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
Flexed belt showing cracks