4.1 SEM-EDX and RBS measurements of modern gold leaf
Table 1 shows that the SEM-EDX observation on the gold (Au) content of modern gold leaf is 96.8±0.2 % by weight (w%) for Spezial-Poliergold and 96.6±1.1 w% for Dukaten-Doppelgold, corresponding to the RBS data of 96.6±0.3 w% and 96.2±0.3 w% respectively. Compared to atomic ratios, weight ratios (w%) are preferably used for the quantification of the gold content of the medieval samples, since they can be easily transformed into carat, which is the most important and frequently used unit of gold purity in most literate records regarding medieval gilding leaves/foils. In the case of modern gold leaves used in our study, their gold purity is about 23.1 to 23.2 carat.
SEM-BSE and RBS measurements further show that the Poliergold (96±9; 92±7 nm) appears slightly thinner than the Doppelgold (116±16 nm; 127±7 nm) by ca. 20–30 nm, but both of them are still within the published thickness range of 100–200 nm for modern gold leaf [21,22]. The high consistency between SEM-EDX and RBS measurements of modern gold leaf samples proves that the samples were precisely prepared, which further confirms the accuracy and reliability of the SEM-EDX observations on the medieval gold leaf samples.
Table 1: SEM-EDX and RBS data of the modern gold leaves used in the models.
Gold leaf type
|
SEM-EDX
|
RBS
|
Cu content (w%)
|
Ag content (w%)
|
Au content (w%)
|
Thickness (nm)
|
Cu content (w%)
|
Ag content (w%)
|
Au content (w%)
|
Thickness (nm)
|
Spezial-Poliergold
|
1.1±0.3
|
2.1±0.4
|
96.8±0.2
|
96±9
|
0.8±0.1
|
2.6±0.2
|
96.6±0.3
|
92±7
|
Dukaten-Doppelgold
|
1.6±0.2
|
1.8±1.0
|
96.6±1.1
|
116±16
|
1.4±0.2
|
2.4±0.2
|
96.2±0.3
|
127±7
|
Fig. 3 presents one example of the SEM-BSE image (a) and a EDX spot test (a) on a sample taken from a small (ca. 10x10 mm), non-burnished gold leaf model made from Dukaten-Doppelgold (laid atop a red bole substrate), compared to the RBS measurements on the same model. Note that the RBS thickness measurement (Fig. 3c) was conducted in the thickness scan mode tracing a 2 MeV beam (in a diameter of ca. 1 mm) along the middle line of the model in steps of 1 mm; and its elemental analysis (Fig. 3d) was performed at the central position of the model with a 5 MeV beam. Here, it is worth noting that no obvious thickness difference has been observed between the non-burnished and burnished states of gold leaf in both SEM and RBS measurements. Indeed, after the high pressure pressing and rolling during the manufacturing of gold leaf, a manual surface burnishing with agate tools should not generate sufficient pressure to significantly modify the leaf form. The same situation should also apply to the medieval gold leaf, which was produced through the gold-beating process [8].
4.2 SEM-EDX measurements of medieval gold leaf
In VLM observations on the sample stratigraphy, two of the 34 medieval gold leaf samples show an overlay of different Fassungs, indicating that they may not have been part of the original artefacts, and so were excluded from further analysis. Fig. 4 shows an example of overlaid Fassungs in a sample stratigraphy (b), compared to a normal gold leaf sample (a). Therefore, a total of 32 samples were measured through SEM-EDX.
Table 2 presents details of SEM-EDX measurements on the 32 medieval gold leaf samples with respect to their gold content and leaf thickness. Note that 7 of the 32 samples contain multiple sub-layers. In 3 cases, the sub-layers can be clearly distinguished and hence the average thickness of the individual sub-layers are included in the summary of basic statistics of the leaf thickness and gold content in Table 3. In the remaining 4 multi-layered samples (marked with “*” in Table 2), the sub-layers are poorly defined and difficult to distinguish, and so these 4 samples were excluded from the statistics of leaf thickness, but still used for the statistics of gold content.
