The Consolidation of Powdery Leather Surfaces Evaluation of the Penetration Capability of Klucel® by Fluorescence Staining Autohrs: Barbara Nydegger (atelier verso, b.nydegger@atelierverso.ch), Prof. Elke Menzel (University of Applied Sciences Bern), Dr. Anke Mondschein (FILK Freiberg Institute gGmbH), Dr. Nadim C. Scherrer (University of Applied Sciences Bern), Prof. Dr. Karolina Soppa (University of Applied Sciences Bern)
leather with powdery surface
insufficient consolidation
successful consolidation
Fig. 1: Problem of the consolidation of powdery leather surfaces: If the consolidant (blue) does not fully penetrate the powdery zone consolidation fails and causes a flaky surface.
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Introduction Powdery leather surfaces are a common degradation phenomenon observed with vegetable tanned leathers. In humid environments they tend to discolour and become brittle. The most popular consolidant applied in this context is Klucel® (hydroxypropylcellulose). It is soluble in organic solvents which cause less or no reaction with the leathe compared to water-based solutions. Klucel has been tested on its stabilising effect, ageing behaviour, flexibility and tackiness of the surfaces upon application, and always performed good or very good (Cains, 2005; Kourgierakis, 2013; Mahony, 2014; Müller et al., 2018; Pesch, 2020). Yet studies examining the penetration capability have been missing, though this is fundamental for a sustainable consolidation effect (Fig. 1).
Klucel G 1% in isopropanol 3x
Klucel G 1% in ethyl lactate 3x
Fig. 2: Comparison of a leather treated with Klucel G 1% in isopropanol (left) and ethyl lactate (right). The solution with ethyl lactate penetrates deeper and spreads better than the one with isopropanol.
Method Several factors to manipulate the penetration were considered in this study: - The viscosity of the consolidant was varied by using different degrees of polymerisation (E vs. G) and different concentrations of the glue (0,5% 2%). - Two solvents were compared: The commonly used isopropanol versus ethyl lactate, which is likewise polar and comparable in terms of the octanol-water partition coefficient (IPA: 0,05; EL: 0,06), but differs in the vapor pressure (IPA: 43 hPa (20°C); EL: ca. 2 hPa (20°C)) (Pesch 2020). - Applications were carried out with a micropipette for reproducibility and with an airbrush, as is the authors chosen method when treating objects. - One to five applications were compared, with and without time for the substrate to dry inbetween. Depth and homogeneity of the consolidant across the penetration zone within the leather were examined by adding 0,01% Rhodamine B to the solution and observing thin sections of the treated leather with a fluorescence microscope (filter: excitation BP 573-613nm; splitter filter LP 510; emission BP 512-542nm) (Soppa 2018)(Figs. 2 & 3). Further tests examined the stability of the leather after consolidation with sticky tapes (M3) and a brush and wether discolouring is apparent by eye or with a spectrophotometer.
powdery area 540μm
Klucel E 2% in ethyl Lactate 3 applications without drying inbetween
Klucel G 0,5% in ethyl Lactate 3 applications without drying inbetween
Klucel G 1% in ethyl Lactate 3 applications without drying inbetween
Fig. 3: Cross-sections of leather with a thick powdery layer under the fluorescence microscope. On the left, the powdery zone is indicated. The three images on the right show the extend of the consolidant (red fluorescence) in powdery leather sections, treated with different kinds and concentrations of Klucel in ethyl lactate. In this case, only Klucel E 2% reaches the necessary depth of penetration for successful consolidation.
Results and Conclusions - The lower the viscousity, the deeper the the penetration of the consolidant (Fig. 3). The less viscous, the more stable becomes the treated area. - The higher the concentration, the more discolouration is observed (Fig.4). - Concentrations lower than 0,5% Klucel G and 2% Klucel E respectively do not achieve sufficient stabilisation of the leather surface. Concentrations higher than 1% (Klucel G) respectively 5% (Klucel E) can not be applied with an airbrush. - Solutions with ethyl lactate exhibit better penetration (depth and lateral distribution)than those with isopropanol (Fig. 2). The former also cause less discolouration (Fig. 4). - Deeper penetration can be achieved with multiple applications as long as the substrate is still absorbing and not dryed. - With more applications more stability can be achieved, yet more discolouration is observed (Fig. 4). Within the frame of this study, the following tendencies were observed: If the leather has a thick powdery layer and deep penetration needs to be achieved, 3-5 wet in wet applications of Klucel E 2% in ethyl lactate are recommended, even though light darkening of the leather may occur. If the powdery layer is thin, about 2-3 wet in wet applications of Klucel G 1% in ethyl lactate achieves similar stability with darkening.
Fig. 4: Surface colour changes in differently treated areas: The consolidants Klucel E and Klucel G were applied three (3x) and five (5x) times with an airbrush without drying inbetween. On the very left, pure water (H2O) was applied. It was observed, that the choice of Klucel (E or G) did not affect the intensity of discolouration. Yet the higher the concentration and with every further application, darkening of the leather increased. Pure water caused the worst darkening and showed, how strongly vegetable leather can react to humidity. Ethyl lactate caused less discolouration than isopropanol.
References: ● Cains, A. (2005). Conservation of leather bookbindings: a mosaic of contemporary techniques - binding solutions to old problems. In M. Kite & R. Thomson (ed.), Conservation of leather and related materials (S. 225–230). Butterworth-Heinemann. ● Kourgierakis, J. (2013). Stabilisierung und Konsolidierung von degradiertem vegetabil gegerbtem Einbandleder. CICS – Cologne Institute of Conservation Sciences. ● Mahony, C. C. (2014). Evaluation of consolidants for the treatment of red rot on vegetable tanned leather: The search for a natural material alternative. University of California, Los Angeles. ● Müller, J., Pataki-Hundt, A. & Brandt, S. (2018). Festigungsmittel zur Behandlung von abgebautem Leder. Conference: Fachgespräch der Nordrheinwestfälischen-Papierrestauratoren. ● Pesch, K. (2020). Evaluation von Lösemitteln bei der Festigung von abgebautem Leder mit Klucel. CICS – Cologne Institute of Conservation Sciences. ● Soppa, K. (2018). Die Klebung von Malschicht und textilem Bildträger: Untersuchung des Eindringverhaltens von Gelatinen sowie Störleim und Methylcellulose bei der Klebung von loser Malschicht auf isolierter und unisolierter Leinwand mittels vorhergehender Fluoreszenzmarkierung-Terminologie, Grundlagenanalyse und optimierungsansätze. Hornemann Institut HAWK, Hochschule für angewandte Wissenschaft und Kunst.