Blackening mild steel

[Neil LickfoldParticipant @neillickfold44316]

Much cation is required, but a hot black process is easy enough. Requirements are a stainless pot/container big enough to hold the parts. Then you mix equal parts Sodium Nitrite, with dissolved, Sodium hydroxide,pearl grade with water. Mix the 1 litre volume of water with 1 litre volume of sodium hydroxide, very slowly, it is extremely caustic. After mix, then carefully and slowly add the sodium Nitrite.

Clean parts thoroughly, with some dish wash liquid and water. Then heat the mixture slowly, until about 140 degC then place the parts in the simmering liquid for about 10 to 15 mins. If the mixture boils too high a temp add more water,after it has cooled down to room temp.Never add any water when it is hot. If the temp is too low, then just let it boil longer. Too low a temp and after 10 mins it will not be black or will be a light brown colour, if too hot, the part will come out with a red brown colour.

Quite a few gun smiths use this method for steel blackening.

As I said , safety gear is required and extreme care is needed. Make sure that your method of putting the parts into and out of the pot can not accidently drop in or splash any fluid what so ever.

Only use mild steel or stainless steel wire and mesh pots to hold parts. NEVER allow any Aluminium or Magnesium or other metals that can react to caustic chemicals.

Neil

Edited By Neil Lickfold on 09/09/2015 12:17:19

Found this website with warnings and his formula, he used Sodium Nitrate instead of Nitrite.

http://www.northeastshooters.com/vbulletin/threads/160066-DIY-Hot-Bluing-for-Dummies-(in-progress)-with-pics

 

Edited By Neil Lickfold on 09/09/2015 12:28:19

[Fowlers FuryParticipant @fowlersfury]

I don’t think reproducing a web article like that is either helpful or desirable. You have written “heat the mixture slowly until about 140degC”. What mixture?? Do you mean “parts” rather than mixture?? How do you heat an aqueous solution to 140 degC at atmospheric pressure?
There’s enough controversy about the carcinogenicity of sodium nitrite to justify taking the simpler option of Gun Blue !
Please also describe a method for the responsible disposal of the resultant 2 litres of NaOH and NaNO2.

[VicParticipant @vic]

Gun blue is the easiest, just wash your hands afterwards as it contains some nasty chemicals.

[Fowlers FuryParticipant @fowlersfury]

Vic is quite correct, most of the Gun Blue formulations contain selenium salts. His advice to wash your hands after use is sensible.

But we shouldn't become 'chemophobic' – selenium is an element essential to life, albeit in trace amounts. As the father of toxicology, Paracelsus said (in loose translation) "The dose alone makes the poison".

Gun Blue used sensibly represents no significant risk. (Certain anti-dandruff shampoos contain more selenium as sulphide salt, weight for weight)

Living close to the NRA at Bisley, I bought my small pack of gun blue gel from their shop. But if you put "Bisley Gun Blue" into Google, there are several suppliers at around £9.50 per pot which lasts for years.

Off topic – the NRA shop also stocks a similar sized plastic bottle of a sister product which blackens brass. That really does require good surface preparation but the results aren't bad.

[Fowlers FuryParticipant @fowlersfury]

Just checked and my little pot of gun blue purchased years ago from NRA at Bisley, was made by the American company Birchwood Casey. (they also made the solution for brass).

There's a YouTube vid of "how to do it" – I don't have the their degreaser or rust remover, but use other products.

**LINK**

[Frank.N StormParticipant @frank-nstorm18349]

Posted by Fowlers Fury on 09/09/2015 14:01:43:
What mixture?? Do you mean "parts" rather than mixture?? How do you heat an aqueous solution to 140 degC at atmospheric pressure?

.

Imho a mixture of 3 substances is a mixture and not a part…

It should be obvious that an aqueous solution of such a high concentration has a much higher boiling point than 100 °C.

Regards, Frank

[MikeParticipant @mike89748]

If using gun blue or black, be careful to follow the instructions after the stuff has been applied. If you don't "kill" the chemicals after the desired shade of black or blue is achieved, the item goes rusty very quickly – sometimes overnight. I know this to my cost….

[Fowlers FuryParticipant @fowlersfury]

What mixture?? Do you mean "parts" rather than mixture?? How do you heat an aqueous solution to 140 degC at atmospheric pressure?

.