Table 2: SEM-EDX observations on the leaf thickness, the content of gold alloying element and the calculated gold purity in 32 gold leaf samples
Object Inv. No.
|
Object title
|
Production data
|
Production regions
|
Sample-taking area
|
Leaf thickness
|
Au content
|
Cu content
|
Ag content
|
avg. (nm)
|
stdev. (nm)
|
med. (nm)
|
min. (nm)
|
max. (nm)
|
counts
|
avg. (w%)
|
avg. (carat)
|
stdev. (w%)
|
med. (w%)
|
min. (w%)
|
max. (w%)
|
counts
|
avg. (w%)
|
stdev. (w%)
|
med. (w%)
|
min. (w%)
|
max. (w%)
|
counts
|
avg. (w%)
|
stdev. (w%)
|
med. (w%)
|
min. (w%)
|
max. (w%)
|
counts
|
AG9061.1
|
John the Evangelist
|
1490-1500
|
Unknown
|
Golden flakes on gown
|
244
|
101
|
223
|
89
|
496
|
113
|
99.68
|
23.92
|
1.05
|
100.00
|
96.53
|
100.00
|
11
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
11
|
0.32
|
1.05
|
0.00
|
0.00
|
3.47
|
11
|
AG9061.1
|
John the Evangelist
|
1490-1500
|
Unknown
|
Green area on garment
|
127
|
40
|
121
|
57
|
250
|
50
|
99.73
|
23.94
|
0.41
|
100.00
|
98.81
|
100.00
|
10
|
0.27
|
0.41
|
0.00
|
0.00
|
1.19
|
10
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
10
|
AG9061.2
(sub-layer)
|
Unknown female saint
|
1490-1500
|
Unknown
|
gown
|
155
|
69
|
139
|
65
|
391
|
171
|
99.96
|
23.99
|
0.94
|
100.00
|
96.63
|
100.00
|
10
|
0.04
|
0.12
|
0.00
|
0.00
|
0.40
|
10
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
10
|
DEP64
|
Virgin and Child with St. Anna; St. Magnus; St. Francis
|
1520
|
Southern Germany
|
gown
|
167
|
54
|
153
|
76
|
318
|
75
|
99.65
|
23.92
|
0.94
|
100.00
|
96.63
|
100.00
|
13
|
0.09
|
0.23
|
0.00
|
0.00
|
0.61
|
13
|
0.26
|
0.94
|
0.00
|
0.00
|
3.37
|
13
|
LM10419
(sub-layer)
|
St. Mary Magdalene
|
1500
|
Southern Germany
|
Golden flakes on edge of gown
|
157
|
44
|
151
|
96
|
292
|
30
|
100.00
|
24.00
|
0.00
|
100.00
|
100.00
|
100.00
|
15
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
15
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
15
|
LM10556
|
St. Bishop Ulrich
|
1500
|
Unknown
|
Gown
|
147
|
66
|
135
|
45
|
436
|
131
|
99.38
|
23.85
|
1.42
|
100.00
|
95.90
|
100.00
|
8
|
0.17
|
0.23
|
0.00
|
0.00
|
0.50
|
8
|
0.45
|
1.27
|
0.00
|
0.00
|
3.61
|
8
|
LM10557
|
St. Bishop Theodore
|
1500
|
Unknown
|
Gown
|
152
|
38
|
151
|
68
|
298
|
91
|
99.34
|
23.84
|
1.05
|
99.48
|
96.49
|
100.00
|
10
|
0.35
|
0.32
|
0.48
|
0.00
|
0.81
|
10
|
0.31
|
0.97
|
0.00
|
0.00
|
3.06
|
10
|
LM11029**
|
John the Baptist
|
1510
|
Central Switzerland
|
Gown
|
146
|
43
|
139
|
71
|
259
|
103
|
96.34
|
23.12
|
2.44
|
95.06
|
93.73
|
100.00
|
14
|
1.18
|
0.40
|
1.26
|
0.00
|
1.63
|
14
|
2.48
|
2.27
|
3.58
|
0.00
|
5.08
|
14
|
LM11266
|
Coronation of Mary with God the Father
|
1515
|
Eastern Switzerland
|
Gown of Mary
|
103
|
38
|
92
|
48
|
237
|
48
|
99.15
|
23.79
|
0.37
|
99.10
|
98.74
