Grinding & polishing glass

Overview

Grinding and polishing glass is similar to doing the same operations on any hard material, be it stone, metal, gemstones. The process is called ‘abrading’ and is defined as ‘scraping or wearing away by friction’.

• Grinding makes use of coarse particles of hard material to rapidly remove large quantities of glass, be it for shaping or for removing gross surface imperfections.
• Polishing uses particles of ever decreasing size and often of softer materials to remove finer surface imperfections, so that any remaining are too small to scatter the light.

The abrasive materials are often attached to paper or cloth backings, or bonded to discs or machined shapes to allow them to be used with power tools.The waste product produced can clog the surface and reduce the cutting action, so water is used to help flush it away. The use of water is important when grinding glass to prevent the localised build-up of heat, which could break small pieces or sheets. and cause fine crazing in larger pieces such as castings. Loose abrasive particles can be used with mops or hand held pads for fine polishing.

Whether grinding or polishing, particles of ever reducing size are used as the process continues, until the final polish is done with materials which appear to be no coarser than fine dust.

The processes are similar to those used by lapidarists and gemologists in grinding and polishing gemstones and mineral specimens, by monumental and stone masons on major buildings or monuments, or by metal workers polishing bright plated parts. 

The techniques and equipment used will vary with the size of the piece and on the time and money available. A laborious job by hand can be done more speedily with the aid of power driven machinery.

Grinding and polishing can be done to pieces large or small; using a range of electric or air powered tools, grinding wheels or discs, polishing mops or hand held pads.

• Mould marks can be removed from glass castings.
• Large shards or cast pieces can be carved or shaped.
• Edges of mating pieces may be ground to improve the fit.
• A slumped piece may have edges re-shaped to remove imperfections.
• A bent glass piece may have surface imperfections removed
• All may be polished to whatever level of clarity and brilliance is desired.

What takes place–abrasion 

When two pieces of material are rubbed together they will both be worn away to some degree. Thus, when walking on a stone or concrete surface, friction will wear away both the sole of the shoe and the pavement.

The rate at which each surface is worn down will depend on their relative resistance to wear, and on the pressure applied. Continuous rubbing of a hard material with a softer one will wear down both; but rubbing with a harder material will remove more, and faster.

Abrasive particles

The ‘Mohs’ scale, ‘rating of scratch hardness’ lists 10 minerals in the order of their ability to scratch or abrade another. It goes from the softest, talc at 1, to diamond at 10.

A material with a high number can scratch a material with a lower number, but not the other way round.

Glass rates a hardness of about 5 – 6, so material best suited to the speedy scratch removal of glass should be toward the top end of that scale.

Readily available materials suitable for scratching glass, and thus for grinding and polishing it, in descending order of hardness, include;

• Diamond.   10
• Silicon carbide.   9.5
• Corundum.   9
• Aluminium oxide / alumina.   8
• Garnet.   7 / 7.5

They are either naturally occurring or synthetic minerals.
As hardness and high melting point are related, good abrasives also have good refractory properties; so it is not surprising that many of the materials mentioned here will appear in other places throughout this site; such as in battwash ingredients, as bedding powders or as kiln furniture.

One characteristic of refractoriness is ‘brittleness’.  The materials have a regular structure and sharp edges. When crushed, they become smaller versions of their larger ‘parent’, retaining the sharp edges and regular structure.

They are ‘multi faceted’, somewhat like a diamond, but the actual shape and arrangement of faces varies with the chemical composition of the particular mineral.

Larger lumps are crushed and then screened into various grades, each grade representing a narrow range of particle sizes. 

Whatever the number and arrangement of the faces on an abrasive particle, a ‘point’ or an ‘edge’ where faces meet becomes a cutting tool to make a gouge or furrow.  

Force is required to make the cut, so the particle must be firmly held, with appropriate pressure applied if it is to dig in and not just slide over the surface and produce heat but no cut.

With repeated cutting the point will wear away and become blunt. Greater force will be required to make a cut. Less cutting will be done and more heat will be produced.

The increased force can be enough to cause the particle to fracture; to cleave along one of the faces where the internal bond is the weakest. The dull piece will fall away and a fresh, sharp cutting tip will be exposed; the process will be repeated until the particle cannot cleave any further.

A single abrasive particle is generally ineffective as a cutting tool, although a single industrial diamond in a holder makes a long lasting glass cutter or dressing tool for grinding wheels. Whilst loose particles are used with pads for hand rubbing, more frequently the particles are:

• bonded to cloth or paper backings which can be formed into sanding belts or other shapes,
• cemented to flat or shaped metal supports for use on tools such as glass spindle grinders or ring saws,
• Formed into wheels or discs for use on power tools.
• Formed into regular shapes such as slip or hand sharpening stones, or as polishing blocks or pads.

The scraping action

Grinding or polishing involves rubbing the glass with abrasive particles so that the glass is scraped away. Pressure must be applied to ensure that the abrasive digs in and does not just slide over the surface.

This can be using an abrasive stone to remove needle points on a fired piece, or cleaning up the edge of a piece after cutting.  A similar job can be done with a cloth backed abrasive sheet wrapped around a sanding block, as is done by a painter or carpenter cleaning up their work.  Simple tasks such as this will not produce much heat, so water cooling of the glass won’t always be needed; the waste can be shaken or wiped off.