Imho a mixture of 3 substances is a mixture and not a part…

_________________________________________________________________________________

Alright, maybe I should have added…. "Do you mean parts to be blued ?" instead of assuming it was obvious. But I now realise that the article did mean mixture (of NaOH and NaNO2).

As for bp of NaOH:-

On first reading that post I thought that the quantities of both salts had been omitted. Though unclear, I now guess the proposed "1 litre volume of sodium hydroxide" might mean "1 litre of solid sodium hydroxide". To that it's proposed to add 1 litre of water making a 50% w/v of NaOH solution, which I now find, boils at nearly 150C.

Ignoring the evident hazards of a 50% solution of boiling NaOH, I've no idea what effect on that bp the additional "1 litre of sodium nitrate" would be.

It's a witches brew nobody should contemplate handling in a domestic environment.

The respected NIH comments thus on sodium nitrite:-

"Used in many industrial processes…………The compound is toxic and mutagenic and will react in vivo with secondary or tertiary amines thereby producing highly carcinogenic nitrosamines……….. will accelerate the burning of combustible material. If large quantities are involved in a fire or if the combustible material is finely divided, an explosion may result. ……….. Prolonged exposure to heat may result in an explosion. Toxic oxides of nitrogen are produced in fires involving this material."

I stand by my original comment "I don't think reproducing a web article like that is either helpful or desirable".

 

Edited By Neil Wyatt on 09/09/2015 19:29:30

[Fowlers FuryParticipant @fowlersfury]

Sorry about above and what happened to formatting – try again !

_________________________________________________________________________________

What mixture?? Do you mean "parts" rather than mixture?? How do you heat an aqueous solution to 140 degC at atmospheric pressure?

Imho a mixture of 3 substances is a mixture and not a part…

_________________________________________________________________________________

Alright, maybe I should have added…. "Do you mean parts to be blued ?" instead of assuming it was obvious. But I now realise that the article did mean mixture (of NaOH and NaNO2).

As for bp of NaOH:-

On first reading that post I thought that the quantities of both salts had been omitted. Though unclear, I now guess the proposed "1 litre volume of sodium hydroxide" might mean "1 litre of solid sodium hydroxide". To that it's proposed to add 1 litre of water making a 50% w/v of NaOH solution, which I now find, boils at nearly 150C.

Ignoring the evident hazards of a 50% solution of boiling NaOH, I've no idea what effect on that bp the additional "1 litre of sodium nitrate" would be.

It's a witches brew nobody should contemplate handling in a domestic environment.

The respected NIH comments thus on sodium nitrite:-

"Used in many industrial processes…………The compound is toxic and mutagenic and will react in vivo with secondary or tertiary amines thereby producing highly carcinogenic nitrosamines……….. will accelerate the burning of combustible material. If large quantities are involved in a fire or if the combustible material is finely divided, an explosion may result. ……….. Prolonged exposure to heat may result in an explosion. Toxic oxides of nitrogen are produced in fires involving this material."

[Neil WyattModerator @neilwyatt]

Ah… just after I fixed your formatting…

For the record, Let's make it abundantly clear that, like many traditional bluing and metal colouring recipes anyone who has common sense would agree these should be done in a proper environment (e.g. a fume cupboard or other set up with screening and forced ventilation) and abundant protection for eyes, skin etc. etc.

Sodium nitrite is used for curing meat, but at somewhat lower concentrations! and it is a powerful oxidiser – as any 70s teenager will be aware – having played with molten potassium nitirite as a lad, my advice is treat these chemicals with great respect.

Neil

[John Ockleshaw 1Participant @johnockleshaw1]

My thanks to Neil Lichfold for his recipe for blackening steel.

I and my friends have used this often to provide a pleasing, tightly adhering black film like you find on some hand tools.

As well if you put cast iron in the bath, say brake blocks, you get a tightly adhering rust coloured film which is very realistic on a model.

If you do the work outdoors, wearing glasses and gloves, you certainly will not drop dead over the pot.

[jaCK HobsonParticipant @jackhobson50760]

My thanks also. I have looked for this information in the past and failed to find.

The nitrate/nitrite confusion could be significant and it would be great to confirm which is recommended. Otherwise I think the information is plenty to get one started in this direction.

There used to be a valuable resource of a list of heat treating salt recipies as a pdf link referenced here http://www.bladesmithsforum.com/index.php?showtopic=6375&page=2

Unfortunately all references to it now seem to be dead links.