|
100.00
|
11
|
0.85
|
0.37
|
0.90
|
0.00
|
1.26
|
11
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
11
|
LM12011
|
Virgin and Child with St. Anna
|
1525
|
Unknown
|
Gown of Anna
|
224
|
69
|
210
|
108
|
486
|
162
|
100.00
|
24.00
|
0.00
|
100.00
|
100.00
|
100.00
|
6
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
6
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
6
|
LM13543**
|
Unknown female saint
|
1400-1410
|
Unknown
|
Gown hem
|
122
|
38
|
115
|
38
|
209
|
85
|
96.47
|
23.15
|
3.39
|
95.11
|
90.00
|
100.00
|
10
|
0.16
|
0.25
|
0.00
|
0.00
|
0.59
|
10
|
3.37
|
3.32
|
4.42
|
0.00
|
10.00
|
10
|
LM17796*
|
Unknown female saint
|
1500
|
Central Switzerland
|
Gown
|
402
|
118
|
395
|
160
|
667
|
78
|
100.00
|
24.00
|
0.00
|
100.00
|
100.00
|
100.00
|
8
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
8
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
8
|
LM18014
|
Descent from the Cross
|
1520
|
Eastern Switzerland
|
Gown
|
184
|
68
|
176
|
83
|
375
|
40
|
100.00
|
24.00
|
0.00
|
100.00
|
100.00
|
100.00
|
14
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
14
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
14
|
LM18015*
|
Crucifixion
|
1520
|
Eastern Switzerland
|
Gown
|
364
|
83
|
357
|
159
|
598
|
79
|
99.66
|
23.92
|
1.09
|
100.00
|
96.55
|
100.00
|
10
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
10
|
0.34
|
1.09
|
0.00
|
0.00
|
3.45
|
10
|
LM18016*
|
Resurrection of Jesus
|
1520
|
Eastern Switzerland
|
Gown
|
304
|
134
|
241
|
181
|
523
|
8
|
100.00
|
24.00
|
0.00
|
100.00
|
100.00
|
100.00
|
2
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
2
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
2
|
LM20886
|
John the Baptist
|
1500
|
Southern Germany
|
Gown
|
155
|
62
|
143
|
52
|
368
|
153
|
99.95
|
23.99
|
0.17
|
100.00
|
99.46
|
100.00
|
10
|
0.05
|
0.17
|
0.00
|
0.00
|
0.54
|
10
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
10
|
LM24174**
|
Half figure of a Holy Pope, possibly St. Paul
|
1500-1510
|
Southern Germany
|
Gown
|
150
|
55
|
141
|
67
|
317
|
78
|
91.77
|
22.02
|
2.71
|
91.50
|
88.36
|
97.08
|
7
|
2.39
|
0.32
|
2.38
|
1.87
|
2.92
|
7
|
5.84
|
2.93
|
5.99
|
0.00
|
9.13
|
7
|
LM5657
|
John the Baptist
|
1495
|
Southern Germany
|
Gown
|
113
|
25
|
108
|
58
|
177
|
55
|
98.71
|
23.69
|
1.25
|
99.32
|
96.60
|
99.50
|
8
|
0.51
|
0.24
|
0.55
|
0.00
|
0.74
|
8
|
0.78
|
1.45
|
0.00
|
0.00
|
3.40
|
8
|
LM6280
|
St. Lawrence
|
1500
|
Southern Germany
|
Gown
|
148
|
48
|
135
|
64
|
282
|
85
|
100.00
|
24.00
|
0.00
|
100
|
100.00
|
100.00
|
11
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
11
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
11
|
LM6982
|
Virgin and Child with St. Anna
|
1500-1510
|
Southern Germany
|
Gown of Anna
|
139
|
59
|
134
|
55
|
291
