Blue mould staining???

[Rowan the Bruce]

Hi all,

 

I've been playing around with an alaskan 36 inch mill for the year and some of the wood has been getting blue mould. I dont have a kiln but have recently been put on to a chap who is running a home made one. Any tips on keeping this stuff away would be good as it isn't as nice to look at as a perfect grain.

[Tony Croft aka hamadryad]

is it oak that is staining?

[skyhuck]

It could be steel or iron in the wood.

[agg221]

The phenomenon that Hamadryad and Skyhuck are referring to is a dark blue/black stain within the wood, which is visible when you first mill it, formed by a reaction between the tannic acid present in some woods (notably oak and sweet chestnut) and iron, usually from nails or bits of fencing.

 

However, what you are referring to is called "blue stain" which is a mould growth on the surface fairly rapidly after milling. This is more likely to form on species without high tannin content.

 

There are several things you can do. First option is to paint everything with borate solution straight after milling. This is a durable inorganic fungicide, and stops the mould growing.

 

Second option which is a bit trickier is to mill only in clear, bright but cool weather - usually September through to April, and lay the boards out up off the ground out of direct sunlight, rather than stacking them immediately, turning every day or so. After a few days, time depending on temperature, light levels etc, you will see a subtle change in colour on the surface as they start to oxidise. In most pale species it's a slight yellowing; oak first loses that pinkish tinge, then goes very pale and then yellows a bit. If you stack the boards when they start to get that slight oxidation colour then they are much less wet at the surface and don't develop mould, however you have to be very careful to avoid either surface checking or case hardening in the process, which is why you have to keep them cool and in the shade.

 

Borate is much easier!

 

The other factor to consider is your stickers. They really do need to be very dry, as they will trap water against the surface of the board which then leads to local development of blue stain, known as sticker stain.

 

Hope this helps.

 

Alec

[Tony Croft aka hamadryad]

The phenomenon that Hamadryad and Skyhuck are referring to is a dark blue/black stain within the wood, which is visible when you first mill it, formed by a reaction between the tannic acid present in some woods (notably oak and sweet chestnut) and iron, usually from nails or bits of fencing.

 

However, what you are referring to is called "blue stain" which is a mould growth on the surface fairly rapidly after milling. This is more likely to form on species without high tannin content.

 

There are several things you can do. First option is to paint everything with borate solution straight after milling. This is a durable inorganic fungicide, and stops the mould growing.

 

Second option which is a bit trickier is to mill only in clear, bright but cool weather - usually September through to April, and lay the boards out up off the ground out of direct sunlight, rather than stacking them immediately, turning every day or so. After a few days, time depending on temperature, light levels etc, you will see a subtle change in colour on the surface as they start to oxidise. In most pale species it's a slight yellowing; oak first loses that pinkish tinge, then goes very pale and then yellows a bit. If you stack the boards when they start to get that slight oxidation colour then they are much less wet at the surface and don't develop mould, however you have to be very careful to avoid either surface checking or case hardening in the process, which is why you have to keep them cool and in the shade.

 

Borate is much easier!

 

The other factor to consider is your stickers. They really do need to be very dry, as they will trap water against the surface of the board which then leads to local development of blue stain, known as sticker stain.

 

Hope this helps.

 

Alec

 

very informative post:001_cool:

[Rowan the Bruce]

Thanks for that Alec, yeah it was in Cedrus deodara and Pinus sylvestris (very wet) and the stickers are very dry. Everything is a learning process.

[Rowan the Bruce]

Anyone know if the above borate solution is normal borax powder with water and what the ratio it says 5% for woodworm etc so maybe thats about right?

Order Woodworm Treatment Products - Boron based Woodworm Treatments

Although that seems a bit pricey compared to:

intralabs | Borax

[Pete Bannister]

Hi Rowan, you're talking about conifers and so the blue stain (if its blue stain fungi and not iron stain) would be associated with the sapwood. In conifers, Ceratocystis spp. are the usual culprits. The stain is not actually a 'stain' as such but is caused by the presence of dark (usually brown!) hyphae within the wood cells. These fungi are using the starch rich contents of the wood cells, rather than the cell walls themselves. Infection may occur prior to conversion, in which case there's not much you can do about it other than perhaps restrict it spreading. If it’s occurring at the point of conversion then as previous posts advise, you should look at the milling and storage environment. The fungi grow best in warm temperatures (20 deg C +). At 10 deg C or below growth is very slow. Timber moisture contents in excess of fibre saturation (say 28% w/w) are required; so rapid drying of fresh cut surfaces will go a long way to controlling blue stain. Rapid can achieve this. Kiln drying is the surest way to prevent fungal activity of any sort. However, it can be managed simply by the management of storage age and hygiene

The financial loss due to blue stain fungi and significant the world over. There is no shortage of academic study and industrial collaborations to address the matter. Alec has already given you some sound practical advice

I’ll make another post with some opinion on the use of Boron/ Borax etc. I hope not to sound like an activist (because I’m not one honest!); I’m no expert; however, I’ve been commercially / professional involved in the subject for over 25 yrs and thats slowly formed my views on the matter

cheers

Pete

[Pete Bannister]

