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Report on the Analysis of Sulphur Free Calf Leather for Bookbinding

  

Report on the Analysis of Sulphur Free Calf Leather for Bookbinding

By René Larsen PhD, MS in Conservation, Bookbinder, Chairman of the Board of Conservation Zealand - Knowledge Centre for the preservation of Cultural Heritage/Bevaring Sjælland - Videncenter for bevaring af kulturarven

 A preliminary report on the analysis of the Sulphur Free Calf leather (SF Calf leather) developed for use in book binding concluded that Relevant tests done by bookbinders to check the organoleptic properties indicated that this SF Calf met and/or exceeded other contemporary calfskins commercially available. These properties are very important to a binder and since the initial reactions were so positive, we decided to develop a more elaborate test regime. Introduced were an internal tropical test (50°C/90%RH/20 days) and an oxidation test (120°C/24-48-96 hours). In addition, to the standard tests a coherence of dry fibres assessment test to assess the freshly made leathers and the aged leathers. The Tropical test (for hydrolysis) and the oxidation test indicated that the SF Calf performed the best. Moreover, a mineral tannins determination (ISO 17072-2:2019) confirmed a very low, insignificant amounts of metals present in SF calf (1).

More recent responses form bookbinders and analysis of new productions of the SF Calf leather confirms that it possesses all the best organoleptic properties for bookbinding and in addition to be sulphur free, the SF Calf is distinguished by the presence of an insignificant amounts of metals.

In addition to the analysis and test mentioned above measurement of colours of SF Calf and three commercial binding leathers were performed. Figure 1 shows  the % change of colours in RGB values (red-green-blue colour space) after the dry heat ageing at 120o C for 96 hours. The aging leads to SF Calf becoming a bit more reddish-brown. However, compared to the other leathers, SF Calf is the most durable with respect to colour under oxidative conditions such as heat and light.

.

Bar Diagram
Figure 1. % Change in colours after the dry heat ageing at 120o C for 96 hours.

 


With respect to the Climate test, the colour changes are slightly less for SF Calf compared to the change it undergoes in dry heat, whereas leather B becomes darker than in the dry heat ageing.

Regarding its long-term durability, an assessment of the coherence of dry fibres after the internal tropical test and the oxidation test showed that the SF Calf performed better than the other leathers analysed, and which are sold as being archival. Table 1 and figure 3 shows the results of the fibre coherence test performed on the flesh side (corium) of the SF Calf and three commercial leathers A, B, C before and after accelerated ageing.

Table 1. The results of the testing the fibre coherence of SF Calf and the commercial leathers before and after accelerated ageing at 120o C for 96 hours and accelerated ageing at 50o C, 90 % RH for 20 days,
respectively.
Coherence categories goes from 1 (intact) to 5 (completely loss of coherence).

Sample

Coherence

Remarks

SF Calf

1

Very difficult to scratch

SF Calf: 120o C/96 hrs

3

Difficult to scratch

SF Calf: 50o C/90% RH /20 days

2,5

Difficult to scratch

A

2,5

Easy to scratch

A: 120o C/96 hrs

3,5

Less easy to scratch

B

2,5

Easy to scratch

B: 120o C/96 hrs

4,0

Easy to scratch

C

2

Easy to scratch

C: 120o C/96 hrs

4,0

Less easy to scratch

C: 50o C/90 % RH /20 days

3,5

Easy to scratch


Figure 2. Scratch test of corium fibres.Microscopic images




Thus, with respect to fibre strength and lightfastness it can be concluded that SF Calf is without doubt the most durable leathers among the tested leathers.

 

  1. Larsen, R., Siegel S., Themmen, E.R. Preliminary Report on the Development of a Sulfur Free Full Vegetable Tanned Archival Calf. Leather and Related Materials Working Group Newsletter, June 2022, 6-7.

 

 


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