|
55
|
99.14
|
23.79
|
0.27
|
99.02
|
98.91
|
100.00
|
15
|
0.86
|
0.27
|
0.98
|
0.00
|
1.09
|
15
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
15
|
LM7108
|
Mary with Child
|
1450
|
Southern Germany
|
Gown
|
127
|
43
|
127
|
50
|
214
|
55
|
100.00
|
24.00
|
0.00
|
100.00
|
100.00
|
100.00
|
8
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
8
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
8
|
LM7202.a
(sub-layer)
|
Mary with Child, surrounded by two angels
|
1490-1500
|
Unknown
|
Gown
|
122
|
39
|
124
|
61
|
195
|
24
|
98.85
|
23.72
|
1.36
|
99.44
|
95.65
|
100.00
|
16
|
0.53
|
0.19
|
0.56
|
0.00
|
0.85
|
16
|
0.61
|
1.33
|
0.00
|
0.00
|
3.69
|
16
|
LM7202.b**
|
Virgin and Child with St. Anna
|
1520
|
Unknown
|
Gown
|
121
|
40
|
108
|
64
|
214
|
29
|
96.90
|
23.26
|
1.63
|
97.00
|
94.96
|
100.00
|
7
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
7
|
3.10
|
1.63
|
3.14
|
0.00
|
5.04
|
7
|
LM8139.2
|
Mourning John the Evangelist
|
1500-1510
|
Central Switzerland
|
Gown
|
160
|
49
|
152
|
60
|
327
|
77
|
99.23
|
23.82
|
0.48
|
99.06
|
98.71
|
100.00
|
16
|
0.77
|
0.48
|
0.94
|
0.00
|
1.29
|
16
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
16
|
LM8554.1
|
Mary with Child (from a figure group “Mary with eight saints”)
|
1510
|
Southern Germany
|
Gown
|
168
|
58
|
157
|
56
|
349
|
74
|
99.69
|
23.93
|
0.90
|
100.00
|
97.01
|
100.00
|
11
|
0.04
|
0.12
|
0.00
|
0.00
|
0.40
|
11
|
0.27
|
0.90
|
0.00
|
0.00
|
2.99
|
11
|
LM8554.3
|
S.t Jeromy
|
1510
|
Southern Germany
|
Gown
|
215
|
81
|
228
|
86
|
402
|
40
|
99.97
|
23.99
|
0.11
|
100.00
|
99.57
|
100.00
|
15
|
0.03
|
0.11
|
0.00
|
0.00
|
0.43
|
15
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
15
|
LM8658
|
Enthroned bishop, possibly St. Theodore
|
1500
|
Unknown
|
Gown hem
|
240
|
58
|
230
|
127
|
400
|
32
|
98.41
|
23.62
|
1.68
|
98.86
|
93.99
|
99.48
|
9
|
1.18
|
0.51
|
1.14
|
0.52
|
2.32
|
9
|
0.41
|
1.23
|
0.00
|
0.00
|
3.69
|
9
|
LM9282.2**
|
God the Father (from a figure group “Coronation of the Virgin”)
|
1500
|
Southern Germany
|
Gown
|
196
|
46
|
203
|
92
|
305
|
95
|
96.21
|
23.09
|
2.68
|
96.30
|
92.85
|
99.02
|
14
|
1.24
|
0.17
|
1.22
|
0.98
|
1.61
|
14
|
2.55
|
2.66
|
2.28
|
0.00
|
5.82
|
14
|
LM9282.3**
|
Christ (from a figure group “Coronation of the Virgin”)
|
1500
|
Southern Germany
|
Golden flakes on gown
|
168
|
55
|
166
|
55
|
360
|
80
|
96.07
|
23.06
|
2.11
|
95.01
|
2.11
|
98.92
|
9
|
1.27
|
0.15
|
1.31
|
0.15
|
1.45
|
9
|
2.66
|
2.02
|
3.66
|
0.00
|
4.69
|
9
|
LM9428**
|
Unknown bearded saint
|
Unknown
|
Unknown
|
Golden flakes on gown
|
159
|
50
|
162
|
48
|
279
|
86
|
94.99
|
22.80
|
2.83
|
93.09
|
90.13
|
98.96
|
17
|
1.98
|
1.01
|
1.96
|
0.00
|
4.45
|
17
|
3.03
|
2.73
|
3.94
|
0.00
|
7.54
|
17
|
LM9484*
|
Holly Kinship
|
1500
|
