DR. C. Ti. PLOWRIGHT ON WOAU AS A BLUE DYE. 
145 
What really takes place in a Woad vat is concisely this : — Indigo 
blue is a very insoluble substance ; it will not dissolve in any 
of the ordinary solvents such as hot or cold water, dilute acids, 
alkalies, alcohol, ether, chloroform, etc. Hence it is a very fast 
dye if it can only be made attach itself to a fabric. In order that 
this may be done it is necessary to dissolve it ; but as we have 
seen, none of the ordinary solvents will do this. What happens in 
the Woad vat is this : the chemical composition of the insoluble 
indigo-blue is altered, it is, as chemists say, reduced to indigo- 
white ; now indigo-white is soluble in weak alkaline solution, 
hence the use of the slaked lime. If a skein of wool be dipped 
into a vat containing indigo-white in this state, the solution soaks 
into the tissues of the wool iibres ; when the wool is taken out 
and exposed to the air the oxygen unites with the reduced indigo, 
and the skein passes from a greenish yellow to a deep blue, the 
insoluble indigo-blue being thus formed and the fabric dyed in 
such a way that no mordant is required. The chemical changes which 
take place in the Woad vat when once started are, that the starch of 
the bran is converted into grape sugar, which becomes lactic acid. 
The lactic acid becomes butyric acid, and in so doing nascent 
hydrogen is liberated, which reduces the indigo to indigo-white.* 
Indigo is soluble in strong sulphuric acid, and there are other 
processes by which it can be reduced, but the above is the rationale 
of the Woad vat, which has held its own from the time when the 
raediajval dyers added a little indigo to the vat to improve the 
colour of the blue down to this present time. It is an expensive, 
awkward, and difficult process, but it Inis this one .advantage : the 
colour produced is extremely durable. In actual practice a little 
madder is added : this is done, the dyers say, “to kill the green” 
in the indigo. 
Woad was used long before indigo came into Europe, not as a 
solvent, but as a dye per se. Woad contains no indigo ready 
formed ; not the slightest trace of any blue colour can be detected 
in it. With water it forms a dark-brown mixture which colours 
woollen fabrics olive-green. In order to dye with Woad all that is 
necessary is to pour boiling water on the Woad, and keep it in a 
well-covered vessel for 15 or 20 hours at a temperature of about 
# Rawson Christopher. Cantor Lecture. ‘Journal of the Society of Arts,’ 
vol. xlviii. Ap. 6, 190J, p. 424. 
