To the saucepan containing the indigo (100 per cent) add 7 fluid ozs. of the caustic soda solution, then gradually add 3-1/2 pints of hydrosulphite solution, stirring gently for 15 to 20 minutes. Heat the saucepan to 120 deg.F. and on no account to more than 140 deg.F.--_overheating will ruin the Stock Solution_--let it stand for half an hour, then test with a strip of glass. This should show a perfectly clear golden yellow colour (turning blue in 45 secs. approx.), free from spots. If dark spots show, this indicates undissolved indigo, therefore gradually add hydrosulphite solution (2-3 fluid ozs.). Wait 15 mins. and test with glass strip; if incorrect continue this every 15 minutes until the glass indicates clear yellow. If the Stock Solution is greenish white and turbid, undissolved _indigo white_ is present. Add then not more than a teaspoonful at a time caustic soda solution until the Stock Solution answers the glass test. The _Dye Vat_ should contain about 10 gallons of water heated to hand hot, 120 deg. and not above 140 deg.F. Add 3 ozs. of hydrosulphite solution stirring carefully, let it stand for 20 minutes; this renders harmless any undissolved oxygen. Add a small cupful of the Stock Solution, stir carefully without splashing. The vats should be greenish yellow and _should not feel slimy_, an indication of too much caustic. The vat is now ready to dye and is kept at 120 deg. to 140 deg.F. Between dips add Stock Solution as required, if the vat goes blue and turbid add 3 to 4 fluid ozs. of hydrosulphite and warm up to 140 deg.F. and wait 30 minutes. As a last resort add caustic soda solution very gradually. This should not be required if the Stock Solution is properly prepared. Start to dye with weak vats, 20 to 40 minute dips, and finish with stronger vats. The more dips given to obtain a fixed shade, the faster will be the yarn to washing and rubbing. The yarn must be oxidized by exposure to the air for the same length of time as dipped. After the final dip, pass the yarn through a 10 gallon bath of water to which is added 3 ozs. of sulphuric acid, pure or hydrochloric. This neutralizes the caustic used. Wash yarn at least twice in water. _Improvement of Defective Indigo Vat Dyes._ If, after washing until clear, the yarn should rub off badly, there is but one remedy. Wash same in Fuller's earth, and if the shade is then too pale, re-dye. If, through bad management of the vats, the yarn is dull, pass the yarn through a hot bath (100% water, 1% acetic acid) and wash in two waters. If yarn is streaky, take 10 gallons of water at 120 deg.F., 1 oz. of hydrosulphite powder, 2 fluid ozs. liquid ammonia fort. 880, and let yarn lie in same for 60 minutes. Wash in two waters. The following facts should be carefully noted:-- The Caustic Soda is the _alkali_ which dissolves the Indigo White. The Hydrosulphite _reduces_ the Indigotine in the Indigo to _Indigo White_.