If any of you are wondering about the possible harm from use of borax of less than Photo grade, whatever that is, I will show you how to reduce impurities in 160 grams of 20 Mule Team or other Technical Grade borax by a factor of at least sixteen at the expense of less than 40 grams of the cheap stuff.
I have recommended making a stock solution of borax and ascorbic acid to which various developing agents can be added to make a complete developer. The stock solution requires about 25 grams of borax per liter.
Weigh out about 200 grams of borax into a bucket big enough to hold 4 liters. Add two liters of ice water. It need not be distilled water at this point, but it will make your conscience feel better if it is water that you would not mind using for D-76. Stir well, let it settle a while and stir it again. This time let it settle while you go watch a TV show or something. What has happened is that only about 40 grams of the borax have dissolved, but all the soluble impurities from the original 200 grams have dissolved. Now decant as much as you can of the clear liquid. You should be able to recover at least 1.9 liters which will contain all but about 5% of the original impurity. That 5% remains in the water that is held by the sediment. You may even be able to squeeze more out by filtering, but do it after decanting. Filtering borax slurry would try the patience of a saint, and I know of no incantation that will speed it up. We shall just have to rely mostly on decantation.
Now the remaining sediment will contain only about 5% of the original soluble impurities in about 80% of the original borax. True, it contains a somewhat higher ratio of insoluble impurities, but need not worry over that. We are not trying to recover powdered borax, but are seeking a solution of borax of certain concentration. Insoluble impurities, by definition, will not dissolve, although the settling time for some may be very long. We will just leave the insoluble stuff behind. So, add 3 liters of water to the sludge in the bucket and stir well. If you do this with water at 17 degrees C (63.6 F) you will saturate the solution at 4 grams/100 grams, using about 120 of the remaining 160 grams.
Again, decant the clear liquid. This time you will get all 3 liters back plus a little because the sludge already was wet from the first solution. The solution is fixed at 4.00% by weight as long as the temperature stays above 63.6 F or 17 C. If it goes below, it will either supercool or precipitate some borax. Measuring the borax will now be done by weighing the solution. Note that neither the water nor the borax had to be measured accurately, but the weight percent is accurately determined by the temperature at which the saturated solution was made, and cannot change once the source (sludge) is no longer present, and as long as the measuring temperature stays above that temperature. 0.1 grams of the solution contains 0.004 grams of borax decahydrate. Thus, measuring the solution to the nearest 1/10 gram is as precise as measuring the powder to the nearest 0.004 grams.
Much of this discussion is of no concern if the solution is only to be used to make the developer's borax-ascorbate stock. Just add another liter of water to the 3 liters of saturated solution you have just made and add 36 grams of ascorbic acid, and you're ready to go.
In a numerical example, suppose 1330 ppm maximum total soluble impurities are guaranteed. Then:
Impurities carried in by 200 grams = 200 * 0.00133 = .266 grams maximum.
2 liters water were added, 1.9 liters liquid decanted. The ratio of impurities dissolved in the output water to those dissolved in the input is 1.9/2 = 0.95. The impurity remaining in the sediment is thus 0.05 * 0.266 = 0.0133 grams. The impurities in the sediment have been reduced by a factor of 20, but only 160 grams remain in the sediment. Thus the total soluble impurities are about 83 ppm, 1/16 of the original.