Will tannin marks on green oak penetrate over time?

[Harrris303]

Hi everyone, I just picked up some fresh-sawn oak posts for a good price on gumtree, with the aim of storing them up to season and use in future. A lot of them have blue tannin marks persumably from where they were sawn, and some have little spots dotted around as well. I'm just wondering if the tannin stains will penetrate deeper into the timber over time if I just leave them as they are. Would it be a good idea to skim them off first before stacking them?

Any thoughts greatly appreciated. Thanks very much.

 

[Woody2Shoes]

Hi - I don't think the stains will get worse. I think the answer to 'what to do' depends on what you plan to use the timber for eventually. If the ultimate aim is to use them outside then they'll weather down to a uniform greyness over time anyway. If you're going to use them indoors, and you're not going to plane the timber first after it's dried out a bit, then it might be worth removing the worst of the blue/black bits (the trick will be to do that without using steel tools and creating more stains when the moisture content is high!), but I really don't think it matters. If you're very fastidious about the stains you can use oxalic acid to get rid of them. Cheers, W2S

 

[Harrris303]

Hi Woody thanks for the quick reply. Yes I couldn't really think why the stains would spread but I've learned never to assume anything!! So I thought I'd see what other people reckoned. I don't have a particular plan for the posts really, just nice to snap up a bit of cheap oak when you come across it! They are quite nice bits of timber actually so if they season nicely then I might end up making windows out of them or something in a few years, but they're 3x3" so I'd be taking quite a lot off them anyway if I did use them like that.
Thanks for your input. I don't think I'll worry too much about it then, and maybe just keep an eye on them as they dry. Cheers.

 

[Woody2Shoes]

If you're stacking them somewhere dry it might well be worth painting the ends with some old emulsion paint, or something similar, so the rate of drying is a bit more steady. Where is the 'pith' i.e. the centre of the tree - is it in the centre of each post - or are the posts 'centre-free'? If they contain the centre of the tree you may well see 'checking' as the posts dry out (which may prevent you using them for fancy joinery but won't affect their structural strength). Cheers, W2S

 

[Harrris303]

I didn't see any taken from the centre of the tree actually so that's good. A couple had a bit of sap wood here and there, and one had a slight waney edge but other than that they're pretty tidy so fingers crossed...
That's a good suggestion about the paint though. They're going to be stacked in quite a dry room so I'll definitely give that a go. Thanks very much.

 

[brianhabby]

Another vote to leave the stains until you know what your are going to use the wood for. If you clean them up now, you will almost certainly introduce your own staining when you work the wood as the tannin is likely to react with any contact with iron or steel tools. Especially if you use a water based glue and steel cramps... DAMHIKT...!

As Woody says, Oxalic Acid will get rid of the stains when you're ready.

regards

Brian

 

[dickm]

Is the BLUE staining an indication of anything more serious? Have never seen it on oak, but softwoods with blue staining are usually pretty ropey.

 

[Woody2Shoes]

I think that mid/light blue staining in random patterns on softwood may indicate mould growth (or tanalith or similar preservative treatment), but I think dark blue/black stains in 'man-made' sort of patterns on freshly cut oak is nearly always tannin stains. Cheers, W2S

 

[RossJarvis]

I've found green oak stains purple to black a lot where iron/steel has been in contact. Planing or sanding will clear it off as it doesn't go in deep. I'd leave finishing till when you use it. If you've not used green oak before, the splits that turn up as it dries may look more worrying, if the posts are 4" the splits will be up to 1/3" and can go from one end to the other. Plus they probably won't be straight after long.

 

[Harrris303]

I've actually used green oak much more than seasoned oak. Spent the last year helping a friend build big frames for a barn conversion in oak he milled up himself not long before we started. The marks are definitely just the normal staining you get from working the wood with steel tools. I said blue but it does really look black on the wood itself to be fair. When you get it on your hands you see it has a blue/purple tinge, a bit like ink, and it stays there for days no matter how hard you scrub!

I only started the thread because I haven't really done much regarding seasoning etc. Wasn't sure if there were long term worries about the staining because we've always sanded any marks off once the frames are built. Just my laziness making me wonder if I could get away with leaving them as they are for now, and that sounds like the general consensus so happy days haha. As for the cracking I'll just have to see what happens. They came pretty cheap so it's no big issue really. There's always a use for a bit of oak!

Thanks for all the input guys. Cheers.