Evaluating preservative systems

Service records

The only completely reliable method for evaluating a preservative system is to observe its performance in actual service. Service records are therefore very valuable in confirming the reliability of an established preservation system but alternative techniques are required to realistically evaluate a new preservative system during its development and before its commercial introduction. All acceptable evaluation techniques attempt to reproduce conditions which have been observed in practice to represent severe deterioration risks. There is thus a danger that these evaluation systems can be too exacting, imposing unnecessarily severe

performance requirements on preservatives which may have been designed for use in far less onerous conditions in actual service. For example, a preservative for the carcassing or framing wood of buildings may be required to give protection against fungal decay, resulting from occasional leaks or condensation, or perhaps give protection only against wood-boring insects, and such a preservative does not need to be assessed for performance in severe ground-contact conditions.

Performance classification

Preservatives are classified in this book into four basic groups, shown in detail in Table A.2 in Appendix A, in order to take account of these various hazards. This classification system is not standard but is similar in many respects to the Nordic system, and the required retentions quoted in Appendix A can be readily related to similar classification systems operating throughout the world. Class A refers to wood in normal ground-contact conditions such as transmission poles, fence posts, railway sleepers (ties), piles and structural foundations. It can also be considered to refer to wood immersed in fresh water as in river defence works and even cooling towers, although in the latter case there is an increased risk of Soft rot attack, indicated in the Danish system by a sub-classification Class AS. Such a subdivision is unnecessary as it is now recognized that even ground-contact conditions introduce a risk of Soft rot damage.

Class B refers to building and construction wood which is not in ground contact but which is still subject to a moderate risk of decay through accidental leaks or condensation. In many respects the risk is the same as for Class A except that significant leaching conditions are not present. This class includes building carcassing and framing, as well as joinery (millwork) and cladding. Class M refers to preserved wood for marine conditions but applies only when there is a risk of attack by marine borers, particularly

gribble Limnoria species. Class I applies only when there is a risk of insect attack, particularly by House Longhorn beetle, Hylotrupes bajulus, in temperate areas and Dry Wood termites in the tropics. Preservatives meeting this classification generally conform with the Australian quarantine requirements which are designed to prevent the introduction of new wood-borers to Australia. These four classes thus define the most important deterioration risks that preservatives must be capable of withstanding. There are several other service situations which need to be considered such as stain, Pinhole borer and Powder Post beetle control treatments that are used in the forests and mills, but these do not involve standard evaluation techniques.

A Class A preservative in normally assessed throughout the world by the performance of the preservative in actual ground contact in stake trials (Fig. 3.17). Generally, the stakes are

comparatively small in cross-section, typically about 50×50 mm (2×2 in), in order to exaggerate natural leaching and the deterioration damage. It is usually considered that the performance of a preservative can be judged reasonably reliably after a period of about five years. Obviously the time factor ensures that this system cannot be used during the development of a preservative.

Laboratory tests

Preservative development normally involves exposure of relatively small blocks of wood to cultures of single fungi in laboratory conditions with decay assessed by the weight loss after a period of perhaps 12 to 16 weeks. This principle is used throughout the world, the tests varying only in the medium on which the fungus is cultured; in the British, Dutch and German systems the fungus is cultured on malt agar

FIGURE 3.17 Simlangsdalen test field in Sweden, one of the sites used to assess Class A preservatives for the Nordic approval scheme.

Evaluating preservative systems

whereas in the Nordic and American systems the fungus develops on a small untreated block of wood resting on soil (Fig. 3.18). This technique is used for evaluation new toxic chemicals; the test blocks are impregnated with different solution concentrations so that a toxic limit or threshold can be established between the concentrations at which the blocks just decay and those at which the blocks are just free from decay. In this way the preservative activity of various toxicants can be compared and concentrations proposed for their use. When complete preservatives are assessed in this way considerable care is required in diluting them in order to define their safety factor, or the amount of dilution that can be tolerated before decay occurs. Although these tests involve increasing dilutions of the preservative solution and results are therefore obtained as toxicant or formulation concentration, it is normal to take account of absorptions and report the results as retentions in kilogrammes per cubic metre kg/m³ or pounds per cubic foot lb/ft³.

Test fungi

The choice of test fungi is also important; for example, Poria species are tolerant to copper and should always be used when a preservative formulation contains this element. Generally the test fungi are defined in the appropriate national standards. No attempt is made in this book to describe these standards in detail as they are being continuously revised and it is always advisable to obtain the current standard from the appropriate national authorities.

Weathering resistance

Laboratory block tests can also be used to assess the performance of a preservative after weathering by leaching or volatilization. If a product has good weather resistance and a wide spectrum of activity against the test fungi it can be considered to be a realistic candidate for a full stake trial, although if it is meant to meet only Class B requirements a block test may be considered adequate in many countries, as in the FIGURE 3.18 Laboratory methods for assessing the efficacy of preservatives against Basidiomycetes: (1) a miniature soil and wood block technique suitable for rapid and inexpensive product development tests; (2) the soil and wood block technique used in the American standard test; (3) the malt agar and wood block technique used in the British, German and Dutch standard tests; (4) the soil and wood block technique used in the Nordic standard test. (Penarth Research International Limited)

Nordic system and in the British system for the evaluation of preservatives for treating joinery (millwork).

Insect borers

If a preservative is also intended to give resistance to insect borer attack, further tests are necessary against the most appropriate species, these tests usually being carried out in the laboratory. In Europe the House Longhorn beetle represents the greatest danger to structural wood yet protection is also required against the Common Furniture beetle which generally has a greater tolerance to preservatives. Performance against termites is also frequently assessed, the subterranean termite, Reticulitermes santonensis, usually being considered the most important species in Europe.

New preservatives

Normally a new preservative is first assessed against a single Basidiomycete fungus such as the Brown rot, Coniophora puteana, and if it proves effective at apparently economic retentions the test is then extended to further fungi such as White rot and Soft rot as appropriate to the intended use of the preservative. Leaching and insect borer tests

follow so that, ultimately, comprehensive information is available which clearly establishes whether the new preservative system is likely to be reliable in service. Some tests are unrealistic in the laboratory such as the ground-contact stake test and assessment tests against marine borers; these tests are best carried out in natural conditions where there is known to be a particular hazard. In all these assessments it is normal to compare a new preservative system with a well-known established system, in order to confirm that the tests are realistically severe and in order to provide direct comparisons for commercial reasons. Unfortunately the establishment of preservation reliability represents only a small part of the the time, cost and effort that is required today to establish a new preservative system, health and environmental evaluation being much more difficult. It is therefore not surprising that new preservatives are rarely introduced; only the largest companies and consortia can afford to develop new products today, and most new systems are simply adaptations, based as far as possible on established knowledge and experience. This is unfortunately a situation which encourages the retention of existing products, even if they would not be acceptable if submitted for approval today, a situation that actively discourages the development of more effective and safer products.