PREVENTION OF SAP-STAIN IN WHITE-PINE.

New Zealand Journal of Agriculture, Volume XXXVI, Issue 2, 20 February 1928, Page 118

PREVENTION OF SAP-STAIN IN WHITE-PINE.

TESTS BY STATE FOREST SERVICE.

C. E. Dixon,

Forest Assistant, State Forest Service, Wellington.

This article presents the results of a series of tests made by the State Forest Service with the object of preventing the sap-staining which occurs in the sapwood of New Zealand white-pine (Podocarpus dacrydioides) during seasoning. The work was carried out under the direction of Mr. A. R. Entrican, Engineer in Forest Products. The sapwood of most softwoods and of some hardwoods becomes discoloured under certain conditions. This discoloration — may be blue, green, brown, or red, depending on the species, ' method of formation, and duration of the discoloration—is known generally by the term "sap-stain.” The causes of this sap-stain are many, but are all included under the headings of (i) chemical action, and (2) fungal attack. Sap-stain caused by chemical action is in the main due -to the enzymes in the wood, and produces discoloration both in the sapwood and heartwood. Such stains give more or less permanent discoloration to the wood, causing degrading, and often resulting in financial losses. Sap-stain caused by fungal attack, however, is the most common one, and that affecting white-pine is known as “ blue-stain,” due to the blue discoloration occurring in the timber, and is caused by several genera of fungi, mainly Penicilliufn and Cladosporium, which feed on the soluble sugars present in the sapwood. These fungi do not confine their attention to the surface, but penetrate the sapwood, and may, if conditions are suitable, completely permeate it.

A large economic loss is apparent every year as a result of degrade in the timber caused by these sap-staining fungi.' It is estimated that an average depreciation in. value of 5 per cent, results in whitepine alone from sap-stain losses, and this is equivalent to some per annum. As white-pine is the staple wood used for the construction of butter-boxes in New Zealand, the close bearing of this matter on the dairy industry, becomes evident.

Laboratory investigations by Dr. J. S. Yeates indicated that borax was the best and most suitable fungicide for sap-stain. As a result the present commercial tests were instituted, using a borax dip as the sap-stain preventive. In addition, methods of piling were also incorporated in the study to determine their effect as control measures. The commercial experiments proved very satisfactory, and it was demonstrated that the borax-dipped timber, stacked in approved fashion, is almost entirely free from the attack of sap-stain fungi. Correct open piling of even untreated timber was also proved to minimize the sap-stain considerably. The cost of treatment is estimated to vary from 2d. to 3d. per 100 ft. B.M., depending on the size of the mill and methods of treating the timber.

Conditions suitable for Growth of Sap-stain.

The development of sap-stain fungi depends on four factors —a supply of air containing the essential. element, oxygen; the requisite amount of moisture ; a favourable temperature ; and the necessary

food substances. Fungi require oxygen for their growth, and even : under storage conditions the supply from the air is ample for their propagation. Stagnant air containing a considerable amount of moisture is favourable to the growth of fungi, in that it prevents the drying of the wood. The extent of growth of sap-stain and mould fungi is largely dependent upon the amount of moisture present in the wood. Tests already carried out in New Zealand by the Forest Service have proved that the moisture content of green sapwood of all softwoods is considerably more than 100 per cent, (based on the oven-dry weight of the wood). ■ Thus while drying under normal conditions it is possible for the timber to retain for a considerable time the amount of moisture suitable for the development of fungi. If, however, the timber can be surface-dried by sufficient air-currents the growth of fungi on it can be rendered almost impossible. It has also been clearly established that the fungi grow most rapidly between certain limiting temperatures, .which, however, appear to include normal air-temperatures. Food is essential for the growth of the fungi, and this is obtained from the starches, oils, and sugars occurring in the green sapwood. When supplied with these essentials for growth fungi develop rapidly, and often reproduce abundantly. Deprived, however, of any or all of these factors, the fungi will cease to grow and 'will eventually die. Under normal air-seasoning conditions it is 'impossible to interfere with the factors temperature, air, and food. Moisture, however, can be regulated to a certain degree by varying the method of piling the timber and thus altering the air-circulation throughout the stacks. Under these conditions growth of the fungi, which, as stated, depends on the moisture. present, can be made partially dependent on the methods adopted in stacking the timber.

In addition, therefore, to treating the timber in order to kill the fungi, experiments were also carried out by altering the conditions of piling, in the hope that the conditions suitable for the growth of the fungi might be modified and the attack rendered impossible.

Material tested.

In carrying out the study it was decided to test as far as possible the type of timber most badly affected by sap-stain fungi. With this purpose in view, white-pine sapwood, I. in. thick, in random widths and lengths, was used throughout. In all 10,000 ft. B.M. was used.

As it had also been proved previously that boards in open-piled stacks become most seriously affected immediately under the fillets, all of the latter used throughout the tests were first treated with the borax dip to remove this factor from the study.

Tests to prevent Sap-stain

Two types of tests were carried out. The first, which consisted of a chemical treatment, was accomplished by dipping the timber in a saturated solution of borax in water (2 per cent, at normal temperature). The green timber on sawing was completely immersed in the treating-bath, immediately taken out again, drained, and stacked as shown in Fig. 1. The treatment thus given consisted of a surface wetting of the timber only. There was no penetration of the solution into the timber.