I tried to source ingredients for low temp salts in the UK and soon decided it was much easier to buy proper heat treating salts … in 20kg bags and £50 for pallet delivery (did split the cost with a friend). I don't know how easy the above components will be to source (I can probably find the water ).

 

Edited By jaCK Hobson on 10/09/2015 08:18:24

[LambtonParticipant @lambton]

Neil Lickfold describes the exact method used by a long establish gunsmith that I worked for part time for a number of years following my retirement. It is used on shotgun barrels that have brazed or silver soldered ribs (most modern guns) but cannot be used on older traditional “English guns” that have soft soldered ribs because the solder will not withstand the temperature involved. It works very well but the temperature has to be controlled within fine limits if a true black colour is to be achieved. The main snag with this process for gun barrels is that it does not colour brass or silver solder.

"English type" guns with soft soldered ribs have to cold blacked which is along and labour intensive job involving swabbing the barrels with a hell brew containing diluted aqua regia and certain mercy salts. The swabbed barrels immediately gain a fluffy black coating which is left for a few hour before being removed using a soft rotary wire brush. After the fluffy coating is removed the steel will be starting to take a black finish. This process is repeated up to probably ten times until a satisfactory black surface has been established.

Neither process confers much rust resistance and the newly blacked finish has to be protected by bees wax or special oils. Many shot gun barrels that I have blacked were rusted in places due to blood from game contaminating the surface.

The small bottles of blueing of blacking solutions sold by some gunsmiths are only intended to do very minor touching up and not to treat large areas. Similar reasoning to small pots of car touch up paint.

We also applied brown finish to Damascus barrels but this is a work of art and need a lot of experience to produce a professional l job.

There is an excellent book “Firearm Blueing and Browning” by R H Angier dating from 1936 that give full details of the various processes and chemicals used . Copies are available from Amazon. However be warned in 1936 people were not afraid of using all sorts of substances that would make a safety office go “up the wall” today.

[VersabossParticipant @versaboss]

Usually it's not my habit to write about something I didn't check out before myself. But in this case I make an exception. I think that I've read in another forum (possible German) that it is possible to blacken steel parts by immersing in hot vinegar. Surely a simple method when (if) it works, but as said I didn't try it. But I will do on the next occasion.

Regards, HansR.

[JasonBModerator @jasonb]

Posted by Rainbows on 07/09/2015 20:54:54:

So all quick change toolposts tend to have this sort of blackened finish to them

Surprised nobody else picked up on the fact that most toolposts don't actually have a black coating, maybe only some of the piston types. Personally I'd just leave it bare metal rather than have it worn off by the swarf

Edited By JasonB on 10/09/2015 13:09:04

[Neil LickfoldParticipant @neillickfold44316]

When I was blackening tooling, I used sodium Nitrite, the very same product that is used as a bacon/ham/salami preservative. I did mention that it was dangerous if you were not careful. I also mentioned that if your procedures were such that a part could free fall or splash going into the hot mixture, you are doing it wrong.

As for the disposal of the very caustic residue, you either have to get it neutralised by an acid, or take it to a place to be disposed of. Where I was blackening the tooling, they also had an acid process. That required it to be neutralised. The waste of the old blackening bath was used to neutralise this acid. So it worked out very well.

What I detailed is not new and was not a copy and paste from a web site. I cam across a site that was basically doing what I was so posted that link.

There are commercial caustic metal black available to buy. They add to the recipe I gave a few other things, but what I posted works very well and is relatively very cheap and easy for someone to set up and use.

One other thing, keep Chlorine away from the mixture and fluoride , distilled water works very well. I used an infra red thermometer to keep a watch on the temperature. But after a while you get to know the bath and when to add a little more Nitrite or Caustic soda or water. Water you will add at the beginning of each days use, and you have to keep a watch on it, even if you are not cooking with it every week.

Neil

[AjohnwParticipant @ajohnw51620]

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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[Neil WyattModerator @neilwyatt]

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

[Fowlers FuryParticipant @fowlersfury]

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.

[AjohnwParticipant @ajohnw51620]

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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[bodgeParticipant @bodge]

"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.

[jaCK HobsonParticipant @jackhobson50760]

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

[AjohnwParticipant @ajohnw51620]

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

–

[dcostaParticipant @dcosta]

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

[jaCK HobsonParticipant @jackhobson50760]

Posted by John W1 on 11/09/2015 09:20:43:

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

–

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?

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