Modification of wood with formulations of phenolic resin and iron-tannin-complexes to improve material properties and expand colour variety

European beech wood (Fagus sylvatica L.) was modified with thermosetting phenol–formaldehyde (PF) resins, which were blended with various additives (ferric chloride and a mixture of ferric sulphate and hydrolysable tannin) to stain the material. The aim was to stain the wood dark and to enhance the moisture-related properties as well as the weathering performance. The weight percent gain (WPG) and solution uptake were only slightly influenced by the composition of the resin formulations. All treatments induced high bulking, dimensional stability and reduced the maximum swelling of the wood. Particularly at high relative humidity the equilibrium moisture content (EMC) of the modified wood was clearly reduced compared to the control. All formulations darkened the wood colour and enhanced the colour stability. The formulations containing ferric sulphate and tannin caused the darkest colour and the highest colour stability in the course of weathering. The modification with all formulations clearly reduced the capillary water uptake compared to the control. The capillary water uptake increased due to weathering. The increase was highest with the control, followed by the samples modified with tannin formulations and those containing only PF as sole phenol component. It is concluded that addition of iron salt as well as tannin can be used to stain the wood dark without negatively affecting the effects of modification with PF resin.