Blackening mild steel

I don't mind people mentioning that substances are toxic but perhaps it would be a good idea to state LD50's to give people some idea just how toxic it is. Sodium Nitrate for instance is 3236 mg / kg. It's classed as an irritant and an oxidiser. Oxidisers can be problematic when mixed with some substances as combustion can result. Bit like gun powder and potassium nitrite which also serves as an oxidiser - they need to be the right other ingredients, another oxidiser may be a very bad idea. The cancer aspects are thought to relate to substances formed when it's used to cure meat.

Sodium hydroxide also referred to a Lye is caustic. Although the wiki disagrees I reckon it's often sold as sugar soap. Best thing for people who want to make use of substances like this is to look at it's material safety data sheet and react accordingly. It's pretty obvious that people shouldn't wash in it or drink it. One thing it mentions is violent reactions when water is added. What this means in practice is that heat is likely to be generated when it's added to water so that needs to be done slowly. Very similar to never diluting strong acids with water, the acid should be added to the water while stirring.

To me though like all metal colouring treatments other than some that very definitely can be dangerous it sounds a bit iffy and people may well finish up with something that isn't the black they want.

One thing concerning both substances is that fairly large volumes of both can be bought mail order. There are pretty extreme rules about what joe public can easily buy and what can be shipped by ordinary carriers. People are still expected to use some common sense.

John

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Sugar soap is pretty weak, I've got sodium hydroxide here for things like making PCB developer - it actually gets colder when you dissolve it, like salt. I've used it to clean up an oven hood as it eats its way through fat (turns it to soap) and it does the same to your skin (de-fats it) and I can tell you that if you don't wash splashes it off quick it makes your hands feel very unpleasant for several days.

Thing thing with molten nitrite is that it is an oxidiser so heat + nitrite + anything flammable = big fire risk.

The final thing is that as these boil at well over 100% c any pure water poured on will flash to steam throwing gobs of concentrated molten salts everywhere which are aggressive irritants, very hot and will stick to you and rapidly burn (heat and chemical) very deeply.

Played about a lot with these sort of things as a teenager and learnt a few lessons from close calls, one of which involved a tin can with a small amount of molten nitrite and <redacted> that went bang, disappeared and dropped right at my feet several seconds later. It had changed shape

Neil

We are getting seriously off topic here, so Neil permitting.........!

"John's" sensible posting above suggests providing LD50s to give "some idea" of toxicity. That bit needs qualification I'm afraid.

LD50 is the dose which kills 50% of animals receiving the substance. "Dose" is critical, it means the amount taken in and not the exposure concentration killing 50% of animals - for that we need the LC50 value and there aren't too many of them published. Most if not all LD and LC50 values are derived for mice and rats. Their relevance to humans is questionable. Apart from WWI data and a few other accidental exposures there are very few chemicals with LC50 data for humans.

LD or LC50 values tell you nothing about chronic effects such as carcinogenicity, solvent neuropathy, lung damage etc.. Yes - sodium nitrite does produce carcinogenic nitrosamines (implicated in bladder cancer) when in contact with amines but we humans are "full of" of amines as amino acids and other compounds.

What's the relevance of all this for engineering operations?

Simply, risk = hazard x exposure.

Hazard is intrinsic toxicity of a substance so reduce exposure and you reduce risk. Carry out an operation outside and wear proper protective equipment and your exposure is reduced. Do the job infrequently and your overall exposure is reduced. Old tar-based cutting oils, fluxes, grindstone grit, mineral acids and solvent vapours are some of the hazards model engineers face. Thankfully exposure to them is usually low and intermittent.

I've got several books from the 1st half of the 20thC advocating use of all sorts of chemicals which were then freely available, including Prussic acid (hydrogen cyanide) for plating in the home workshop. No doubt many users of the recipes lived long and happy lives but sensible precautions today to reduce risk are always worthwhile as "John" and others advise.