Southern Germany
|
Gown of Mary
|
388
|
124
|
404
|
144
|
730
|
93
|
100.00
|
24.00
|
0.00
|
100.00
|
100.00
|
100.00
|
11
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
11
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
11
|
LM9485
|
St. Dorothea
|
1490-1500
|
Unknown
|
Golden flakes on gown
|
238
|
75
|
217
|
111
|
467
|
118
|
100.00
|
24.00
|
0.00
|
100.00
|
100.00
|
100.00
|
6
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
6
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
6
|
* gold leaf samples containing indistinct sub-layers
** gold leaf samples showing relatively lower gold purity (22.02 – 23.26 carat)
Table 3 exhibits the basic statistics of the leaf thickness and gold content of medieval gold leaf samples based on their SEM-EDX data presented in Table 2. Note that the average (avg.), standard deviation (stdev.), median (med.), minimum (min.) and maximum (max.) values in the statistics are based on the averaged measurements of each sample.
Table 3: Statistics of SEM-EDX observations on the leaf thickness and gold content of medieval gold leaf samples.
Measurement
|
Avg.
|
Stdev.
|
Med.
|
Min.
|
Max.
|
Counts
|
Samples
|
Thickness
|
162 nm
|
39 nm
|
158 nm
|
103 nm
|
244 nm
|
2235
|
28
|
Cu content
|
0.43 w%
|
0.63 w%
|
0.07 w%
|
0.0 w%
|
2.39 w%
|
342
|
32
|
Ag content
|
0.84 w%
|
1.43 w%
|
0.13 w%
|
0.0 w%
|
5.84 w%
|
342
|
32
|
Au content
|
98.73 w%
|
1.93 w%
|
99.65 w%
|
91.77 w%
|
100 w%
|
342
|
32
|
23.70 carat
|
0.46 carat
|
23.92 carat
|
22.02 carat
|
24 carat
|
4.2.1 Gold content
As shown in Table 3, the average gold purity of the medieval samples is around 23.70 carat, based on 342 measurements of 32 samples, which is close to the historical records of florins (23.75–24 carat) and ducats (23.75 carat). The main alloying elements observed in the sample leaves include copper (Cu) and silver (Ag), with an average content of 0.43% and 0.84% respectively. However, these are both unevenly distributed in the gold leaf, demonstrated by the relatively high standard deviations. Details regarding the Ag and Cu contents of each sample are presented in Table 2.
It is worth pointing out that 7 of the 32 samples show relatively lower gold quality of 22.02 to 23.26 carat (marked with “**” in Table 2), while the rest of the samples exhibit very high gold purity of 23.62 to 24 carat. We are not certain about the gold source for these 7 samples, which could be some other gold coins such as Rhenish gold with its early compositions. Evidence of using Rhenish gold for gold leaf can be found in some German work contracts of the early 16th century [15]. Compared to the florin and ducat, the quality of Rhenish gold was not stable, and its gold content sunk from 98% when it was first minted in 1354 to only 79% by 1419 [15].