The second type, which consisted of a physical test, was carried out by varying the method of stacking, by which means the physical conditions most suited to the development of the fungi can be modified or removed. Two types of piling were accordingly adopted. In one the timber was block-stacked —that is, the boards were piled directly in contact and no air-spaces left anywhere in the piles. The width

of the piles was in., the height 16 in., and the average length 11 ft., although the latter varied, due to the boards supplied being in random lengths, the stacks thus having overhanging ends. A diagrammatic sketch of this type of pile is shown in Fig. 2.

In the other type of piling adopted the timber was open-box-piled — that is, each layer of boards was separated from the layer immediately above and below it by means of fillets. The latter consisted of 3 in. by 1 in. borax-dipped white-pine sapwood, placed on the flat and spaced 4 ft. apart down the length of the stack. The front fillets were placed flush with the ends of the stacked timber, but each succeeding fillet in the height of the stack was placed at a distance of in. forward •or in front of the fillet immediately beneath it, thus giving the stack a decided forward lean. These stacks had no overhanging ends, the pile being built up, as its name implies, in the form of a box. To accomplish this the stack was formed ■ from both ends, two boards always lying in the length of the stack, thus allowing the free ends of the boards to occur in the centre of the pile. This minimizes the staining which occurs on overhanging ends, as illustrated in Fig. .7. Additional air-spaces were provided when, due to their length being too short, boards forming the length of the stack did not quite meet. In addition, air-chimneys 6 in. wide, running throughout the length of the piles, were also provided. The finished stack, which is shown in Fig. 4, was 7 ft. wide, 6 ft. 6 in. high, and 22 ft. long, and contained approximately 4,000 ft. B.M. A section of the pile is shown in Fig. 3, and photographically in Fig. 4.

To facilitate inspection each of the boards in all stacks formed was marked with a consecutive number (commencing from the bottom of the stack), and also designated according to the stack to which it belonged. Thus “ C.U.” represents boards from the close-stacked untreated pile, “ C.T.” represents boards from the close-stacked treated pile, “ O.U.” represents boards from the open-stacked untreated pile, and “ O.T.” represents boards from the open-stacked treated pile.

Representative boards from each stack were examined every week for the first two months, and every month thereafter, for the purpose of determining the extent of stain occurring in each stack. ” Five representative boards were used for each close-stacked pile, and ten for each -stacked pile, and were chosen, as illustrated in Figs. 2 and 3, so as to be representative of all parts of each stack. These boards were sawn in. less in thickness, and were of less width than boards directly above them, in order that they might be removed and examined when required. They were further each designated with the letter “ R.” At the end of eight months the stacks were dismantled .and every board examined in detail for signs of sap-stain.

A further series of experiments has also been instituted to determine the effect of the treated timber on butter packed in boxes manufactured from it and exported overseas. As this test will not be finalized until a grading report has been received from London, it will be some time yet before the results will be available. It is confidently anticipated, however, that, as the treatment is only a surface one and butter-box material has to be planed before use, no damage will arise from the use of the treated material.

Analysis of Results.

When the working-plan was drawn up for the study it was laid down that the extent of the stain should be judged on an area basis,' and measurements taken . accordingly. In the test, however, due to the stain appearing in scattered spots and streaks, &c., as shown in Figs. 5 and 6, this was found impossible, and it was necessary to resort to descriptive methods and photography to obtain an idea ..of the extent of the stain. The nature of the stain was accordingly described as “streaked,” “spotted,” or “blotched.” Stain occurring in the form of an area in which the length was greater than four times the width was described as “ streaked stain ” ; that occurring in the form of an area more or less circular in shape but under in. in diameter was denoted as “ spotted stain ” ; while that occurring in the form of an area over | in. in diameter was described as “blotched stain.” The degree of the stain was also recorded, and was described as “ light,” moderate,” or “heavy,” depending purely on its appearance —that is, whether it was light, medium-coloured, or dark. On some boards a combination was found to occur, and in such cases was described accordingly.

Table 1 presents the results obtained from the closely stacked untreated pile. The staining in this pile commenced almost immediately on stacking, and by the end of a fortnight all boards examined had been lightly stained. During the succeeding seven weeks the staining became more intense and darker in colour, indicating that the stain was penetrating the timber. During the next four months, until the dismantling of the stack, there was little increase in the staining. . The results of the final inspection are tabulated in Table 5. Blotched stain,

which represents” the largest and deepest type of stain, occurred on 68-6 per cent, of [the boards, and is shown in Fig. 5. The stain in these cases was often Jin. in depth, and the timber was practically useless. Only 0-7 per cent, of the boards were free from stain. (To be concluded.)

Note.—Stack badly sap-stained.

(Five boards examined.) Date. "Sap-stain Occurrence. 1927. 25th March Stack formed. 2nd April Staining, nil. th April Staining,’nil. ' ’ ' Scattered, uneven, light-brown spotting, all boards. 16 th April Spotting become larger and darker. 23rd April ,, )) 30th April ,, >> 7th May Spotting now covering both sides of all boards. 14th May t . . Staining increased considerably ; all boards in stack becoming .very dark in colour. . 21st May very dark in colour. Spots going darker, but not increasing appreciably in extent. 28th May No further increase of stain. ' ■ ‘ 2nd July No increase in stain ; two boards turning yellow in places. 30th July No increase in stain. 27th August No increase in stain. ' ■ ' . ' >> 24th September ,, 27th October . . Slight increase in stain. ■■ •

Table I.—Occurrence of Sap-stain in Close-stacked Untreated Pile. (Five boards examined.)