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Okay, this is a weird one. Was studying dyeing techniques in Youtube videos, had looked at vinegaroon, and came across a claim that sodium bicarbonate alone can react to veg tan to create a brown color. The person in the video went well beyond the saturation point and had undissolved sodium bicarbonate, and dunked his project in it without filtering. The result did look brown, but was also covered in residue. Was thinking that since sodium bicarbonate can be used as a abrasive, that wasn't a good thing. Still, it did seem to turn the leather brown, and filtering and maybe washing later might remove the grit.

However, if this is a chemical reaction to the tannins, what does it do to the leather long-term?

Has anyone tried this? Did it turn out good or bad? Will be trying it on a tiny scrap I was going to toss, but it's already had too much neatsfoot oil and that might affect results.

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Update:

I tried it by making what I hope was just under a saturated solution and dipping the leather. Here I messed up: I should have timed it. I waited at least a minute, maybe two, and removed as the solution began wicking through the scrap. Dried and set aside until completely dry. The portion that was in the solution is a medium brown. The leather doesn't feel stiffer, but note it was subject to neatsfoot oil a week ago.

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Unlike the color black, the browns represent a wide spectrum of colors.  I would think it difficult to control the shade of brown color by exposing leather to sodium bicarbonate.  Baking soda is used to extract tannins out of tanned leather, but I haven't seen anything about using it to color leather.   Recipes for brown dyes in old books on harness making typically include saffron, annotta, and/or tree barks to achieve different mixtures of yellow, red, and black-brown.  Here's some examples.   

 

From The Harness Makers Illustrated Manual, second edition (1886).  W.N. Fitz-Gerald. 

STAINS. The use of russet and brown leather for reins, etc., necessitates the employment of stains of various shades in the workshop, in order that the reins or other straps may be of a uniform color after being worked. In most cases rein leather is stained by the currier, but when worked, the freshly cut edges, etc., need to be stained to correspond with the grain. The stains used are generally made of Spanish saffron and anotta, or of saffron alone, made up in various ways, the most common and reliable being the following : Boil a given amount of saffron in water until the color is extracted ; cut a quantity of anotta in urine and mix the two together, the proportions of each determining the shade ; the more anotta used' the darker is the color.

Another manner of preparing this stain is to boil 1/2 ounce of Spanish saffron and 1/4 ounce of anotta in water until the dye is extracted, to which must be added some alcohol to set the color. To make a stain of saffron alone, boil a quantity in water until the dye is extracted ; strain off, and, when cold, add alcohol in order to set the color. The shade may be changed by adding oxalic acid in varying quantities according to the color required. The proportions can not be given with any degree of accuracy, as the color is a matter of taste, and can be regulated by using greater or less proportions of each article. Another saffron stain is made by boiling saffron in a small quantity of water until the color is extracted, and reducing with urine. In using any of these stains, apply them with a cloth, and, when nearly dry, rub with a woolen rag slightly waxed.

A yellow stain is produced by boiling fustic berries in alum water ; the shade may be darkened by the addition of a small quantity of powdered Brazilwood boiled with the berries. Another yellowish red stain is made of Brazil wood and yellow berries in proportion to suit, boiling them in water until the coloring matter is extracted. This can be applied to sides that have not been stained, when intended for flat reins, halters, etc., in the following manner : Lay the leather upon a table, and rub the flesh side with a warm stretching iron; turn it over and moisten the grain side with water, and rub with a copper stretching iron until the leather is nearly dry ; then apply the coloring matter to the grain, and rub with a copper slicker. When the leather is perfectly dry, rub the grain with a glass slicker. An edge stain is made by adding a small quantity of alum to the above mentioned ingredients.

A brown stain is made by boiling equal parts of pine and alder barks in six times their bulk of water until all the coloring matter is extracted, and when cold adding a small quantity of alcohol. Saffron boiled for twelve or fifteen hours gives a good brown stain, to which alcohol must be added to make it set. Picric acid and water, in proportions of 1 to 10, heated to a blood heat, makes a good yellow stain. Wold boiled in water also makes a yellow stain. An orange yellow is produced by boiling fustic berries in alum water. This stain may be converted into a rich brown by washing the leather to which it has been applied, before the stain is fairly dry, with an alkali. A red stain is produced by boiling Brazilwood in lye. If mixed with wold, it produces a brown.

 

Edited by TomE

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Thanks. Started thinking about the reaction with tannins and the resulting products. Most of what little chemistry I knew is long gone, and tannic acid turns out to be complex ( C76H52O46 ). Three elements but a whole lot of them, reacting with NaHCO3. A search turned up the obvious: tannic acid in leather is a preservative, raising the question about removing tannic acid from leather. That also has me wondering about vinegaroon for black.

Will likely just use Fiebing's Pro Dye to be on the safe side.

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Vinegaroon is acidic, sodium bicarbonate basic. 

I know very little about leather, but quite a lot about the protein fibre wool (and hair, and silk). It stands up to acids pretty well, but not to alcaline substances. 

Given that leather is protein-based material, I would expect somewhat similar behaviour. I also wonder whether plant dyes used normally for wool might work  - the above recipes for leather use some of the same plants. Brazilwood, saffron, tree barks, wold (Reseda luteola) - does anybody know what fustic berries are?

 

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Veg tanned leather is mildly acidic -- usually around a pH of 3 in my experience. I'm no tanner but I'd be willing to bet that there's a good reason behind that. In the trade certain premature degradations of the leather are associated with low pH ("red rot") and with excessively high pH. For comparison human skin is around pH5.5, regular vinegar pH2-3, distilled water neutral at pH7 and a saturated solution of baking soda mildly basic at pH8-9. I think it's a very good idea to keep leather as similar as possible a pH as it came out the tannery when we're mangling it, for longevity.

I don't remember the pH of iron acetate (the active component of vinegroon, i.e. the result of reacting iron with acetic acid [vinegar]), nor can I find it from Google. The pH of any particular batch of vinegroon might be anything within a wide range though, dependant on things like the amount of unreacted vinegar present.

The simple version of the chemistry of vinegroon and other similar strikers is that excess tannic acid in the leather (left over from the tanning process) reacts in the presence of water/moisture with iron to form iron tannate, which is a blue-black in colour. Iron tannate is not water soluble, so can't wash out of the leather. This same reaction is used on tannin-containing wood like oak and chestnut.

There's nothing magic or unique about vinegroon -- many different ways of getting iron to react with the leather work. Iron filings are a common accidental reaction in workshops and tanneries. Blood is also common, though usually again an accident. I've used iron sulphate crystals from the garden centre before, and have heard of good results from dietary iron supplement tablets (same stuff). Will Ghormley simply uses a barrel of rusty water. I greatly prefer the idea of using either largely neutral reactants (like rusty water) or consistent off-the-shelf components like ferrous sulphate, for which there is no brew/wait time, are consistent in their behaviour, whose pH can be predicted, measured and controlled. (Incidentally a 10% solution of ferrous sulphate in water has a pH of around 3.5, which is very similar to veg tanned leather.)

Immersion of leather in basic solutions of unknown or uncontrolled strength doesn't seem like a good idea to me. I've tried it as the "received wisdom" way to "neutralise" a vinegroon bath and ended up with stuff that seemed fine initially but cracked and split far too early. No amount of grease will knit broken leather fibres back together.

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