Blackwood Stain.

[woodrat]

[Hi Bruce....Thanks for your experience with oxalic acid....I think vinegar would be the neutralising agent to use though....
)

Vinegar is also an acid, and hence will not neutralise oxalic acid. You would need to use an alkali.

Opps Of course ...Thanks Jeff...

[Nick]

If you are looking for something to neutralize an acid, use baking soda.

Cook on a high heat for 20 minutes then remove from oven and place on tray until cool. Then proceed with icing of your choice. Sorry Lillian couldn't help myself, welcome back, good to see your scrawny avatar grace these pages once again! And your sage advice is bang on as usual, we have bucket loads of the stuff on all our laboratory levels for just such occasions as an acid spill, which students are prone to do being students an' all.

[Lillian]

If you are looking for something to neutralize an acid, use baking soda.

Cook on a high heat for 20 minutes then remove from oven and place on tray until cool. Then proceed with icing of your choice. Sorry Lillian couldn't help myself, welcome back, good to see your scrawny avatar grace these pages once again! And your sage advice is bang on as usual, we have bucket loads of the stuff on all our laboratory levels for just such occasions as an acid spill, which students are prone to do being students an' all.

Medium heat Nick. High heat will make it too crispy.

Thanks, its good to be back.

[Trevor Gore]

Hi Bruce....Thanks for your experience with oxalic acid....I think vinegar would be the neutralising agent to use though....

Vinegar is also an acid, and hence will not neutralise oxalic acid. You would need to use an alkali.

If you are looking for something to neutralize an acid, use baking soda.

I think a quick chemistry overview is in order....

Blackwood goes "black" due to tannins, iron and an alkali environment. There is usually enough tannin and iron around in the native wood for an alkali environment to turn the wood "black" (where "black" also means the dirty green, brown etc. that blackwood and your hands will turn).

An acid environment (particularly oxalic) will turn the wood back to natural, too much will turn it reddish. If you neutralise with an alkali, like baking soda, the wood will change colour to "black" again. So don't neutralise with baking soda (unless you like that effect!)

If you want to retain the natural colour you have to "neutralise" whilst still leaving an acid environment, otherwise the wood just turns "black" again. Therefore you have to "neutralise' using a weak acid, like, for example, vinegar. i.e., you're just washing the residual strong acid off without risking going alkaline, which if you're in this situation is likely due to the alkalinity of the water supply you're using.

I've bent quite a bit of blackwood and never really had a staining problem. I use an old style bender with light globes as heaters, with stainless steel slats. The bit of water I use is highly filtered rain water (where I live, there is no mains water). The pH of the water is neutral. If you sweat on your wood, it will change colour. How much and what colour depends on the pH of your sweat.

[woodrat]

Hi Trevor, thanks for your insight...there is a lot to think about there:)

....crying over the side would be similar to sweating on it I would hazard to guess!

John

[jeffhigh]

So Not really neutralizing the Oxalic with the vinegar just redisolving it in a weaker solution to enable it to be wiped off?

Would it work as a preventive measure to add a dash of vinegar to any moistening water used during bending to keep the ph from becoming alkaline?

[Kim]

Medium heat Nick. High heat will make it too crispy.

Thanks, its good to be back.

Hey Lillian!!! Great to see you posting again. Your absents has been felt by all..welcome back mate.

Cheers

Kim

[Lillian]

Thanks for the welcome back. Its good to be home again.

[Trevor Gore]

So Not really neutralizing the Oxalic with the vinegar just redisolving it in a weaker solution to enable it to be wiped off?

Yes, but you don't necessarily have to let the oxalic acid dry. Put the oxalic on, wait until the colour is what you want, then get it off quickly. If you know your water is neutral-acidic, you can wipe off with water, because there will always be a little oxalic left to keep you safely on the acid side.

Would it work as a preventive measure to add a dash of vinegar to any moistening water used during bending to keep the ph from becoming alkaline?

Never tried it, but keeping things slightly acidic should stop the really dark stains.

[kiwigeo]

If you eat alot of oranges your urine becomes alkaline

[auscab]

Rhubarb leaves are high in oxalic acid, I just did a search to find out how strong, I've always thought it would be interesting to try on a stain. 0.5% !! thats strong.

interesting safety info as well.

Here is some of the link.


