Restaurierung Rönsch

Scientific examination

To begin with the types of wood used for the bureau were determined.  As there was only replacement work to be done on the bottom of the bureau and on the cylinder slide only this part was determined microscopically (The wood was determined microscopically based on Grosser, D. (1977). Die Hölzer Mitteleuropas. Berlin, Heidelberg, New York: Springer.). It turned out to be pinewood (Pinus sylvestris L.) for the bottom and pearwood (Pyrus comunis) for the slide. The type of wood of the veneer is cherry (Prunus avium). The surface was determined employing various techniques. First of all it was examined using ultraviolet light (The ultraviolet lamp used: UVAHAND 250 GS Fa. Hönle, uv-technology, Gräfeling/München), which showed that the majority of the varnished surface has an orange fluorescence that is typical of natural resins, especially shellac. The areas that had already been conspicuous during the optic examination by differing in gloss level and colour showed a blueish-milky fluorescence that indicates a modern nitrocellulose lacquer. Inside the corpus as well as on the outside of the bottom there are large patches of a substance with whiteish fluorescence which leads to the assumption of a rework using generous amounts of modern glue.

The different areas of the surface were then tested regarding their solubility in order to find a solvent to clean the varnished surface. The best results were achieved with aqua dist., the varnish with the orange fluorescence is ethanol soluble which supports the assumption of it being based on natural resins. In the areas that showed blue fluorescence the lacquer is stable to the common solvents such as aqua dist., ethanol, white spirit (100-140°C), isopropanol, Shellsol T. To further determine the latter finish a test for cellulose nitrate was performed that had a positive result (A scrape sample of the lacquer was dissolved in a few drops of amyl acetate in a micro test-tube. Some diphenylamine crystals were added. Slowly a drop of concentrated acetic acid was poured into the mixture, instantly initiating a blue staining.). Lastly cross-section samples of an area with orange fluorescence as well as one with blue fluorescence were analysed microscopically. Both showed only one layer of clear varnish with an orange respectively blue fluorescence. This indicates that the areas that have an orange fluorescence are likely to still be the original varnish whilst the areas that have a blue fluorescence were completely replaced. This applies to the moulding on the right side of the corpus and to parts of the black balustrade.     

Restoration

The first and most important step was to remove the fungal infestation, which was taken off with a vacuum cleaner equipped with a HEPA filter (Muntz-Museums-Staubsauger 555 MU E HEPA). The areas of raw wood were then cleaned with a mixture of water and isopropanol (3:7). To have better access to the inside of the bureau the back panel was removed by sewing through the modern glue that held it in place. Afterwards the constructional work that mainly concerned restoring the smooth running of the cylinder in its slide was done. As there was no possibility to dismantle the cylinder the needed replacement had to be done on the slide. The slide was then cleaned and then reinforced with a fitted replacement made from pearwood onto which some wax was applied. The stoppers on the bottom of the writing surface were replaced as well. As a result the cylinder was sitting firm in the slide and running smoothly again. The area of woodworm manifestation was first evened out with little rods and then replaced with a fitted replacement in pinewood. It was stained with the water-based stain Körnerbeize (Nussbaumbeize by Kremer Pigmente) to match the surrounding wood. To diminish the mechanic stiffness of the large drawers the bottoms of these were firmly glued afresh. The removal of the back panel of the bureau allowed to take out the little racks containing the secret drawers. That way the drawers could be removed, the rack was cleaned from excess modern glue using acetone gel and glued together anew minding the drawers to fit in. Loose parts such as the moulding on the right and left were glued down as was the loose veneer. The glue was massaged into cracks of the slightly warmed veneer which was then pressed down for two hours with hand clamps and plexiglass interleaving Hostaphan foil. This was opened to remove the excess glue and then pressed down again over night. The missing skittle-shaped ornaments were replaced with ones that were turned taking the remaining one as model. They were stained with a mixture of Körnerbeize (Nussbaumbeize by Kremer Pigmente) and Irgasperse (a water-soluble metal complex dye by BASF) and finished with orange shellac (by Kremer Pigmente). Hide glue (skin and bone 1:1) was used for bonding.

With all the constructional work done the work concerning the surface was next. The entire surface was cleaned with aqua dist., in some areas saliva was used additionally, thoroughly afterwards cleaning with aqua dist. The matte layer was taken off with white spirit (100-140°C). To achieve an even gloss level micro-crystalline wax polish (Renaissance Wax) was applied in some areas. The result was the nice, semi-gloss finish of an aged shellac varnish. Several layers of orange shellac (by Kremer Pigmente) were applied to the replacements with unsuitable colour and gloss level and burnished afterwards using abrasive linen (Micromesh, grain 4000-12000, by CS Cybernetic-Systems GmbH).

The marble paper was restored by first consolidating it with wheat starch paste. This type of glue was chosen due to its long processing time and its elasticity. The paste was prepared as a 40 % solution and heated in a bain-marie frequently stirring until transparent. After cooling down completely the paste was applied under the paper using a spatula, diluting it with a small amount of water. It is important to use just as much water as needed to make the paste processible while avoiding stains due to water spotting. The paper was pressed down using blotting paper to immediately absorb any excess water. In areas of decolouration, mainly on the edges of the paper, shellac (bleached and dewaxed, by Kremer Pigmente) was applied. These areas were then neutrally retouched with watercolours (by Schmincke). The textile loops were replaced with a cotton ribbon stained with Irgasperse (by BASF) to fit the remains of the original loops.