It is essential that the indigo used should be of the best quality, and ground to so fine a powder that it will float on water. Coarsely ground indigo will never reduce and can be found at the bottom of the vat unchanged. It should be so fine that no roughness is felt with the tongue. Buy the best quality indigo ready ground, and if possible mixed to a paste with water. A 20% paste, _i.e._ 20% of indigo and 80% of water, is a usual quantity. If indigo powder must be used it must be mixed to a paste very carefully, as it will, if properly ground, fly about like dust. The easiest method of mixing is to pour the required amount of boiling water into a jar (previously heated), then put in the indigo. Close the vessel tightly. The steam which rises will moisten the indigo so that it loses its tendency to fly about. After 10 or 15 minutes it can easily be mixed with a stick. The zinc dust should be dry and not caked. _The lime_ should be in hard lumps. It should be bought from a reliable chemist in a sealed container, and kept sealed till wanted. If it is crumbling and cracking it has been exposed to damp air, and is partly slaked already, and therefore more or less useless. As the indigo is more quickly reduced in a concentrated solution, a stock vat is first made and this is added to the dye vat as required. The vessel for the stock vat should have a well-fitting lid. A stoneware jar with a bung will do very well. To make a stock vat sufficient to furnish a dye vat containing 15-20 gallons use:-- 10 oz. Indigo 20% paste (or 2-1/2 oz. indigo pasted with 7-1/2 oz. of water), 1-1/2 oz. zinc dust, 4-5 oz. quick lime, 4-5 pints of water. Mix the zinc dust to a paste with a little of the water, gradually add the indigo and the rest of the water. The heat of the water should be not less than 160 deg.F. as it will cool while the lime is being prepared. Slake the lime in a separate vessel by pouring about 5 oz. of water over it. When it begins to hiss and break, add more water little by little. When all the lumps have cracked up stir till a thick even cream is made. Add this to the other ingredients in the stock vat. Stir well. The stock vat should have a temperature of 120-140 deg.F. It should be stirred at intervals. The vessel should be stood in hot water to keep the temperature as near 120 deg.F. as possible. In about 5 hours the mixture has a pure yellow colour and is ready to add to the dye vat. (There is of course a blue-black scum of indigo on top.)