How to Prepare Abrasive Powders

Alvaro Amaro de Azevedo

Alvaro's article on grinding miniature lenses "The Challenge of Grinding a Miniature Biconvex Lens ," The Citizen Scientist, 13 January 2006) so intrigued Shawn Carlson and me that we asked him to provide this technical note on how he prepared his grinding compounds. Editor.

Arrange about 200 g of washed dried sand and put it in a ball mill (see here or search here). Let it crush for one hour or so and then transfer the flour to a 200 ml glass. Fill the glass with distilled water (Fig. 1) and stir vigorously until all sediments get suspended.

Figure 1. Place 200 g sand crushed in a ball mill into an empty glass.

Stop stirring, wait for 30 seconds and then pour quickly the content of that glass to another empty glass the same size (Fig. 2).

Figure 2. Crushed sand after being stirred into suspension.

Stir again the suspension and let it rest for one minute. Then pour the content to another glass the same size. Repeat this procedure but now wait 2 minutes to the next transfer. Every time the suspension is transferred, you have to wait the double of the previous waiting time. After repeating the process six times you will have six empty glasses in the bottom of each lying a layer of sediment (Fig. 3).

Figure 3. Powder having increasingly smaller sizes is sorted in these empty glasses.

Let these sediments dry out over night and transfer each one to a vial having an identification number (Fig. 4).

Figure 4. Use marked vials to store the various grades of grinding compound.

The coarsest powder will be the one that is decanted first, while the finest powder will be the one that is decanted last, after resting 16 minutes.

The estimation of the accurate granulometry values is not so relevant as long as you use the powders in progressive order (from the coarsest to the finest) during grinding.