Not sandblasting

What's in a name

The title was shotblasting, but I am informed that glassies would not recognise the term as they sandblasted; but not with sand! Having made a concession to common usage, please indulge me if from here on I use shotblasting as best describing the use of different types of shot in the blasting process.

The blasting process may be described as hurling rocks at the glass with demonic force, with the objective of knocking lumps off it. Effectively it is a form of etching.

The process is widely used in industry with a wide variety of materials being used as the abrasive, but sand is no longer one of them; because of the very real health risk it presents. In the blasting process the grit particles break up into ever smaller fragments and, were sand to be used, the air would be full of potentially hazardous respirable silica particles. In many areas, OH&S laws ban the use of sand altogether.

Shotblasting is an abrasive process, and as has been mentioned elsewhere, for maximum cutting efficiency an abrasive medium should be at least as hard as the material being cut.

Shotblasting Glass

Glass is a hard material, and garnet is sufficiently hard to be an effective cutting medium. Industrial grade garnet is a mined material, of reasonably uniform shape, easily crushed and screened and relatively inexpensive.

Silicon carbide is another suitable material but is more expensive, so is rarely used for shotblasting glass.

Not that using garnet completely eliminates the ‘silica’ problem. Garnets are themselves a group of silicate minerals, but at least they do not break up into pure silica dust.

However, the extremely fine glass particles blasted free are silica, so extreme care should be taken to keep them under control.    

Wikipedia calls it sandblasting and describes it as ‘forcing solid particles across the surface’ and likens it to a wind storm and sand erosion.  This description is adequate for commercial operations such as cleaning buildings metalwork and foundry castings, but does not cover the more intricate operations performed by glass artists.

To them, besides creating an overall frosted appearance of uniform or varying texture, the gun can be used as a tool to carve small or large areas of the glass  to varying depths; to create areas of differing texture so as to transmit varying amounts of light.

The gun or nozzle can be used in a highly directional manner so as to cut either square-on to the glass, or at an angle to undercut; to produce sharp lines or softly rounded feathered and hazy outlines.   

Resists

Whilst being directional, the stream of abrasive is not highly focused, but is scattered over an area with the principal cutting action concentrated in the centre.

To achieve sharp lines, a resist or mask is used to confine the cutting action to a selected area of the glass.

There are many materials which can be used as a resist. They include:

• Adhesive backed film as used by the sign writing industry
• Masking tape
• Duct (silver gaffa) tape
• Cellotape
• Electrical insulating tape
• PVA glue
• Silicone

This list is far from exhaustive.

Adhesive film

Time was when adhesive plastic film of the type used to cover kitchen shelves and school books was used as a resist. One popular brand was Contact.

This was applied to the glass, a design was drawn and cut with a sharp blade and the areas of glass to be etched were ‘weeded’ out. All this is similar to techniques used for masking for acid etching or painting.

However, the Contact film was thin and the adhesive sometimes unable to resist the blast of air, so that it lifted at the edges; lines became uneven and edges blurry.

The film was often unable to resist the impact of the abrasive gri, so that pinholes were punched into it and damage was done to supposedly safe areas of glass.t.

More recently, heavier films with much more tenacious adhesives have been developed for the signwriting and graphic design industries, so wavy edges are less of a problem.

Concurrently, computer aided graphics allowed the signwriting industry to shift from paint pot and brush toward machine cut stick-on lettering.

Which film to use?

First, it should be said that the term resist is almost unknown to the makers and sellers of self adhesive films. To them, they are ‘graphic films’ which can be translucent or opaque and offered in a vast range of colours; as well as black  white and clear.

The films can be vinyl or, less frequently, polypropylene.

Some films are chosen as much because they can be bought by the metre as for any knowledge of their suitability for the application.

Among the properties cited for graphic film will be suitability for external use, resistance to UV and weathering, years of reliable performance. There is bemusement when a life of about ten minutes is stated for a shotblast resist. There are a wide range of adhesives used, some with the most surprising names. Neither colour nor name of adhesive gives any indication of resistance to perforation nor lifting.

There is much more on this topic in the course of preparation. Watch NEWS for more!