4.2.2 Leaf thickness
Most of the medieval gold leaf samples (Fig. 5b-d) were observed to be slightly less even than the modern gold leaf (Fig. 5a); and their thickness varies significantly. The basic statistics in Table 3 show that the average thickness of medieval gold leaf is 162±39 nm based on 2235 measurements on 28 gold leaf samples, with a median value of 158 nm, a minimum and a maximum value of 103 nm and 244 nm respectively. This observation is close to the literature record (possibly a calculated leaf thickness) based on Vasari’s treaties (154 nm for gold leaf of the 16th century) and ca. 1.4–1.7 times as thick as modern gold leaf that we have observed in Spezial-Poliergold (96±9) and Dukaten-Doppelgold (116±16).
The seven sample gold leaves that contain multiple sub-layers (Fig. 5e) show an overall thickness of 388±45 nm, ca. 2.5 times larger than the average thickness of normal gold leaves. Measurement data of these seven samples are presented in Table 4. Such thick, multi-layered gold leaf can be sometimes misidentified as a folded normal (i.e. single-layered) gold leaf (Fig. 5d), which usually occurs in wrinkled areas of a gilded surface. The observation of the multi-layered gold leaf reminds us of the special type of “fine reinforced gold leaf” recorded in artists’ supplies in Dijon in 1400. Since very little space is observed between these sub-layers, we believe that the artisans must have combined a few thin gold foils in the later stages of beating, in order to ensure a desired final thickness relative to the normal gold leaf. This is different from Cennini’s suggestion of reducing the number of the gold leaves produced from one gold coin for greater leaf thickness, and also unlike Theophilus’s recipe of applying multiple layers of gold leaf with adhesive. SEM observations on a sample taken from a model applied with double layers of modern gold leaf (model produced in Part I) provide evidence supporting our hypothesis. Fig. 5f shows that when adhesive (1.5 w% rabbit skin glue in this case) is used for laying one gold leaf above another, a relatively large space is present between these two leaves, even in a burnished state. Based on our experience in the production of models, overlaying gold leaves without adhesive is not practical, as the upper leaf does not adhere and is removed with the lightest touch.
Table 4: SEM observations on leaf thickness of 7 medieval gold leaf samples containing multiple sub-layers.
Object Inv. No.
|
leaf structure
|
Measurements of the overall thickness (nm)
|
Avg.
|
Stdev.
|
Med.
|
Min.
|
Max.
|
Counts
|
AG9061.2
|
3 distinct sub-layers
|
428
|
131
|
412
|
231
|
750
|
49
|
LM10419
|
2 distinct sub-layers
|
390
|
92
|
362
|
295
|
698
|
39
|
LM17796
|
Up to 2 indistinct sub-layers
|
402
|
118
|
395
|
160
|
667
|
78
|
LM18015
|
Up to 2 indistinct sub-layers
|
364
|
83
|
357
|
159
|
598
|
79
|
LM18016
|
Up to 3 indistinct sub-layers
|
304
|
134
|
241
|
181
|
523
|
8
|
LM7202.a
|
3 distinct sub-layers
|
439
|
160
|
385
|
217
|
862
|
45
|
LM9484
|
Up to 2 indistinct sub-layers
|
388
|
124
|
404
|
144
|
730
|
93
|
More detailed information is presented in the thickness histogram in Fig. 6, which was constructed by summing the set of clipped normal distributions corresponding to the average, standard deviation, minimum and maximum values calculated from the measurements on each medieval sample shown in Table 2 (four samples marked with “*” were excluded). The histogram of medieval gold leaf samples shows a half-peak range of 85–216 nm with the peak value of 138 nm, indicating a much broader thickness range compared to the modern samples of Spezial-Poliergold (76–115 nm with the peak at 94 nm) and Dukaten-Doppelgold (92–144 nm with the peak at 112 nm).
