o< MONTHLY. I>^ 
XXIII. 
COLOMBO, JULY 1st, 1903. 
No. 1. 
TANNING AND TANNING 
MATERIALS.* 
O much has the " new chrome 
process of tanning " been re- 
ferred to lately, without ex- 
plaining what that process is, 
that it may be of interest to 
give some information about 
tanning materials in general. 
It is not so very many years 
ago that all kinds of Tanning material, — contained 
in oak bark, oak galls, valoni*, myrabolans, wattle, 
tea-leaves, and elsewhere, — were supposed to consist 
of a single similar principle, which was denomi- 
nated Taniin or Tannic acid, and was supposed to 
be a compound of a kind of sugar (called Glucose) 
and Gallic acid. This was found to be a mistake; 
and it was soon recognized that the tannin in oak 
galls difiered from that of oak bark, and these 
difiered from that in tea and coffee, and so on ; so 
that that of galls was called Gallo-tannic acid, of oak- 
bark qnerci-tannic acid, of coffee oaffe-tannic acid, 
and so on ; almost every kind of taunin being 
separate and distinguished by its special name. 
At the same time, although they have different 
chemical constitution, they are all more or less 
closely allied, and have very nearly the same pro- 
parties. They all possess a powerfully astringent, 
but not bitter, taste ; they all form with solution 
of Raits of peroxide of iron a bluish-black, or greenish- 
black, ink solution ; they all precipitate albumen, 
and combine to form insoluble compounds with 
gelatine tissue, the principal ingredient of animal 
skins, and thereby convert such skins into leather; 
they are all colourless (or feebly coloured) slightly 
acid compounds, consisting of organic acids or anhy- 
drides (acids from which component water has been 
extracted), soluble in cold water and alcohol, uniting 
with alkalies to form soluble salts which readily 
•Specially written by an Anglo-Indian Officer for 
the Troptcai AgnovltViVist, 
absorb oxygen from the air and produce dark-coloured 
products. They are found in almost all the organs 
of plants, but chiefly in the bark, wood, leaves and 
fruits. They are generally associated therein with 
colouring matters, which are also allied to the tannin, 
so much so that on decomposition they often yield 
the same products as the tanuin ; and very possibly 
this association has a good deal to do with the 
smaller differences in tannins of the same class. 
These colouring assooiates, like the tannin itself, 
are also soluble in water ; and when tannin is ex- 
tracted from the bark, leaves, etc., the extract consists 
of taunin and non-tannin, the latter being everything 
dissolved out by the] water that is not tannin. The 
value of an extract depends not only on the per- 
centage of tannin present, but is depreciated by 
an unduly high proportion ol! uon-taunin which should 
not exceed one-half the percentage of tannin, and 
depends also on the colour, a s; .ndard now being 
adopted to measure the coloui. 
There are two principal classes of tannins : pyro- 
gallol tannins and catechol tannins. The first class 
include digallic, ellagic or ellagi-tannic, quercitannic, 
f^allo-tannic, rnfigaline acids, phlobaphen, etc., which 
on decomposition produce pyrogallol, a substance 
much used in photography ; ihese usually give a 
light green or yellowish colour, and are mostly 
produced in fruits and seeds. The second class 
include catechine, catechu-tannic, mimo-tannic, kino- 
tannic, etc., acids, which on decomposition produce 
pyro-satechin and phloroglucin and not pyrogallol j 
these usually give a dark reddish colour, and ara 
mostly produced in barks and woods. Leaves pro- 
duce sometimes the first, and sometimes the second, 
class oc taunin, and if they produce the former, 
«-!^y are usually suited for the miiuufactare of 
morocco leather. 
All these natural tannins are very complex organic 
compounds of carbon, hydrogen and oxygen, and are 
very liable to decomposition, especially ucder the 
influence of air and moisture. Complex organic 
bodies are always more liable to decomposition than 
