Making the Point - why sapphire?

Since the demise of the Apollo / Transco lacquer plant and ruby cutting styli production in February 2020, we are frequently asked why Sapphire? and occasionally why not Diamond for cutting lacquer masters.

Synthetic Sapphire and Ruby, also known as Corundum, have the same chemical composition, crystal structure and properties of the natural gemstones.  Synthetic Sapphire is 99.99% pure AI₂O₃ and is clear and colourless.  Ruby contains a trace of chromium to produce its red colour. 

Defining properties include durability, hardness (measuring 9 on the Mohs scale, Diamond is 10 and is 4 times harder than Sapphire), non-porosity corrosion resistance and chemical resistance, has excellent insulating properties and high thermal conductivity.  A very pure material, it has good wear resistance and dimensional stability.

Diamond is generally not a good material for a cutting stylus but is excellent as a playback cartridge (more on this to follow).  Cutting styli for professional use are made of corundum, better known as Sapphire or Ruby, which will outperform a diamond and produce superior recordings.  Since those early recordings on wax, recording blanks have been called by various names including instantaneous discs, lacquers, acetates, soft-cuts, and others.  The most accurate is probably “lacquer”.

There are two basic problems when cutting lacquers with a diamond.  First of all, it must be remembered that a diamond is one of the best heat conductors there is, and early examples of diamond cutting styli were prone to causing heating wire fractures, sometimes after only a few hours.  Another problem is that the heat generated could go straight to the cutter head and cause considerable and expensive damage.

Static is the other problem, and if not earthed there is a tendency for the swarf (chip) to get wrapped around the stylus - in fact anywhere except into the suction tube.   Sapphire and Ruby do not generate a static charge.

Micro-point cutting styli now give an average of up to 50 hours cutting.  With reports that some experienced mastering engineers are achieving almost double that.