This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Sulphate of copper ... ... ... 16 lb.
Quicklime ............ 11 „
Water............... 100 gall.
Place the sulphate of copper in a coarse sack, and hang it just below the surface of a few gallons of water in a cask until melted. In another cask slake the lime gradually until it is reduced to a creamy consistency. When both are thoroughly dissolved, each should be made up to 50 gall, with water. Then pour the milk of lime and the solution of sulphate of copper slowly into one vessel, and stir the whole thoroughly for five minutes.
For determining whether the mixture is safe to use, the usual test is to place the blade of a knife in the solution for one minute. If the blade remains unchanged, the solution is safe to use; but if the blade becomes coated with copper, more milk of lime should be added. A more certain test for the presence of free copper in the mixture is to put a few drops of a solution of potassium ferrocyanide, along with some water, into a white saucer, and to drop into this some of the clear Bordeaux mixture. If the liquid becomes red or brown it shows that there is copper in solution; whereas if there is no change of colour, the mixture is safe to use. The addition of lime, as before, corrects the mixture.
The preparation of Bordeaux mixture is given because it is very important to remember that the home-made material is much superior to any of the commercial mixtures on sale. It must also be borne in mind that reliable Bordeaux mixture can only be made from good materials. Sulphate of copper is often adulterated with other substances, more especially with sulphate of iron, which is useless as a fungicide. In purchasing sulphate of copper, 98 per cent purity should be insisted upon. The quality of the lime is also an important point. It should be quicklime in the proper sense; partly air-slaked lime results in scorched foliage. The lime should be in lumps, and when slaked should form a creamy mass devoid of lumps and grit, otherwise the nozzle of the spraying machine will become clogged.
A modified method of preparing Bordeaux mixture has recently been devised by Mr. Spencer U. Pickering, F.R.S., of the Woburn Experimental Fruit Farm, which promises to supersede the older method. The point of difference consists in using lime water instead of milk of lime or dissolved lime. This results in a clear liquid which cannot clog the nozzle of the sprayer. It acts at once as a fungicide; whereas ordinary Bordeaux mixture has no fungicidal action until it has been on the foliage for some days. Finally, there is less danger of scorching. It is prepared as follows: Dissolve 6 lb. of sulphate of copper, in the way advised under Bordeaux mixture, in a wooden or earthenware vessel. Take 3 lb. of good quicklime and slake it in a little water; then put it into a tub with 120 gall, of soft water. Stir thoroughly, then leave the mixture to settle. The clear liquid is known as lime water. Add this lime water to the dissolved sulphate of copper to make up 100 gall. This will have the same strength as the Bordeaux mixture previously described. The same test should be applied to find out whether it is safe to use without scorching the foliage. If copper is found to be present, add more lime water.