4.3 Further discussions
The SEM-EDX observations of the late medieval gold leaf samples are in high agreement with related literature records, often exhibiting greater leaf thickness and even higher gold purity than some modern gold leaves. Combined with the known optical properties of thin gold films excluding significant transparency in gold films thicker than 100 nm [23] and our findings through modern gold leaf models in Part I, our observations of medieval gold leaf samples indicate that no perceptible reflected light from the substrate could be able to penetrate through the medieval gold leaf to the surface, except for through cracks or holes inside of the leaf. In Part I we determined that such crack and holes in the gilding typically do not make up a significant enough fraction of the surface area to make a perceptible difference to the colour. Therefore, we conclude that the substrate colour does not play any role in the visual appearance of late medieval artefacts gilded with gold leaf. However, why has this misconception been well accepted since at least the Middle Ages and circulated even till now? We believe that the main reason is very likely related to the historical development of medieval polychromy. According to Brachert and Kobler [24], in the time period from the second half of the 12th century till the early 14th century, white ground was generally used for glossy gilding in northern Europe, while the use of Armenian bole started in the South. During the trend of “Ideal polychromy” (translated from Ideale Fassung in German), which appeared in the first half of the 14th century, differentiated gold effects were preferred: for example, hair was always in matte gold, while gown could show a contrasted effect with glossy gold on white ground and matte gold on yellow substrate. By the end of the 14th century, gold leaf was generally applied on a yellow, seldom white substrate, and red Poliment (German term, refers to fine clay bound with organic media and often mixed with pigments, similar to bole [26]) became popular since the middle of this century and dominant in later epochs [17,24,25]. Although the “yellow substrate” is not clearly specified by the authors, from its usage for matt gilding we believe that it refers to yellow mordant. Now we see a clear chronological sequence of using different substrates for gilding in the North of medieval Europe: white ground – mixture of white ground and yellow mordant – yellow mordant – red Poliment.
Since ground gilding can only offer a poor surface burnishing and mordant gilding is not burnishable, gold surfaces of North European sculptures in the time periods prior to the introduction of red bole/Poliment, must appear relatively matte and yellowish. Following the wide use of red bole, the quality of the surface burnishing was strongly enhanced, and the gold surface hence exhibited high metal gloss and more depth in its colour appearance, rendering a warmer and more saturated feeling to human eyes. From this perspective of working with a limited set of substrate materials, there is a clear correlation between the substrate colour and the gilding surface, and so it would be easy for medieval artisans to fall victim to the fallacy that the correlation implies causation. However, it is only a coincidence that the colour tone of these substrate materials correlates with the mechanical support and surface roughness properties that are the true cause of the mirror-like smooth surface of the gilding that provides the desired visual appearance. This was demonstrated in Part I with the use of a wider range of substrate material types and colour combinations than would have been available to artisans at the time that the substrate colour theory was being popularised. The occurrence of very fine gold leaf in the late Middle Ages could further strengthen people’s “faith” in using certain substrate colours for gilding, because they believed that the gold leaf was too “thin” to block the substrate colour.
Another contributing factor for artisans’ misconception could have been related to the parallel development of painting techniques in the late medieval period, especially the wide application of stand oil as a pigment binder for the late Gothic’s artworks [26]. Such heat-treated oil (e.g. linseed oil, walnut oil) can be dry within 24–48 hours in the paint layers and is thus especially suitable for thin layers over a white grounding [26]. It was also commonly used for the half-transparent colour glazing or varnishes [10], due to its closer refractive index to many pigments for reduced light scattering, compared to water-based binding media [27]. For example, aluminium hydroxide was a common substrate for the organic lake pigments that were extensively used in coloured glaze; its refractive index (ca. 1.5) is very close to that of linseed oil (1.48) and significantly larger than that of glue (ca. 1.35) [27]. Additionally, some mineral pigments and fillers such as ultramarine, smalt, chalk and gypsum also show similar refractive indices as oil (1.5, 1.49–1.52, 1.5–1.64, 1.53–1.62 vs. 1.48) [27]. The practical experience of dealing with oil containing binding media might have helped to build the fallacy in minds of the artisans that a thin gold leaf should behave similarly to the oil paint layers and also be unable to fully block the colour of an underlying layer (e.g. a red bole). The transfer of such ideas could occur especially easily in workshops where gilders and Fassmaler (German term, refers to painters working on sculptures) were working together, which was fairly common at that time.