Adhesives

For the Conservation of Fine Furniture

Norbert Gutowski

 

Mahogany chair with detached rear leg lying in front of it  
A 19th-century gothic revival mahogany hall chair with a broken leg – see case study below (All photos: Norbert Gutowski)  

This 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 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.

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.

The cleaned joint where the chair leg attached to the seat Close-up of underside of chair seat showing repaired tenon and filed heads of two carbon fibre rods
The chair leg after removing old glue and the broken tenon from the mortice The damaged tenon pinned in place with two carbon fibre rods

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 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 below give a short overview of glues and adhesives and their use in conservation and/or fine furniture making.

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
compo (mix of hide and rabbit skin glue, rosin, linseed oil and water) 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 lightweight 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 lightweight material can be cut to precise shape and reactivated with a tacking iron; reversible with hexane or acetone none known

CASE STUDY: USE OF ANIMAL GLUE ON ITS OWN AND AS A BARRIER LAYER

A 19th-century mahogany hall chair (top illustration) 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.

Furniture repair materials  
Glass fibre rods, carbon fibre rods and carbon fibre mat (from left to right) used for reinforcing broken and damaged furniture; used correctly carbon fibre is stronger than steel.  

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.

As this case study demonstrates, the conservation of fine furniture requires a sound understanding of the many glues and adhesives available. Where in the past many losses would simply have been replaced with ‘close-to-original’ material, today the conservator’s challenge lies in selecting the right repair materials to allow as much original material as possible to be retained while giving a sense of completeness.

 

 

The Building Conservation Directory, 2017

Author

NORBERT GUTOWSKI, programme tutor for furniture and related objects, has been teaching furniture conservation-restoration and making at West Dean College since 1996.

Further information

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