How toxic is rhubarb?
Oxalic Acid %
Leaf blade Petiole
Water soluble 0.46 - 0.51 0.23 - 0.32
Water insoluble 0.13 - 0.21 0.16 - 0.22
Total 0.59 - 0.72 0.39 - 0.54

From an MSDS (Material Safety Data Sheet) for Oxalic acid, LD50 (LD50 is the Median Lethal Dose, which is the dose of a drug or chemical predicted to produce a lethal effect in 50 percent of the subjects to whom the dose is given) in rats is 375 mg/kg. So for a person about 145 pounds (65.7 kg) that's about 25 grams of pure oxalic acid required to cause death. Rhubarb leaves are probably around 0.5% oxalic acid, so that you would need to eat quite a large serving of leaves, like 5 kg (11 lbs), to get that 24 grams of oxalic acid. Note that it will only require a fraction of that to cause sickness

And the link

http://www.rhubarbinfo.com/poison

[auscab]

Oh no, that’s half of 1% . I’ve got to be more careful around all you hair splitters .

[Trevor Gore]

BTW, if you're using oxalic acid, wear gloves, or you might be in for an unscheduled acid peel.

[kiwigeo]

An old thread but thought Id revive it as Ive just bent some blackwood sides and had some issues with staining.

Side 1 went into the bender wrapped in kraft paper and stacked stainless steel slat - wood - stainless steel slat - blanket - spring steel slat. Blanket got heated to 150degC and cauls were clamped down. Blanket went off for and hour and then on again for 5 minutes and then off for two hours before pulling the sides out of the bender. Wood was spritzed with demineralised water before wrapping in kraft paper.

Side 1 came out with moderate green staining on the inside and minor staining on the outside. Most of the stain came out after washing with oxalic acid.

Side 2 got bent the same way as side 1 but this time I wrapped the side in aluminium foil after a light spritz around waist area of wood. Result was much better than for side 1 with only one tiny patch of discolouration on the inside of the side which sanded out.

[charangohabsburg]

BTW, if you're using oxalic acid, wear gloves, or you might be in for an unscheduled acid peel.

I had success brushing on a saturated solution of edible, non poisonous ascorbic acid (also known as vitamine C).

It worked a treat on some spots that seemed to have been caused by tiny remains of a freshly sharpened saw blade. In such a case (of tiny solid steel particles still in the wood) it might be recommendable though to first sand an scrape the surface to get rid of the particles, or you might end up "washing the spots right through the wood", letting them appear on the other side of the veneer!

The following three pictures display the wood before, while, and after (after 15 minutes) partial ascorbic acid treatment:



Cheers,

[Kim]

Blackwood goes "black" due to tannins, iron and an alkali environment. There is usually enough tannin and iron around in the native wood for an alkali environment to turn the wood "black" (where "black" also means the dirty green, brown etc. that blackwood and your hands will turn).

That is the simple truth of it right there. The green/black discolouration has very little to do with slats, paper, or anything else. The two culprits, mineral deposits + alkaloid tannins are already 'in' the wood itself to varying degrees.

After wetting, those elements if present, will go into solution. When heat is then applied the solution is bought to the surface where oxidation occurs almost instantly causing the colour change. The more water applied, the more minerals and tannins will be dissolved and therefore the more pronounced the problem is likely to be.

Tannin leeching was a problem noted by the timber flooring industry with the introduction of water-based finishes and it is not restricted only to blackwood. As I recall from my reading a few years back, blackbutt and spotted gum are very prone as are a good number of the lighter colour European species commonly used for flooring in and around Germany.

The fact that this was a problem in Europe was actually a very good thing for the floor finishing industry in AU. It meant that all the research to resolve the problem was done for them. I think it was the German company 'Bona' who came up with a clear polyurethane (there's that word again) primer which effectively isolates these soluble elements within wood from contact with water contained in epoxy finishes while retaining excellent adhesion between substrate and the finish film.

Doing that solved the problem well enough to allow these wondrously tough abrasion resistant single and two pack water based epoxy floor finishes on 'any' type of wood without staining, even blackbutt....So how does this help you?? Well it doesn't just say'in tis all.

Mind you it does verify that, unfortunately, water and blackwood 'do' mix, but in an ugly way...so we do need to be careful. Distilled water won't make any difference because we now know the problem is 'any' water....So I wonder how things would go if you were to pre-coat the sides with Bona Prime Intense prior to bending?? Everything indicates that if you use as little water as possible when bending blackwood, you will minimise the undesirable affect. Then if you do strike a problem, at least it will not be so bad and should allow a milder solution of oxalic acid to be applied to rectify.

