FIGURE 17B.-Transverse section of a 3-inch elm plank showing "interior dote." The rot represented by the white areas surrounded by dark lines is seen only in the central part of the plank. The faces of the plank give no indication of the rot within. The scars show evidence of some injury caused by lightning, fire, "bird peck," or other agencies. In this particular sample there is a possibility that the infection entered these wounds in the living tree noted that the fungi under test showed definite reactions in their growth relations to the amount of moisture in the wood. For instance, a blue-stain fungus developing on pine sapwood exhibited no signs of growth below a moisture content of 20 per cent (ovendry weight basis), stained the wood heavily at 65 per cent, and at 90 per cent again showed few signs of development. (See Table 2.) Apparently the lower moisture contents gave insufficient water with FIGURE 18.-Transverse sections cut from 6/4-inch sap gum boards improperly steamed and then air seasoned for about one year. Note the central dark areas which represent the blue-stain infection. The faces of these boards before resawing were bright and free from defect a large amount of air, the optimum or middle range furnished the favorable amount of moisture, while at the highest moisture content there was too much moisture and too little air. From the data thus far accumulated (Table 2) it would seem safe to assume that at and below a moisture content of 20 per cent of the oven-dry weight, there will be no danger of the development of many of the various sap-staining fungi on any wood species, and FIGURE 19. This shows a peculiar type of sap stain, called "interior blue stain," in the center of inch and a half western yellow pine stock. The stock was air seasoned and because of unequal drying the center area remained sufficiently moist to favor the development of blue stain. The grounds for complaints when such material is resawed are obvious consequently no staining. This means that stock properly dried to a low moisture content and kept dry will be free from stain. Moreover, proper air-seasoning or kiln-drying to a low moisture content insures against subsequent staining during the storage or shipment of the stock, provided the stock is kept dry. TABLE 2.-Relation between sap-stain fungi and moisture content of wood (ovendry basis), including some wood-rot fungi for comparison Heat treatments remove the moisture necessary to the growth of the mold plants, and if sufficiently high temperatures are used the fungi or spores present on the wood are killed outright. Chemical dips, such as solutions of soda, bring about changes in the sap which either make it an unfavorable medium for the development of molds or are poisonous to them. Poisonous chemicals check the growth of the sap-stain fungi or kill them. Rapid removal of the moisture. from the surfaces of boards by means of the best air-seasoning methods leaves a dry surface condition unfavorable both to the germination of the spores and to the further development of the plants. THE EFFECT OF SAP STAINS ON WOOD PROPERTIES STRENGTH PROPERTIES Molding and staining do not greatly affect the strength properties of wood. Blue stain is not an early stage of decay. However, the value of the product is often lowered for use where discolorations are objectionable, or where wood is to receive a "natural" finish. On the other hand, there is no reason for condemning blued stock without a hearing if it is free of decay and is to be covered, stained. |