Rowan,

Chemical surface treatment is widely used on green timber. Boron in the form Sodium Octaborate Tetrahydrate is commonly used at 5% - 10% and is effective. However, consider this: you are introducing a pesticide into the production of a natural material. Boron wood preservatives have just had to be re-classified in the EU as 'Toxic' because of evidence related to mammalian toxicity ability to cross the placenta and an association with male infertility). Formerly they were classified in the UK as 'Hazardous' and I can remember when they were introduced for preservative wood treatment in the UK in the 1980's they were widely billed as 'green' or 'environmentally friendly' etc. Whilst this may be the logical option for you, you should in my view consider that boron is not chemically fixed to the timber following application. So wetting may leach it out. Where does it go? Also, when the wood is further milled, what happens to the waste? It now contains a pesticide. I'm not advising that you don’t use boron (I very occasionally I specify it myself) and its true that some timbers are at least as ‘toxic’ in the context. However, it’s important to only employ it where there is no other practical option; and if you do use it you should look at the wider ‘life cycle’ of the product (i.e. where does it go to eventually)

There has been a suggestion that a broad-spectrum pesticide is used. E.g. one that kills both insects and fungi. There’s a simple rule when using pesticides (and it is part of your statuary obligations) and that is to target the pest. If the problem is fungal then that’s the target. Woodworm (insects) are not the target. Broad-spectrum wood treatments will usually contain more than one pesticide. For Woodworm Permethrin or Cypermethrin are often found in formulations. Both are extremely toxic in aquatic environments and so you must consider what happens to run-off and to waste.

cheers

Pete

[agg221]

Hi Pete, interesting post.

 

I've got some current professional involvement with one of the major wood preservative manufacturers - we're trying to develop a non-biocidal alternative on their behalf, so are also working with TRADA and some of the major end users.

 

The fundamental issue is that the very factors which make timber an attractive option at the same time make its use a direct competition with nature - as trees have evolved, so have agents of decay as a fundamental lifecycle and you're trying to stop the process for as long as possible.

 

The factors which generate decomposition/degradation are environmental, and as you highlight relate to temperature and moisture content. The options for preventing it are therefore either to control the environment or to use chemical inhibition (biocides). Environmental approaches are ideal where possible, but they do require extremely tight control to be exercised when crossing from conditions which promote decay (usually the high moisture content when first milled for example) to conditions which inhibit it. In the commercial environment, kiln drying is a highly successful approach - the operation at a mill somewhere like BSW Timber in Carlisle for example is staggering in its efficiency, and facilitates transfer to a kiln kept at optimum conditions (which are sufficiently high in temperature to kill latent fungal infections) within less than an hour of a log entering the saw line. They achieve this through economy of scale - there's a log going in about every 30 seconds, 18hrs a day.

 

For the hobby miller, a different approach is required!

 

The use of a biocide is an effective approach to controlling the 'difficult' stage. It's also useful in controlling any other issues that may arise during the drying process, due to inability to control the conditions to closely.

 

Considering types of biocide - the highly persistent ones are inorganic while the organic ones tend to degrade over time. The other choice is fixed or mobile biocides. The fixed inorganic type are ideal for permanent exposure to environments where there is a risk of decay (class 5 would be the technical definition). The best of these was copper chrome arsenate (CCA) which is now banned. Other copper-based treatments, and some zinc-based have previously been used successfully, and are chemically fixed, but they have virtually been withdrawn. One issue with fixed type preservatives is that they are only operational within the region of the timber which they penetrate during initial treatment. Results suggest that subsequent development of surface checks requires that a minimum of 5mm penetration is achieved, which requires pressure treatment unless repeat applications are to be made at regular intervals throughout the life of the structure and often difficult to access the critical region which is usually concealed, which is why it remains sufficiently wet to promote decay.

 

The advantage of the non-fixed biocides, such as those based on boron, is that they remain water soluble so are taken further into the structure in the presence of water, which makes them long-term active in the event of irregular exposure to decay conditions. The down-side is that they leach under permanent exposure, so they aren't useful for class 5, more class 3 or 4. Practically speaking, OK for bits of house or shed, no good for fence posts!

 

As you point out, the big issue is end of life disposal. One of the biggest problems with CCA is that when the timber is eventually broken down (decay or combustion usually) it results in leaching of arsenic and chromium compounds. Copper and zinc based products are less severe in their effect, but the presence of copper results in highly toxic dioxin formation during combustion under the wrong conditions.

 

Fixed organic biocides are usually rather chemically reactive, usually through radical formation, which means they tend to have relatively short lifetimes. The advantage is that they do not usually have the same problems in terms of end-of-life disposal of the timber structure as they tend to break down either in service or through combustion, but the down side in terms of the lifetime of a structure is that they tend to require re-treatment, which is often not possible in the critical regions. The durability of modern softwood fence posts is highly questionable for this reason - I have spoken to some manufacturers who have significant concerns. Whilst this is unlikely to result major issues, childrens' play equipment is a very real problem.

 

In practical terms, a relatively short-lived fixed organic biocide would be the ideal choice for milled timber which is to be post-processed, since the biocide will chemically decompose (possibly accelerated by combustion) without leaching, and will last until the timber is sufficiently dry to inhibit decay. If it is to be used in an indoor (class 1 or 2) environment it will then be fine.

 

In an exterior environment, the ideal solution is to use naturally durable species such as oak, sweet chestnut or cedar.

 

However, the use of boron-based preservatives, with good understanding of their method of action, is probably one of the most effective approaches currently available in respect of its combination of effectiveness, aesthetics and toxicity.

 

Sorry for the very long post!

 

 

Alec