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Achieving the recommended retention levels of chemical is a common challenge for wood treaters, and one that is exacerbated by the effects of a changing climate on the structure of timber.However, the challenge can be addressed by careful quality control. In this article, our analytical chemist and lab manager Navi Moodley unpacks the problem and explains how to achieve optimal retention.There are several factors influencing the extent to which your timber retains wood preservative chemicals during the treatment process. Drier climactic conditions, which have affected many parts of our continent recently, result in trees with a greater proportion of heartwood and a lower proportion of sapwood than periods of normal rainfall, explains Navi. Poor soil also leads to a greater proportion of heartwood.The sapwood ratio is an important factor affecting retention because wood preservative chemicals can penetrate sapwood but not heartwood (for further discussion, see http://www.dolphinbay.co.za/about-us/news/unpacking-the-sapwood-ratio). If your timber has a greater proportion of heartwood than you anticipated – compared to times of higher rainfall, for example – your retention levels are bound to be lower, unless your calculations take the new sapwood ratio into account.“Instead of 50% heartwood there might, for example, be 60%, which will have a considerable impact on your calculations,” says Navi. “The solution to the problem is to do all the necessary measurements and calculations accurately.”The first step towards is to work out the proportion of sapwood in your timber using a sapwood-heartwood indicator. This should be done regularly, or at least whenever there is a change that might affect your timber, including a change of season, timber species or supplier. If you do not have sapwood indicators, Dolphin Bay can provide you with them.The sapwood ratio is proportional to solution uptake, and enables you to calculate the maximum solution uptake – the volume of solution that your wood can absorb – using treatment charge sheet data.Individual pieces of timber will have differing solution uptakes, and therefore this value is expressed as an average per unit volume of timber. The relevant SANS require treaters to take two sample logs, and work out the solution uptake from three portions of each log.Taking into account the solution uptake, and the recommended retention levels, you will be able to calculate the solution strength that will ensure adequate retention.Of course, if your solution strength is not accurately worked out this, too, could also result in inadequate retention.In order to help our clients achieve proper retention levels, Navi has produced a technical report which explains the relationship between retention, solution uptake and solution strength, and also provides the necessary equations, along with examples of the calculations.The report also contains a table of minimum solution strengths for various retentions at different solution uptake values.“Our aim, in producing the report, is to brings clarity to a relatively technical issue,” says Navi. “We hope it helps those of our clients who are re-assessing their retention levels.”Your Dolphin Bay sales representative will be pleased to provide any other information you require.