Sounds a little more like nitrate to me Neil. Something that wont be so easy to buy. When it's sold as fertiliser something else comes with it. That is a cracking oxidiser on it's own. There is some really bad press about some of these as they miss the fact that they need something to oxidise to explode etc - Special branch etc might get attracted to this thread.

The action sodium hydroxide on skin is probably similar to KOH which is much worse. Skin gets a sort of soapy feel to it - skin cells. Best wash thoroughly asap. I know from personal experience that KOH is still ok if you fail to notice for a short period. Hot stuff - well most people are fully aware that even water can burn and heat will make any effects caused by what's in it worse.

Being realistic though - would some one boil this concoction on the stove? Suppose some might with care turning it down as soon as it simmers but I would expect the mix to have a pretty high boiling point and personally I would use a lab type hot plate or similar. Would people take some precautions - gloves etc. I would hope so. Would any one in their right mind melt the solids first and then add water - I'd hope not. Sounds like the nut cases may go to the right place. .

My comment about heating effects comes from one of the MSDS sheets concerning cleaning up large spills of the solids.

The web is a dangerous place in some respects in areas concerning chemicals. Some years ago I came across a method of oxidising diatoms. This does need powerful oxidisers even mixes of them. No signs of it on the web. It came from a book intended for older school children under supervision. I mentioned it on the web and soon it was all over the place. No mention of what was going on and possible problems and worse still some one thought that one of the chemical would be best diluted as it must be safer. Trouble is that it doesn't work at all then so people might play around with quantities - that really could cause problems. A paper was even produced stating that it should be diluted. The other problem is that it's for small scale use. Again no mention and if silly things are done it could result in a strong acid being sprayed all over the place or even minor explosions. The fact that it was diluted wouldn't help at all. It's really best to try and find original sources of information in these sort of areas from reliable sources. That can be difficult.

John

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"Special branch etc might get attracted to this thread" .

Enough key words, vague /possibly cryptic details ,

Time of year ! . the psycho evaluation crew too, bit a luck they will think your just trying to get seven hundred postings in, in four months . Good luck .

B

ps Fowlers Fury ---Luv ur avatar.

pps almost forgot , Blacking or Bluing as its usually known ,yes i have few bits & pieces that were blued from a long time ago at BAC before elf & safety got in the way, though i don"t think e&s would be the problem .But the bluing medium used would be . Whale oil, sperm whale oil, to be accurate . The bits & pieces ! well they have stood the test of time, and still black. But as i remember it , its not a heat ,dunk,& go. Involves several immersions & holding at temp, for some time. Anyway whaling & whale products not very PC in this day & age rightly so.

Bodge.

Tannic acid converts brown rust to black.

I use a Japanese technique for aging steel where you dip in nitric acid and sponge off then hang in a humid atmosphere. Every day, give it a good going over with a stiff brush. You want to develop a thin, hard, brown coating - not crusty. After many days, when you think you have a thick enough (but still thin) and even patina, boil it with old tea bags for an hour (you could use new ones, but old ones seem to work fine). Any tea - doesn't have to be japanese green tea

The black stands for this lathe were done this way - no paint.

I don't think you _need_ the nitric dip. So this is safe and environmentally friendly - even recycling used tea bags. Everyone is happy!

Theobromine LD50 about 1,000 mg/kg for humans. Much worse for dogs.

Edited By jaCK Hobson on 11/09/2015 08:17:17

Edited By jaCK Hobson on 11/09/2015 08:19:25

Edited By jaCK Hobson on 11/09/2015 08:19:42

Edited By jaCK Hobson on 11/09/2015 08:23:24

Neat Jack. For people who might want to use nitric acid first which is probably partly intended to clean it any idea of the concentration? It's usually low as in anodising.

John

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Hello all, good morning.

