Iron Black Dyes

The easi­est col­ours to dye leath­er in medi­ev­al times were brown and black. Those are by far the most com­mon col­ours for shoes in art­work I have looked at, but I haven’t done a numer­ic ana­lys­is of foot­wear col­our by art and peri­od yet.

Leath­er tanned by the medi­ev­al pro­cess in pits of bark liquor is nat­ur­ally brown of vari­ous shades, but the tan­nins in the leath­er react to UV light (sun­light) by turn­ing much dark­er.

This effect is extremely obvi­ous on mod­ern rus­set veget­able tanned leath­er, which is a very light col­our to facil­it­ate dye­ing with mod­ern dyes. If you leave a piece of this leath­er in the sun it will go a red­dish brown col­our. This is why you should alway store you leath­er out of dir­ect sun­light unless you want to make brown or black things.

When you mix iron salts with tan­nin, you get black. The chem­istry of this is explained on the Wiki­pe­dia page on iron gall ink but the short ver­sion is iron salt + tan­nic acid + light = black. This is the same pro­cess that causes black rings around nails in wooden floors.

Veget­able tanned leath­er is extremely easy to dye black by the simple applic­a­tion of an iron salt solu­tion but you need to be care­ful. If you get it wrong and have too much iron and too little tan­nin you will des­troy the leath­er.

That’s a belt I made about 10 years ago. The grain sur­face of it is peel­ing off the high-wear area and the whole belt is dry and cracked. This is the res­ult of too much iron and not enough tan­nin. Sev­er­al oth­er pieces I made in that time peri­od have suffered the same fate.

The solu­tion to this is to use a suf­fi­ciently dilute iron solu­tion, and to add addi­tion­al tan­nic acid. In the middle ages this would come from oak galls or bark, but in more mod­ern dye­ing log­wood extract is used.

The fol­low­ing para­graphs are tran­scribed from the 1925 book Leath­er Dress­ing includ­ing Dye­ing, Stain­ing & Fin­ish­ing by M. C. Lamb and explain the import­ance of log­wood (a new world plant) in dye­ing blacks. Emphas­is is mine:

Applic­a­tion of Log­wood. — In employ­ing log­wood for the pro­duc­tion of blacks on leath­er, it is cus­tom­ary to apply the log­wood infu­sion first to the leath­er and the mord­ant after­wards.

The most import­ant use in con­nec­tion with leath­er to which log­wood is put is in the dye­ing of blacks in con­junc­tion with an iron mord­ant. For brush dye­ing or stain­ing, a strong infu­sion of the log­wood or log­wood extract is employed, pre­vi­ously made slightly alkaline with ammo­nia or sodi­um car­bon­ate, in order to increase the dye­ing power and to assist in “cut­ting” the grease in the case of greasy leath­er. Anoth­er func­tion of the alkali is to pre­vent the log­wood infu­sion from strik­ing through, that is, from pen­et­rat­ing the leath­er. The alkali is not so import­ant when the log­wood is used for dye­ing blacks in the bath.

From the leath­er dresser’s stand­point, the para­mount use of log­wood is in the stain­ing of blacks on veget­able tanned leath­er, and the dye­ing of blacks on chrome tanned and alum dressed leath­ers. When using log­wood for black­ing goods, in con­junc­tion with some iron salt, a suit­able strength of solu­tion is 5 lbs. log­wood extract, with 1lb. fustic extract, dis­solved in 10 gal­lons water, to which is added after­wards 2 ozs. wash­ing soda. This log­wood-fustic solu­tion is first applied, and then the solu­tion of iron. For the iron solu­tion, with the com­mon­est class of goods, “Cop­per­as” (fer­rous sulph­ate) is employed; nitrate of iron, or iron acet­ate, is employed for bet­ter qual­ity goods. 5lbs. cop­per­as and 1/2 lb. cop­per sulph­ate or acet­ate per 10 gal­lons water makes a solu­tion of con­veni­ent strength.

In stain­ing blacks it is very neces­sary that plenty of the log­wood infu­sion should be applied to the leath­er, espe­cially if this is at all lightly tanned. Unless there is plenty of tan­nin and col­our­ing mat­ter to unite with the iron, the iron will com­bine with what there is of tan­nin mat­ter in the leath­er, and render it brittle and liable to crack. If too much iron is used, the leath­er may be com­pletely ruined. The writer has seen many cases where leath­er has been rendered too brittle by to little log­wood and too much iron.

As log­wood gives blu­ish or viol­et blacks, it is neces­sary, when a jet black is required, to add a yel­low or brown col­our­ing mat­ter to tone off the blue. Fustic, sumach, quer­cit­ron bark, galls, etc., are often used for the ton­ing.

Cop­per mord­ants with log­wood give very deep blu­ish blacks, which are faster to light than the viol­et blacks pro­duced with log­wood and iron. The use of a little cop­per sulph­ate or cop­per acet­ate togeth­er with the iron in dye­ing blacks is use­ful, as help­ing to pro­duce a black which is fast to light. Brazil wood extract may be added to the log­wood infu­sion in order to improve the fast­ness to light of the black.

Source: Leath­er Dress­ing includ­ing Dye­ing, Stain­ing & Fin­ish­ing by M. C. Lamb, F.C.S, third edi­tion. Lon­don: The Anglo-Amer­ic­an Tech­nic­al Co., Ltd,. 112 Tower Bridge Road, S.E.1., 1925

I take from that that part of my prob­lem with my belt is that I used too strong an iron oxide (steel wool dis­solved in vin­eg­ar) and didn’t add any addi­tion­al tan­nins to the leath­er first. The effect is par­tic­u­larly bad on mod­ern veget­able tanned leath­er which has far less resid­ual tan­nin than pit tanned leath­ers do.

This is also the reas­on that you don’t use iron or steel nails or riv­ets in leath­er goods. The leath­er sur­round­ing the riv­et or nail will become brittle and crack and the riv­et will fail. Because the leath­er itself has been dam­aged the leath­er must be cut out and replaced.

Cop­per­as is iron (II) sulph­ate, you get it from your garden centre as “sulph­ate of iron”. It’s really cheap I paid NZ$5 for a 4kg bag. Be care­ful with it though, it per­man­ently stains vari­ous things, includ­ing con­crete.

The gal­lons used above are likely to be UK liquid gal­lons (giv­en that this is a book pub­lished in Lon­don in 1925, well pre decim­al­isa­tion) so that equates to 45.46 litres (thank you Wolfram Alpha).

If we divide the quant­it­ies above by 45.46 and con­vert them to met­ric, 5 lb. cop­per­as and 1/2 lb. cop­per sulph­ate in 10 gal­lons of water turns into 49.89 grams of cop­per­as and 4.989 grams of cop­per sulph­ate per litre, which we can prob­ably safely round to 50 g and 5 g respect­ively. The cop­per sulph­ate is option­al.

This is far less than I had been using in the past.

I have no ref­er­ences for medi­ev­al black dye, so I don’t know wheth­er they used a bark liquor or oak galls in lieu of log­wood, or if medi­ev­ally tanned leath­er just had enough resid­ual tan­nin in it that you can safely dir­ectly apply a solu­tion of iron salts to dye it, but I am sure that they were smart enough to work out that leath­er dyed badly would not last.

More pic­tures of the belt

Iron-dam­aged grain sur­face

Pic­ture 1 of 5

Severe dam­age to mod­ern veg tanned leath­er caused by iron oxide dye. The sur­face has com­pletely delamin­ated from the area of the belt where it goes through the buckle. The under­ly­ing leath­er is brown, not black indic­at­ing that the dye did not pen­et­rate that far.


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