OK, so you have your master tape - check, so, how do we get from this to a vinyl record then? Let's for the moment ignore the Direct Metal Master (DMM) process and stick with the more traditional blank acetate process which starts with a circular aluminium blank of 2mm thick aluminium in a diameter choice of either 16. 12.5, 10.5 & 7.5 inches. This is taken and one side of it is mechanically polished until a 1P non directional polished reflective finish has been achieved.
Following polishing, the disc is carefully degreased using a three stage process that uses an ethoxylate surfactant aqueous wash, a water rinse and finally a polar organic solvent rinse to ensure the polished surface is scrupulously clean. After drying, the disc is placed shiny side up on a pedestal placed on a conveyer belt and sent through a curtain coater where a layer of nitro-cellulose lacquer is applied to the surface.
The nitrocellulose lacquer is rather similar to the cellulose paint that was used in the painting of cars ( and still is by some of us classic car enthusiasts) but differs as it has a much higher loading of a proprietary plasticiser based on an n- butyl phthalate admixture. In the early days, castor oil was used as a plasticiser but as this rapidly oxidised and as this mankiness progressed ( technical term!), the lacquer surface was marred by crazing and decomposed areas that looked as though they were mould riddled so this didn't last long. The result of this application can be an absolutely exquisite coating which is even in thickness, mirror finished and beautiful to behold. To me, it really is a marvel of surface chemistry that man designed such a perfect coating system.
Following coating the polished aluminium blank is is then termed an 'acetate' which in itself is a misnomer, adopted from the early days of record pressing where the coating composed of an acetate material and the so produced 'acetate' was going to be used to make a shellac 78 record!.
Once coated, the 'acetate' went through a tunnel drier to set the lacquer surface and each one was 100% inspected by keen eyed QC staff and a reject rate of 50% was not uncommon.
The rejected acetates were recycled for further use, whereas, the approved items were fitted with a rim protector that allowed the acetates to be stored and stacked with a 1mm gap to prevent chafing and damage to the mirrored surface upon storage and transit.
The last stage was to centre the coated acetate on a hydraulic press and press a centre hole.
In the next blog entry on this subject, I'll let you know what then happened to the acetates in the convoluted journey from recording to record.