But to take things a step further I note that Nick had mentioned in his acetone/rosewood experiment how he had been surprised by the amount of water extracted in that process from what he thought had been quite dry wood. Also Bob Taylor's factory bending process does not incorporate water at all, he explains that his process "relies on the moisture trapped in the wood even when it is fully dry".

When you think about it, even wood with a reading as low as 8% moisture, the joinery standard for good stability, still holds quite a bit of water...errrm about 8% to be precise. So even at that point, every KG of wood is retaining 8 grams of water..That may not sound like very much but it will produce quite a lot of steam...The point is that even if you use 'no' water when bending blackwood, there remains every potential, all be it to a lesser degree, for mineral/tannin staining to still occur as these elements are taken into solution by this "trapped" moisture being released to the surface by heat.

If this Bona Intense primer/sealer can withstand the heat and maintain its good adhesion properties and continue to do its job of preventing contact between the mineral/tannins and oxygen in the atmosphere, it would presumably eliminate the problem entirely...it may also go a good way toward filling the grain too...just thinking out loud.

If anyone is interested I have attached a link to this Bona product. Of course Bona is probably not be the only company making this kind of stuff but the link should support the fact that tannin leeching is nothing new and is not restricted to only those of us who bend wood.

http://www.bona.net.au/Architects/Produ ... t=Coatings

Cheers

Kim

[kiwigeo]

That is the simple truth of it right there. The green/black discolouration has very little to do with slats, paper, or anything else. The two culprits, mineral deposits + alkaloid tannins are already 'in' the wood itself to varying degrees.

After wetting, those elements if present, will go into solution. When heat is then applied the solution is bought to the surface where oxidation occurs almost instantly causing the colour change. The more water applied, the more minerals and tannins will be dissolved and therefore the more pronounced the problem is likely to be.

My research indicates that reaction between tannins and iron does contribute to staining...not saying it is the only source of same.

[Kim]

That is the simple truth of it right there. The green/black discolouration has very little to do with slats, paper, or anything else. The two culprits, mineral deposits + alkaloid tannins are already 'in' the wood itself to varying degrees.

After wetting, those elements if present, will go into solution. When heat is then applied the solution is bought to the surface where oxidation occurs almost instantly causing the colour change. The more water applied, the more minerals and tannins will be dissolved and therefore the more pronounced the problem is likely to be.

My research indicates that reaction between tannins and iron does contribute to staining...not saying it is the only source of same.

It would be nice if you could provide some refferance Martin. Bona are one of the, if not the largest supplier of clear wood finishes on earth introducing waterborn coatings to the market in the late 70's

[kiwigeo]

That is the simple truth of it right there. The green/black discolouration has very little to do with slats, paper, or anything else. The two culprits, mineral deposits + alkaloid tannins are already 'in' the wood itself to varying degrees.

After wetting, those elements if present, will go into solution. When heat is then applied the solution is bought to the surface where oxidation occurs almost instantly causing the colour change. The more water applied, the more minerals and tannins will be dissolved and therefore the more pronounced the problem is likely to be.

My research indicates that reaction between tannins and iron does contribute to staining...not saying it is the only source of same.

It would be nice if you could provide some refferance Martin. Bona are one of the, if not the largest supplier of clear wood finishes on earth introducing waterborn coatings to the market in the late 70's

Alot of information off google plus consultation with my niece who's halfway through a chemistry degree.

Here's a clip from wikipaedia: "Tannins are incompatible with alkalis, gelatin, heavy metals, iron, lime water, metallic salts, strong oxidizing agents and zinc sulfate, since they form complexes and precipitate in aqueous solution."

And then there's this link where the use of tannin plus iron salts as black dyes by the Romans is discussed: "http://www.google.com.au/url?sa=t&rct=j ... f9JnnqssWw

Note that after bending up side 1 yesterday the kraft paper was covered with black lines that matched creases in the paper. I obviously used too much water as the kraft paper was damp. The black colouration and its presence on the outside of the paper suggests contact with the stainless steel slat it was pressed against was a factor.

Note that Im not disputing your original post...Im simply pointing out that using steel slats and water containing metals may contribute to some of the staining.

[Kim]

No offence Marty but I'll stick with what Bona suggests as being the root cause of the problem being discussing here. Bona have more in the game than anyone both in terms of R&D investment targeting this very topic and in terms of the market share they would stand to lose if their specialist did not get it right.