I found a formula for blackening ferrous metals which uses four ingredients: distilled water, tannic acid, ethanol and diluted phosphoric acid.
The formula is from the Canadian Conservation Institute and can be downloaded from ***here*** .
Two years ago I experienced subjecting various steel items to the solution and it worked. Some items were slightly marred by a less dark black.
I have not used in everyday the objects subject to treatment, so I do not know what the durability of the black layer is.
The document has a chapter dedicated to conservation called "Care of Objects Treated with Tannic Acid"

Extracted from that document here is the recipe for one litre of 10% solution of tannic acid:
100gr tannic acid
about 900ml of deionized or distilled water
50ml ethanol
Aprox 2ml of dilute phosforic acid (H3PO4), if available

I hope that the formula has some use to anyone.
Dias Costa

Edited By dcosta on 11/09/2015 10:46:03

I checked... the advice it to use HCl to remove scale... So not nitric at all. I thought the acid may be to encourage a bit of rust, but cleaning makes more sense now you have pointed it out.

I probably used the right thing (HCL) at the time, and got it from the local pound shop (from the cleaning chemicals isle).

There are more advanced Japanese blackening techniques involving mixture of things including rat poo... you can usually get this by asking in pet shops. I'm not sure of the recipe at the moment and I'm guessing no one here is really interested?

Being realistic though - would some one boil this concoction on the stove? Suppose some might with care turning it down as soon as it simmers but I would expect the mix to have a pretty high boiling point and personally I would use a lab type hot plate or similar. Would people take some precautions - gloves etc. I would hope so. Would any one in their right mind melt the solids first and then add water - I'd hope not. Sounds like the nut cases may go to the right place. .

Sadly experience suggests that there are plenty of people who need to be told these things explicitly - they tend to think "after all if you can buy a pot of sodium hydroxide for a quid in the cheap shop it can't be that dangerous". Another proportion of people will simply ignore advice, relying that "providence takes care of children, fools and the United States".

Neil

Like the warning in a site for paper aeroplanes, warning that scissors are dangerous, and require supervised use, this was for 10/12 year olds, I remember using scissors when I was a Kindergarten, at about 7yrs we used a fret saw on craft day, no close supervision.

Some thing about cotton wool, and being wrapped in it.

Ian S C

The easiest source for dilute HCL is cement cleaner. If the item is greasy a pre treatment with an alkaline even washing soda followed by a wash and dry would probably help. HCL removes rust and leaves metals grey. It rusts pretty quickly then. I tried it on a reamer once. No change in size I could measure and it was still sharp.

yes - "modern" things in the directions outlined are a bit strange. People are inclined to want specific instructions on all sorts of things. This seems to discourage thought and lead to an inability to think for themselves and maybe a lack of confidence as well in some areas.

John

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I think the problem today is that schools do their 'experiments' with the kids allowed to use nothing more hazardous than lemon juice and a bit of zinc, or worse still just a computer screen.

If you see what happens when concentrated sulphuric acid hits sugar first hand, you are more inclined to believe what the nasties can do, and you handle them with more respect. I still use the back of my hand to wipe my eyes because 'if you use your fingers you're more likely to get something nasty in them'.

On the other hand, if they do use chainsaws or nasty chemicals in their work they are probably much better trained and aware of the risks.

But the long and short of it, is we now have a new generation who, paradoxically, take health and safety precautions more seriously, but haven't the broad knowledge of how to handle anything they haven't been trained to use.

Neil

From one of my old books: Bluing without heat. Mix finely powdered Prussian Blue with rather thin shellac varnish; gently heat the steel, and apply the varnish.

Ian S C

Maybe some one fancies trying another way

**LINK**

John

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I bought a Bohm hot air engine, and it came with this warning. Caution: The engine must be operated by or under the supervision of persons over 18 years of age.

Flammable objects must be kept from the vicinity of the engine. Do not touch the engine since this could lead to serious burns or injuries. Takegreat care when handling alcoholic spirits. Never leave spirits bottles open. Inappropriate handling of the Stirling Engine may cause fires.

This is not as bad as a bag of tennis balls from China; May cause choking, not suitable for under 36 months. Same label on a Frizby.

Ian S C