BCD 2017

166 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 7 heating to remove any excess water in a vacuum evaporator, drying trays and moulds. From c1925 onwards drying machines were used to accelerate the process. Having undergone a lengthy, but controlled manufacturing process, the final products can be manufactured as cubes, pearls/granules, sheets or in powder form. Characteristics and defining differences Hide glues are both immensely strong and reversible/re-treatable when in contact with moisture/water. Their strength can be partially explained by their triple stranded helix coils being stretched during preparation for gluing by soaking in cold water, melting and simmering at 55–60°C and the subsequent rapid cooling/ drying phase, during which the strands re-coil. ADHESIVES for the conservation of fine furniture NORBERT GUTOWSKI T HIS ARTICLE provides an introduction to the use of glues and adhesives in the conservation and repair of historic furniture and cabinet making. It focusses on three aspects of this subject: historic adhesives, including their manufacture and role in fine furniture making; principles for selecting adhesives for conservation and repair work; and an overview of the adhesives commonly used in the conservation and repair of fine joinery today. ANIMAL GLUE Collagen-based animal glues have been widely used as adhesives, binders and fillers for nearly all organic and inorganic materials. Their use can be traced back to ca 3000 BC. Well-known examples include bone glue and hide glue, also known as Scotch glue. Glues have also been, and continue to be made from tendons, cartilage and connecting tissue, while more refined glues are produced from isinglass (a substance obtained from the swim bladders of fish) and gelatine (extracted from a range of animal sources). The initial stages of manufacturing bone glue involve degreasing crushed stock by high speed agitation in warm water, then passing the ‘glue liquor’ through 4–8 connected pressure cookers successively. After it has been steamed under pressure (15/20psi) for a set time, the glue, by then becoming higher in concentration, is extracted in ‘runs’ and any loss replaced with fresh water. Although this process is relatively straightforward, the high temperatures needed degrade the physical properties of the collagen and make the glue relatively weak from the onset. By contrast, the raw material for manufacturing hide glue, mostly bovine skins, comes from tanneries. It is placed in large vats of milk of lime (water and calcium hydroxide) and left for several weeks. To remove the lime, the skins are treated with hydrochloric or sulphuric acid and washed many times, until they are pH-neutral. In extraction pumps with fresh water and increasing heat, runs are ‘skimmed off’, producing glues of various strengths from gelatine to glue. The final stage of the manufacturing process for both bone glues and hide glues involves filtering and further controlled In furniture, this ability to contract has made hide glues excellent for constructional and decorative work, often without the need for pressure from clamps or presses. However, their weakness is that they have poor gap filling properties, and they can become brittle and discoloured over time. There are many possible causes of partial or complete failure of work or repairs made using glue. These might be because of poor manufacture, hurried preparation or being used too soon without reaching a certain degree of ‘maturity’, combined with inherent weaknesses and flaws in design, poor selection of basic substrate materials (wood), poor execution of work, glue starvation, inadequate environmental conditions or mould and insect infestation. Also, one type of glue cannot necessarily cater for all materials, each of which has its own behavioural characteristics. By the same token, well-built fine, period or new furniture made using animal-based glues, if kept in a stable and controlled environment of 16–20°C, 45–65 RH, moderate exposure to UV light and regularly monitored, will last for a very long time. There are plenty of good, surviving examples to demonstrate this fact. GLUE AND CONSERVATION The main aim in conservation is to minimise the number of treatments needed during the lifetime of an object by following an ethical and professional code of practice. As such, new glues and adhesives for conservation work have been carefully sourced or specifically formulated and are age-tested for stresses, embrittlement and colourfastness. Solvents which are used to undo joints should ideally be fast-acting and volatile to leave no trace and avoid damage to the object. The consensus among conservators today is that it is better to make use of a whole palette of glues and adhesives rather than relying on just a few. There are glue systems that complement each other, such as animal glues used as a barrier layer followed by epoxy resins. An example of this can be seen in the case study below. Where inherent strength has gone for good, many of the losses of original material need to be replaced with bulking A 19th-century gothic revival mahogany hall chair with a broken leg – see case study opposite (All photos: Norbert Gutowski)