general article information about Franz Hoffmann© archival- / restoration and conservation leather

• Work in the water workshop (beamhouse) pertaining to liming (decomposition of skin) and unhairing occur in combination with soaking and liming with NaSH, Na₂S, and CaOH₂ and non-ionic synthetic fatty alcohols as wetting agents.

• Exceptions occur for Archive-Spezial® formulations for calf leather and goatskin. These are unhaired in pure CaOH₂ liming.

• Natural, hydrolysable, vegetable tanning agents are used or chemically pure aluminum sulfate (Al₂O₃) for traditional alum-tanned leather. The old Alum- tawing formulations.
  

• Special fatliquoring agents and lightfast tools allow for the desired finishing, such as fiber- drawing, sharpening, clean cutting (without fraying), embossing, hand gilding, and dyeing with aniline dyes, without having a negative impact.

• We grant maximum usability, satisfaction, and quality of leather products and employ current standards of technology. We reserve the right for specific on the spot tests by our local customers according to the terms of delivery (see order release). Our goods do not contain toxic substances/pollutants in accordance with German law.

article segment Art.- No. 433 (standard vegetable tanned calf restoration leather)

• Tanning recipe formula containing a combination of predominantly condensable, non-acid forming vegetable pyrocatechin tanning agents, from which ca. 17% is quebracho wood, 23% mimosa bark, 8% hemlock bark, 44% light tanning gambier wood, and 6–8% a phenol as a synthetic tanning aid to improve the overall tanning and subsequent restoration colorability. Later fatliquoring affects the particular physical properties of the leather, such as in terms of elasticity, tear strength, and water wettability. In bookbinding and restoration leather, this is carried out with sulfited vegetable oils and sulfated train oils, which are particularly suitable for vegetable-synthetic leather. The combination of a synthetic fatting- or fatliquoring agent and emulsifiers are also used to improve electrolytic stability. The increase in fine pores, light fastness, resistance to aging and absence of acid of leather are likewise very positively influenced. We must expressly highlight that our bookbinding restoration does not contain sulfited fish oils since these, according to the most current state of technology, generally impact negatively on the aging process of the leather. The oxidative degradation “red rot” of the leather fiber advances too rapidly due to the higher acid number of fish oils. Fatliquoring ditto. in contract productions of our leathers will be running mainly by the use of non-ionic fatacid emulsions in the tannery.
  

article segment Art.- No. 438 (archive vegetable tanned calf restoration leather, standard formulation)

• Tanning recipe formula containing a combination of predominantly condensable, non-acid forming vegetable pyrocatechin tanning agents, from which ca. 8% is quebracho wood, 13% mimosa bark, 18% hemlock bark, 44% pale yellow-tanning sumac leaves, and a phenol as a synthetic tanning aid to improve the overall tanning and subsequent restoration colorability. The acid forming properties of the sumac leaves as the only hydrolysable pyrogallol tannin are best neutralized by aluminum sulfate. This results in the leather being very resistant to aging.

• Fatliquoring ditto.

article segment Art.- No. 438 (Spezial® vegetable tanned calf restoration leather)

• Tanning recipe formula containing a combination of non-acid forming vegetable pyrocatechin tanning agents, from which ca. 8% is quebracho wood, 8% hemlock bark, 79% pale yellow-tanning sumac leaves, and 5% aluminum sulfate (AL2O3) as a mineral tanning aid to improve the resistance to aging. The acid forming properties of the sumac leaves as the only hydrolysable pyrogallol tannin are best neutralized by aluminum sulfate. Thus, this formula meets the highest requirements in aging resistance, which the international CRAFT-Standard set for the self-dyeing of restoration leather. The sumac leaves, as the main component of this tanning, are crushed in special ceramic grinders to guarantee absolutely no contamination from metals. The purity of all added chemical additives and their very precise implementation in this tanning is done to the very highest quality of German standards.

• Fatliquoring ditto.

article segment Art.- No. 132 (archive vegetable tanned goat restoration leather, standard formulation)

• Tanning recipe formula containing a combination of predominantly condensable, non-acid forming vegetable pyrocatechin tanning agents, from which ca. 13% is quebracho wood, 13% mimosa bark, 18% hemlock bark, 44% pale yellow-tanning sumac leaves, and 12% phenol as a synthetic tanning aid to improve the overall tanning and subsequent restoration colorability. The acid forming properties of the sumac leaves as the only hydrolysable pyrogallol tannin are best neutralized by the phenolic tanning agent. This results in the leather being very resistant to aging.

