BCD 2017

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 7 167 INTERIORS 5 and/or reinforcement agents such as glass fibre or carbon fibre rods and fabric mats of the same materials. For the conservation of fine furniture, the following principles should be followed: • retain as much of the original material as possible • adjust new to old material, not old to new • develop a thorough understanding of an object before treatment commences • record all processes as a log of your own work and to inform that of future conservators. There are many variables which need to be taken into account when selecting glues and adhesives, these include: • toxicity, hazard to health and environment • molecular weight and compatibility • re-treatability • mixability and workability • wet and flow characteristics and coverage • open time, working time and drying time • elasticity/flexibility • colour and colour-change • storage • cost. The tables opposite give a short overview of glues and adhesives and their use in conservation and/or fine furniture making. CASE STUDY: USE OF ANIMAL GLUE ON ITS OWN AND AS A BARRIER LAYER A 19th-century mahogany hall chair (illustrated opposite) was received with one rear leg detached. A broken half tenon was still lodged in the mortice of the detached leg (illustrated overleaf). The adhesive that had been used was Cascamite (Polymite) which is recognisable by its light yellow colour and crumbly appearance. No commonly used solvents are strong enough to dissolve it, and in this case steam, which is often the most effective method, could not be used due to the risk of damage to the surrounding surfaces. Instead of steam, wet strips of cotton rag were pushed into the mortice and wrapped around the tenon. After 15–20 minutes the Cascamite could then be prised off with dental picking tools. The broken end section of the tenon was reunited with the remainder of the tenon using strong but thin animal glue. The end of the tenon was tapped with a light-weight hammer to ensure that the wet fibres would interlock. When fully cured and dry, two 2.5mm holes were drilled through the end of the tenon at a slight angle towards each other for better anchorage, and 25mm holes were drilled into the rear seat rail. Carbon fibre rods were then inserted and their ends filed flush. To prevent absorption of the epoxy filler, thin animal glue was applied to all surfaces as a barrier layer. When almost dry, Bencon was added where losses had occurred. Remaining areas were coated with full strength animal glue. The leg was reattached and held in place with padded blocks and a light-weight sash clamp. Standing upright on a flat MDF board the chair was stood between two pine battens for alignment and weighted down. Excess glue was removed and the chair left to dry for 48 hours. SOLVENT-BASED GLUES Type Suitable for Advantages Disadvantages hide glues, high strength, undiluted fine woodwork very strong; like others, hide glues can be adjusted in strength to suit a given situation need to be prepared carefully; can shrink, become brittle and discolour hide glues, high strength diluted barrier layers allows epoxy-based infills to be removed easily at a later date can weaken an otherwise strong joint, so other reinforcement may be necessary cold hide glue (modified) joints and small repairs to woodwork ready to use as dispensed from a bottle; long open and working time long drying time; preservatives (often unspecified) may be harmful rabbit skin glue gesso for gilding very strong, yet retains flexibility; very little discolouration needs to be prepared carefully; can cause voids under gilded surfaces hide or rabbit skin glue flakes delicate surfaces at risk of being discoloured through glue spillage flakes can be pushed under loose and lifting veneers, heated and weighted down difficult to control excess glue mix of hide and rabbit skin glue, rosin, linseed oil and water; known as compo mouldable, decorative material strength and flexibility becomes hard and brittle due to ingredients used fish glue delicate assembly work long open time long drying time isinglass delicate and gilded surfaces greater tack, low viscosity brittle and highly moisture sensitive SOLVENT-BASED ADHESIVES Type Suitable for Advantages Disadvantages Ethylene Vinyl Acetate (EVA), similar to PVA, but does not contain plasticiser repairs to paper, cardboard and leather good penetration for light- weight materials can cause swelling or discolouration Lascaux 304/498 repairs to paper and paper lining extremely elastic; can be used as contact adhesive none known Lascaux 4176 consolidation of gilding low viscosity, excellent penetration sometimes too weak Plextol B 500, aqueous acrylic dispersion lining of paper or canvas high viscosity, good for penetration and consolidation of fibrous material; dry adhesive may be used as heat- activated film can cause swelling or discolouration Paraloid B72 with ketone solvents consolidant, binder and coating viscosity/strength can be adjusted to suit; ‘quick release’with acetone only suitable for light repairs Polyvinyl Acetate (PVA), an aliphatic synthetic thermoplastic polymer containing a plasticiser general woodwork and jigs; where reversibility is less of an issue low viscosity; can be diluted to suit; semi- transparent when dry normally cures irreversibly, but can be softened with steam Titebond general and fine woodwork ease of use, dries within 30 minutes, full hardness after 24 hours grips quickly so adjustment can be difficult Araldite standard and 5-minute epoxy resin and hardener, 1:1 repairs to metal useful where heat (eg soldering) would compromise colour or function drying times vary depending on medium used Araldite 2020 epoxy resin and hardener wood, metal and plastic clear resin but can be dyed to produce specific colours long drying time Akemi epoxy resin and hardener repairs to marble and stone good where high strength is needed in relation to weight long drying time Bencon 20 epoxy resin and hardener, 5:1 consolidant for large areas of weakened timber through rot or woodworm low viscosity; almost colourless may not be viscous enough to penetrate deep galleries or rotted wood Bencon 22 epoxy paste, resin and hardener, 1:1 decorative loss replacements light and dark brown in colour; cold setting can dry out when stored for too long, best to buy in small quantities HXTAL NYL-1 resin and hardener, 3:1 glass and ceramic repairs crystal clear, formulated to match refraction index of glass; long open and working time long curing and drying time Polyvinyl Butyral (PVB), available as white powder and miscible with IMS fibrous material, eg woodworm galleries high to low viscosity depending on concentration needed; effective as consolidant very short working time West System epoxy resin and hardener, 5:1 areas where larger quantities of epoxy are needed low viscosity; almost colourless; medium open and working time medium drying time NON-LIQUID ADHESIVES Type Suitable for Advantages Disadvantages BEVA adhesive film lining for paper, cardboard, fabric or similar light- weight material can be cut to precise shape and reactivated with a tacking iron; reversible with hexane or acetone none known