Preservative treatment: processes, penetration and species treatability

Preservative families, in broad terms

The portal's article groups preservatives in broad families. The principles matter more than product names, because approved products change.

• Tar oils (creosote): heavily restricted in Europe.
• Organic-solvent and water-based organic biocides: used in low-pressure treatment.
• Borates: effective but non-fixed, so they leach and suit only above-ground use.
• Copper-based water-borne systems: widely used for pressure treatment.

The article's sources show that efficacy and leachate behaviour do not always move together. Do not read this lesson as a list of approved products; check current approvals.

Treatment processes

• Full-cell process: preliminary vacuum, filling, pressure, final vacuum. Normal for water-borne solutions.
• Empty-cell processes (Rueping, Lowry): no initial vacuum; deep penetration with limited retention.
• Double vacuum (low pressure): air evacuated, vessel flooded under vacuum, a second vacuum removes excess. One manufacturer's guide states that such timber cannot be used for use class 3.2 and 4, suits class 2 and 3.1 with a maintained coating, and needs a maximum moisture content of 28 % before treatment.
• Dipping and coating do not penetrate deeply. Pressure treatment is the industrial method, mainly for pine.

The FAO manual (1986, global, dated) says wood must be dry before pressure treatment and recommends below 30 % MC, with usual pressures of 800-1400 kPa. Plan the drying line and the treatment plant together.

Penetration versus retention

Penetration is where the preservative is in the wood; retention is how much of it there is. They are separate conditions. The European standard EN 351-1 classifies treated wood by penetration; it is a classification, not a treatment specification. The only class the portal verified is NP5, full sapwood penetration, used in a Nordic certificate for Scots pine. In the Nordic system, NTR/AB covers sawn and planed wood up to 38 mm thick (or up to 50 mm thick and 125 mm wide or less) and corresponds to use class 3.2 per an industry body, and NTR/A covers all other dimensions and corresponds to use class 4.

Treatability of the main species

This table is a compilation, not standard classes. Pine illustrates the key idea: because the sapwood takes preservative and the heartwood does not, full sapwood penetration is what matters, and pine is the standard treated softwood.

Spruce takes only a limited amount of preservative, so treated spruce protects less than treated pine. Small-sample laboratory work on spruce and larch sapwood found that incising (mechanically or by laser) or bioincising can raise uptake. These are laboratory results, not plant settings.

Worked example: can this load go to the double-vacuum plant?

A customer wants spruce deck boards for a roofed terrace with good drainage, and a second batch of fence posts for ground contact.

1. Boards: use class 3.1, kiln-dried to 16 % MC. 16 % is under the 28 % limit and double vacuum suits class 3.1 with a maintained coating. Acceptable, subject to the plant's authorisation.
2. Posts: use class 4. Double vacuum is stated not to be suitable. They need a deep-penetration pressure process, and spruce is a poor choice, since it is hard to treat. Pine with full sapwood penetration is the natural material.

Contract impregnation checklist

If you buy treatment as a service, agree:

• species, dimensions and moisture content limits (30 % and 28 % appear in the sources);
• identification of sapwood material for pine;
• incising or dimension limits for spruce and larch;
• drying time after treatment;
• certification (marks are tied to a certified site and species);
• who is liable for what: the use class is the customer's requirement, treatment quality is the treater's.

Key points

• Preservative families have different leaching behaviour; borates suit only above-ground use.
• Double vacuum suits use class 2 and 3.1 only; ground contact needs deep-penetration pressure treatment.
• Wood must be dry before treatment; 28-30 % MC limits appear in the sources.
• Penetration and retention are separate; for pine, full sapwood penetration is the goal.
• Pine sapwood treats well; spruce and larch heartwood are hard to treat.