Modern adhesives have made it possible for amateur carpenters to construct wooden furniture which in the past only skilled craftsmen could have tackled. Work which would have once needed precise joints can now be joined simply with strong wood adhesive – using cramps to hold the wood in place while this is drying. You can choose from a variety of different cramps according to the wood size.
The term glue properly refers to pure animal or vegetable glue. Other types, which are resin based, are known as adhesives. The success of your gluing depends on choosing the right kind of glue for the job from the many different types available.
Choosing the right adhesive
The glue or adhesive you use will depend on the type of wood you want to join together, the surface conditions, the kind of stress the join will be subjected to and whether a temporary or permanent join is wanted.
Pay particular attention to the temperature in the workshop. Most adhesives need a warm atmosphere in which to set. Make allowances too, for hard or resinous woods: these require a sap-resistant adhesive.
Polyvinyl Acetate (PVA)
PVA adhesive is among the most useful for general-purpose woodwork. Available ready mixed in easy-to-use plastic bottles, it is applied directly to the wood which is then cramped. At normal room temperatures, the setting time varies from 30 minutes to three hours depending on the brand used.
Although PVA is a strong adhesive and does not stain timber, it is not completely waterproof and will not adhere very readily to resinous or very hard wood such as teak. It is not suitable for outdoor work.
Diluted with 20% water, PVA can be used to apply wood veneers. Damp the veneer with a sponge and apply the PVA evenly over the surfaces with a brush. Position the veneer then place a sheet of brown paper on the top and iron over it with a medium iron.
Urea formaldehyde adhesive
This is a good adhesive for hard, resinous woods. Its gap-filling properties also make it ideal for gluing loose-fitting joints – such as those found in furniture making and repairs. It resists moisture better than PVA.
This woodwork adhesive provides the greatest strength and is the most water resistant, so it is ideal for outdoor work and boatbuilding. It is also a good gap-filling adhesive.
When adhesives have to be applied to timber treated with preservative. Use urea formaldehyde or resorcinol.
Animal glue (Scotch glue)
This is old fashioned glue made from animal pelts and bones. It comes both in cake or granular form and in liquid form. The cake or granular varieties need to be melted down in a glue pot with water at a temperature of 65 C and used while still hot. These do not stain timber and dry to a medium brown colour which matches most polished or stained woods. They are not waterproof.
Unlike the granular variety, liquid animal glues can be used cold. Again they are not waterproof and do stain some woods – especially oak – so excess glue should be wiped off with a damp cloth before it can do any damage to the wood.
Animal glue has largely been made obsolete by more modern adhesives but is still the best to use when constructing frameworks for upholstered furniture. The stresses which the frame undergoes while being upholstered may cause adhesive-made joints to fracture.
For mending or renovating antique furniture, use an alburnum (sapwood) based Scotch glue. This has the same expansion and contraction rate as old-fashioned Scotch glue, which is different to that of modern adhesives. Never mix old and new glue on one piece of furniture.
These glues are now quite difficult to get and have largely been replaced by urea formaldehydes. Casein glues are made from milk and are available in powder which you mix with water. They are water resistant when set but stain a lot of timbers.
Synthetic resin cements
This type of adhesive is almost completely waterproof and comes in a variety of forms – powder, semi-liquid or two parts (adhesive and hardener). It can be used for outdoor work and small boat-building. The setting time depends on the temperature of the surroundings – the warmer the air around the workpiece the faster it sets.
Impact (or contact) adhesives
Wood stuck to wood with this type of adhesive tends to move after a time. It is therefore not strong enough for furniture making and is better suited to fixing laminates and tiles.
The surfaces to be stuck together are both coated with the impact adhesive and are then left to dry separately according to the manufacturer’s instructions. Afterwards, the two surfaces are brought together to make an instant, strong bond.
Impact and contact adhesives are the only types which do not require cramping while they set. But as a bond is formed instantly, you should ensure that the mating surfaces are aligned.
Almost any material can be stuck with epoxy adhesives but for woodworking, PVA and formaldehyde adhesives often prove to be both cheaper and easier to use. Epoxies are made up of two parts – a resin and a hardener – which must be mixed together. Setting time can vary between one hour and 24 hours.
Extra care must be taken when using these new ‘super glues’. Manufacturers claim that one small spot of the adhesive and a little pressure will stick almost anything to anything in just a few moments. They will certainly stick skin to skin permanently, so be careful and make sure that children do not get anywhere near them.
Many adhesives are highly inflammable and some also give off toxic fumes. Make sure that you work in a well ventilated space and that there are no naked flames about.
Anyone who is prone to dermatitis should wear gloves or a barrier cream as glues contain skin irritants. If the glue should get on to unprotected skin and cause burns or an allergy rash, seek medical advice. If it gets in your eyes, wash it out with plenty of warm water and, again, seek medical advice.