Impregnation of wood and the like

A method of impregnating wood with preservatives in a treatment solution in which the wood and treating solution are subjected to a schedule including cycles of alternate phases of high pressures and low pressure wherein the pressures in the high pressure and/or low pressure phase are changed in succeeding cycles whereby the differential of pressure between the low limit of the low pressure phase or the differential of pressure between the mean of the low pressure phase and the mean of the high pressure phase is progressively increased or decreased in succeeding cycles, or the limit of the pressure in the high pressure phase is progressively increased or decreased in succeeding cycles while the differential of pressure between the high limit of the low pressure phase and the low limit of the low pressure phase remains constant, with the variation of the pressure limits and/or the variations of the differential of pressures being controlled for ensuring that aspiration or absorption of a critical amount of air from within the wood cells is not exceeded before the wood is as deeply penetrated and as heavily loaded with the preservative as is desired.