Wood surfaces

Michael Pearce <michael.pearce [at] scotland__gsi__gov__uk>

>Would anyone have an opinion on treating the surface of wood
>destined for display in cases in museum conditions? We are going to
>display a number of very old oak carvings which have apparently
>never been waxed or painted. It is not our intention to apply a
>surface treatment for any aesthetic reason; we would be interested
>to hear if there is a non-aesthetic argument for applying any
>surface coating for the sake of preventive conservation.

Surface coatings are normally applied to wood to protect it from
water and other solvents and also for aesthetic reasons, to saturate
the colour.  If environmental conditions are being controlled
surface coatings are unnecessary.

The effects of incorrect water activity on wood are two-fold with RH
which is either too high or too low.  Under specific circumstances
many wood species can decay rapidly.  Damp conditions, RH
consistently above 70%, especially with poor air circulation,
encourages fungal activity and wood boring insects. Too low RH will
normally cause shrinkage and possibly other distortions.  If the
structure is constrained in any way i.e. the wood is not free to
expand or contract with a changing moisture content in the air, then
either splits can occur, or case hardening can be induced with the
crushing of wood fibres. Splits can lead to further unanticipated
distortions as wood not previously exposed to air starts to lose
water. Also the modulus of elasticity varies directly with water
content.  Therefore in very dry conditions wood can become more
brittle.

The other side of the coin is that if you avoid too damp or too dry
conditions then wood is a very durable material, in fact it will
last indefinitely.  My experience is that most wooden objects show
no appreciable deterioration if kept within 45% to 65% relative
humidity, and especially if the environment is stable.

Ian Fraser
Conservator of Furniture
Leeds Museums and Galleries
Temple Newsam House


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                  Conservation DistList Instance 14:62
                   Distributed: Tuesday, May 15, 2001
                       Message Id: cdl-14-62-005
                                  ***