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April : How to achieve optimal retention

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19 April, 2017

How to achieve optimal retention

Navi Moodley


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 chemicals during the treatment process. Drier climactic conditions result in trees with a greater proportion of heartwood and less sapwood than periods of normal rainfall, explains Navi. Poor soil also leads to more heartwood.

The sapwood ratio is an important factor affecting retention because wood preservative chemicals can penetrate sapwood but not heartwood (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, your retention levels are bound to be lower, unless your calculations take the change into account.

"The solution to the problem is to do all the necessary measurements and calculations accurately," says Navi.
The first step  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 using treatment charge sheet data.

Taking this into account, along with the recommended retention levels, you will be able to calculate the solution strength that will ensure adequate 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.

The report also contains a table of minimum solution strengths for various retentions at different solution uptake values.

Your Dolphin Bay sales representative will be able to provide you with more information should you require. Alternatively, our technical representative will be able to assist you onsite.

Source: Dolphin Bay Chemicals


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