Linde A is a graded calcined alumina (aluminium oxide) polishing compound with a nominal particle size of approximately 0.3 micrometres. Originally produced by the Linde Air Products Company (a division of Union Carbide, now part of Praxair-Linde plc following corporate evolution), Linde A became the de facto standard fine alumina for lapidary final polishing during the mid-twentieth century, alongside its coarser counterpart Linde B. The name remains in active use in the trade despite multiple changes in corporate manufacture and supply.

Composition and grade

Linde A is calcined alpha-alumina, the same crystalline phase as natural corundum, refined to a tightly controlled particle size distribution centred on 0.3 micrometres. The particles are angular rather than spherical, providing the cutting action required to remove fine surface irregularities. The grade was originally specified for metallographic sample preparation in industrial laboratories and was adopted into lapidary use because of its consistency and availability.

Lapidary application

In facetted-gem cutting Linde A is used in the final polishing stage on tin, lead, zinc, or composite laps following coarser cutting and pre-polishing with diamond compounds in graded sequence. Typical application is as a slurry mixed with water or a glycol-based vehicle, applied sparingly to the lap. The polish is suitable for corundum (sapphire and ruby) where its fine cutting action removes residual diamond marks, and for many other species including spinel, garnet, and softer materials. For diamond, where Linde A is too soft to polish the harder material effectively, diamond compounds remain the standard.

The principal advantage of Linde A over diamond compounds in the final polishing stage on softer-than-corundum species is the absence of the very fine pitting and residual scratching characteristic of diamond polish on these stones. The disadvantages include slower stock removal and greater operator skill required to maintain a flat facet.