• Fatliquoring ditto.

article segment Art.- No. 132 (Spezial® vegetable tanned goat restoration leather)

• Tanning recipe formula containing a combination of non-acid forming vegetable pyrocatechin tanning agents, from which ca. 8% is quebracho wood, 8% hemlock bark, 79% pale yellow-tanning sumac leaves, and 5% aluminum sulfate (AL2O3) as a mineral tanning aid to improve the resistance to aging. The acid forming properties of the sumac leaves as the only hydrolysable pyrogallol tannins are best neutralized by aluminum sulfate. Thus, this formula meets the highest requirements in aging resistance, which the international CRAFT-Standard set for self-dyeing of restoration leather. The sumac leaves, as the main component of this tanning, are crushed in special ceramic grinders to guarantee absolutely no contamination from metals. The purity of all added chemical additives and their very precise implementation in this tanning is done to the very highest quality of German standards.

• Fatliquoring ditto.

article segment Art.- No. 139 (traditional alum-tawed goat restoration leather, old Alum- tawing formulation)

http://www.feinleder-hoffmann.com/en/articles/article-range/goat-leather/alum.html

• Pure white tawing as traditional “old-alum tanning” with approximately 19% aluminum salts (aluminum sulfate Al₂O₃) which is applied in a highly viscous pulp consistency comprising of approximately 4% egg yolks, about 12% flour or sugary bran, and approximately 20% salt calculated in pelt weight to the still untreated, depilated, completely decalcified pelt which is very well bated. The approximate values of the mixed pulp consistency may vary and depend on the current density in °Bé (Beziers grades). The pelts rest in this system (in a tanning drum/or reel) for 5 to 8 days. During this procedure, they are mechanically moved at certain intervals, to accelerate the penetration of aluminum sulfate (AL2O3) into the cross-section of the skin. Only then, for the improvement and fixation of the tanning effect, it is made alkaline by adding alkalis and extending the tawing fleet with water in the moving system. After three hours of moving the pelts in this salt mixture, tanning is completed. The white tawed pelts are then suspended without prior washing for slow drying at 40° C in well-ventilated and clean rooms. The dried leather is still somewhat stiff and hard and has to be lubricated afterwards on the flesh side with a fine, mineral solids-free, anionic, synthetic oil dissolution. After repeated putting up of the leather to air it on the toggle frame, it is mechanically staked and made softer by grinding it from the back.

ditto. article segment Art.- No. 410 (traditional alum-tawed pig skin restoration leather, old Alum- tawing as well)

ditto. article segment Art.- No. 430 (traditional alum-tawed calf restoration leather)

Summary occur for Archive-Spezial® formulations for calf leathers and goatskins. These are unhaired in pure CaOH2 liming except our current leathers for modern bookbinding, interior and portefeuille (fine- leather goods).

Our Archive-Spezial® leathers for purposes in restoration- and conservation are totally free of free- acids which may influence any O2 oxidation for red- decay in the leather fibers to negative in different worldwide climates.

As there is no standard testing procedure for the analysis of free acids in materials (leather).

In the absence of a specified method, the standard testing that we could currently offer includes:

·         Heavy metals via ICP
·         pH (which we could perform before and after ageing)

With regards to ageing, it would be advantageous to understand the environmental conditions that these leathers would typically be exposed to i.e. in the Vatican library, in order to optimise the ageing parameters.

Be advised that we are discussing this testing request with peers; regarding how we could potentially determine this and develop a suitable technique.

It may be possible to adapt the ASTM D6410-99(2009) method even in our modern times and standards, which is the determination of acidity of vegetable tannin liquors.

You will still find our so called modern standard book- restoration German heifer leathers in many colors with still better technical results as follows:

https://www.feinleder-hoffmann.com/en/articles/article-range/aniline-dyed-heifer-leather.html 

                                                                                                                    TS Stand 05/2019

archival leather / product specification