferometric microscopy measurements on surface roughness of models
The quantification of surface roughness was determined using interferometric microscopy. Measurement spots that represent general surface conditions were selected for each of the three areas in the models: “nb”, “b” and bare substrate.
Fig. 9 shows radially averaged 2D power spectral density (PSD) of the surface topologies of the three types of water gilding models investigated in this study as a quantitative measure of surface roughness , including models with yellow bole (a), red bole (b) and blue-grey bole substrates (c). A common trend of the roughness change between the bare substrate, unburnished (“nb”) and burnished (“b”) areas can be easily observed in each type of the models: the bare substrates show the highest roughness, while the “nb” sub-models show a similar roughness on the longer length scales (following the shape of the substrate) and a strong roughness decrease at short length scales, and the “b” sub-models show decreased roughness across all length scales.
The water gilding models with the blue-grey bole substrates (“b1_dg”, “b4_dg” and “b2_dgx2”) are further analysed in Fig. 10. Comparisons with respect to “nb”, “b” and bare substrate are presented in Fig. 10a-c. The surface roughness of the “nb” sub-models in Fig. 10a shows that “b1_dg_nb” is significantly smoother than “b4_dg_nb” and “b2_dgx2_nb” at the length scales larger than ca. 0.05 mm; while below 0.05 mm the surfaces show very similar roughness. A similar situation is also observed in their bare substrates in Fig. 10c where “b1” shows a lower roughness than “b4” and “b2” at the larger length scales, while in the smaller scales there is no difference. Comparing these to the “b” sub-models in Fig. 10b, we observe that burnishing causes a strong reduction in the surface roughness and now the three “b” sub-models show similar roughness in all length scales, indicating the performance of a good-quality surface burnishing on all these three models. The output of the roughness measurements is consistent with our observations through digital microscopy and colorimetry, providing strong evidence for a correlation between the surface roughness of the substrate and the colour appearance of the gold leaf. For example, the substrate “b1” was sanded to obtain a smoother surface (to a similar level as the yellow bole), and the gold leaf laid above (“b1_dg_nb”) correspondingly shows the lowest roughness and smallest colour difference (compared to the target “y7_dg_nb_sp1”) within this set of three models; after surface burnishing its surface roughness becomes similar to the other two models.
Fig. 10d-f shows the comparison between “nb”, “b” and substrate for individual models. It is clear that all “b” sub-models exhibit the lowest roughness, followed by the “nb” sub-models and the bare substrates are the roughest. The roughness change in this set of models is similar to the trends observed in Fig. 9.
Fig. 11 presents the radially averaged 2D PSD roughness profiles and the corresponding interferometric microscopy images of three models produced with different gilding techniques (i.e. “b1_dg”, “w1_dg” and “w4_dg_oil”), which were also observed through digital microscopy. Note that measurements of the substrate roughness of “w4_dg_oil” was performed on the chalk ground rather than the oil-based gold size due to the tacky nature of the latter. From the images shown in Fig. 11b-d, we can see that the three “nb” sub-models show different large-scale textures, however these differences are not reflected in the corresponding PSDs shown in Fig. 11a, due to a lack of statistics in the large length scale region. A similar situation is also observed in the roughness graphs of their bare substrates in Fig. 11h. However, the colour measurements show that the ∆E*ab values of “w1_dg_nb”, “w4_dg_oil_nb” and “b1_dg_nb” are 6.08, 4.41 and 3.10 respectively; the relatively higher ∆E*ab value of the former is mainly attributed to its higher L* value compared to those of the other two (74.09 vs. 67.36 and 68.28), indicating a slightly more diffuse light reflection. In this case, the PSDs in Fig. 11a are not sufficient to explain the observations in colour measurements. We expect that the diffuse light reflection from “w1_dg_nb” could be enhanced by the light scattering through many fine horizontal lines on its gold surface, which we have observed in the DM image (Fig. 8b) and expect are the result of sanding marks on the chalk ground. Such fine horizontal lines can be also observed in the topography images of the “nb” and “b” sub-models of “w1_dg” (Fig. 11c, g), as well as the ground substrates of “w1_dg” and “w4_dg_oil” (Fig. 11j, k), but are not observable in the corresponding PSDs due to poor statistics at length scales above about 0.2 mm. The vertical lines in “w4_dg_oil_nb” that have been observed in the DM image (Fig. 8c) can be also seen in its topography image (Fig. 11d); and the small wavy wrinkles present in the same image appear consistent with the drying process of the oil contained in the gold size.
The roughness comparison between “b1_dg_b” and “w1_dg_b” (Fig. 11e) seems rather clear, with “b1_dg_b” showing significantly lower roughness than “w1_dg_b” at length scales below 0.1 mm. This lower roughness can be expected to result in a much less diffuse light reflection. Indeed, our colour measurements indicate that the L* values of “b1_dg_b” and “w1_dg_b” are 36.76 and 53.10 respectively, which result in a significant colour difference by ca. 16 units (∆E*ab value of 32.62 vs. 16.23). The comparison of these sub-models is a clear demonstration of how a soft substrate (i.e. bole) produces a superior burnished surface than hard substrates (i.e. polished ground).