I do agree other factors such as interaction between metallic slats and minerals already in the water being applied to the wood as you are suggesting 'could' possibly come into play. But such a reaction would then stain 'all' types of wood and this discussion is specific to "Blackwood Stain" and blackwood stain, that black, green and dirty brown happens without anything but exposure to air on the surface of a freshly cut blackwood billet, but when its on the surface of a billet, its simply refer to it as plain old oxidation.

The bottom line is that 'water' is allowing this reaction to take place. So in terms of eliminating or reducing that reaction, the less water there is in the bending process the better....and as always, we cannot blanket this, because each stick of blackwood is going to differ from the next and some will contain more tannins than others.

Cheers

Kim

[kiwigeo]

You aint reading my posts Kim......Im not diasagreeing with oxidation with the tannins causing most of the colouration and the presence of water serving to worsen the issue by leaching out said tannins from the wood. What I am adding is that metal slats in contact with the wood or water in or on the wood can cause black colouration due to reaction between the metal and the tannins.

I do believe Ive actually proved this during my bending session when I noted a concentration of black colouration on creases on the OUTSIDE of the kraft paper...a surface in direct contact with a stainless steel slat. The black discoluration on the creases didnt match up with any of the minor black discolouration on the underlying wood so the reaction was happening on the outside of the paper. Unfortunately I threw the paper away so cant provide photos of this...youll just have to trust me on this one.

[Kim]

Note that after bending up side 1 yesterday the kraft paper was covered with black lines that matched creases in the paper. I obviously used too much water as the kraft paper was damp. The black colouration and its presence on the outside of the paper suggests contact with the stainless steel slat it was pressed against was a factor.

I don't know about that. To me the lines you describe indicate a double fold of paper to which a concentration of tannins in solution would naturally be driven by heat from the bending process. As these areas would be the very last to dry out, perhaps the stain you mention was all from the wood as the solution discussed had been wicked by these double folds and forced through the wet paper by steam at those points. If you still have the paper some images from both sides would be useful.

By the way, I think the reaction between tannins and wet metal is already well understood and I have read your post, but I reiterate that the topic of this discussion is "blackwood' stain, not simply wood stain. Also your using "stainless steel". I am not really understanding how this would be causing a reaction?

Cheers

Kim

[Paul B]

The first blackwood side I ever bent turned green. I'd wet it down and wrapped it in aluminium foil. The next one I wrapped in this new silicone baking paper (yes, silicone, but not THAT silicone) worked the same but without the green crap. But then the sides were mismatched - hours of sanding later...I turned out good. My understanding is that it is the tannins (tannic acids) the the wood react with metals, so you have to prevent hot wet wood contacting metal while bending.

[kiwigeo]

The first blackwood side I ever bent turned green. I'd wet it down and wrapped it in aluminium foil. The next one I wrapped in this new silicone baking paper (yes, silicone, but not THAT silicone) worked the same but without the green crap. But then the sides were mismatched - hours of sanding later...I turned out good. My understanding is that it is the tannins (tannic acids) the the wood react with metals, so you have to prevent hot wet wood contacting metal while bending.

I dont think aluminium reacts with tannins to produce discolouration.

[MBP]

Blackwood goes "black" due to tannins, iron and an alkali environment. There is usually enough tannin and iron around in the native wood for an alkali environment to turn the wood "black" (where "black" also means the dirty green, brown etc. that blackwood and your hands will turn).

That is the simple truth of it right there. The green/black discolouration has very little to do with slats, paper, or anything else. The two culprits, mineral deposits + alkaloid tannins are already 'in' the wood itself to varying degrees.

After wetting, those elements if present, will go into solution. When heat is then applied the solution is bought to the surface where oxidation occurs almost instantly causing the colour change. The more water applied, the more minerals and tannins will be dissolved and therefore the more pronounced the problem is likely to be.

When you think about it, even wood with a reading as low as 8% moisture, the joinery standard for good stability, still holds quite a bit of water...errrm about 8% to be precise. So even at that point, every KG of wood is retaining 8 grams of water..That may not sound like very much but it will produce quite a lot of steam...The point is that even if you use 'no' water when bending blackwood, there remains every potential, all be it to a lesser degree, for mineral/tannin staining to still occur as these elements are taken into solution by this "trapped" moisture being released to the surface by heat.


Cheers

Kim

It would actually be 80 grams of water. 8 grams would be .8% which sounds like and really is not much.

From what I gather if alkaloids and tannins binded they would not go into solution in water.

No matter how much water is used it doesn't seem to be a problem on the copper pipe unless it actually burns.