Abrasive particles can be pressed into the surface of a fine cork or soft plastic block and used to grind or polish; these can be flat or contoured to suit the piece. Similarly with a cloth pad which is sometimes used wet to help retain the particles. Blocks of various shapes and sizes, impregnated with diamond  grit, are available in grit sizes as fine as 1800 mesh.

Diamond coated tools of complex shape are available to smooth awkward spots. They are the equivalent of the special files used by toolmakers and are available from specialist or large industrial hardware suppliers.

Cross contamination

The ultimate objective of grinding and polishing is to achieve a perfectly smooth and transparent surface.

Each of the abrasive particles will carve a score or groove, and the larger the particle the deeper the score. As the operation progresses, particles of reduced size are used to  produce an ever finer and smoother surface.

Extreme cleanliness should be exercised when progressing through these stages of grinding or polishing. Make sure that every trace of coarser abrasive or glass waste is removed before commencing the next stage; else a coarser particle ‘puts a creek bed in a ploughed field’.

• Different mops pads and cloths should be used for each grit size, and be stored in plastic bags or sealable containers when not in use.
• Be careful of what coarse residue may remain in water containers.

Because of this risk of cross contamination, it is usual to use only a limited number of different grit sizes, else one be continually cleaning. In some cases the choice will be made by the maker of a piece of machinery such as a belt sander, who will supply belts in only one or two grit sizes. In some large operations a number of machines will be used, with a different grit used on each machine. All that then needs doing is to wash the glass thoroughly between machines. Oh, such luxury.

Water cooling

As mentioned previously, water is used to keep the glass cool and to flush away glass particles and worn or broken abrasive. Water is an amazingly efficient coolant, so not a lot is required for that purpose; but a greater flow may be needed for flushing. As most power tools use belts or discs running at a high speed, water can be thrown far and wide.  When using hand-held tools, the wearing of wet weather gear can improve ones personal comfort considerably.  With fixed machinery, that is less of a problem but attention should be taken to ensure that water does not splash onto or into electrical components.

Power tools

All these operations can be speeded up considerably by the use of power tools, which can take many forms. They will usually comprise some form of tool with disposable cutters which will be discussed under ‘consumables’ below.

Hand held power tools

Water and electricity don’t always mix. Fortunately, there are better quality electric power tools made to be used in damp conditions. In addition, most types of hand held power tool are available as compressed air driven as well as electric. For those contemplating extensive hand-held power tool work on glass, the investigation of air driven tools is recommended.

The most common hand held tool for grinding is the ubiquitous angle grinder. This can be used with cloth backed discs of numerous grit sizes as well as bonded discs.

Smaller models accepting 100mm (4 inch) or 125mm (5 inch) discs are so cheap that they have become a throw-away item. However, they have an open case with a fan to draw cooling air through the motor. Moisture getting inside the motor can cause electrical breakdown in such situations. Fortunately, centrifugal force throws the water away from the machine, so they will work OK provided the dampness level in the air is kept down.
Sometimes a power drill may be used with discs attached to a spindle held in the chuck.  This is awkward to use, as both the tool and the position in which it is held gives little control. There is an ever present risk of the edge of the disc digging in and making a gouge which can be time consuming to remove.

Many machines have been developed for the commercial glazing industry such as belt sanders, flat bed polishers, circular and band saws. Smaller versions for the art glass market have been available for some time, and the increasing demand has seen drastic reductions in price.

As these have applications other than grinding or polishing, they will be covered elsewhere under  ‘Power tools for glass working’.

Grinding

Unless one wants a matt or frosted finish, grinding will usually be the preliminary to polishing. The objective may be to smooth out the surface, remove bumps and blemishes, get rid of residue from a mould, maybe remove some devit before flame or hand polishing.

In other cases a large block of glass may be carved by grinding to create a sculpture; in which case unevenness may be desired; creases in a face or in a fold of fabric.

Grinding can proceed through a number of stages where abrasives of increasing fineness are used. The first and last grit sizes chosen can vary widely, depending on the process which is to follow.

• If the piece is to be polished, then the surface will be taken down to a high level of fineness with extremely fine abrasive. 
• If the grinding is to clean up cut edges before firing, then a medium to fine grade abrasive stone can be used.
• If the purpose is to remove needlepoints or to reduce edge sharpness on a fired piece, then a fine grade abrasive stone can be used, followed by polishing if considered necessary.

Polishing

The most widely used abrasive for polishing glass is cerium oxide. It is a processed product and varies from brown to almost white in colour, depending on purity. Impurities don’t impair its effect as a polishing agent, so the brownish material is OK. It’s also the cheapest.

Rouge was previously used for this purpose, but is now difficult to obtain as it has been declared to be carcinogenic. It was originally known as jewellers rouge and consisted of very fine particles of ferric oxide, (iron oxide red as used as a pigment in paints; it is the common red rust on steel) mixed with organic materials to make a waxy block. It was convenient as it was easy to apply, and stayed where it was put.

Apart from the polishing medium, the other important ingredient is patience. Polishing glass is a very time consuming process and cannot be rushed.

Consumables

Abrasive grits

As has been seen, glass is a minute portion of the market using abrasive products, so the cost of suitable products is considerably lower than it otherwise may have been.

The two most readily available materials are silicon carbide (grey) and aluminium oxide (white) and these are available in a large number of size ranges; 12, 16, 24, 36, 50, 60, 80, 100, 120, 150, 180, 220, 360. Thus, 50 grit will include all those particles which passed through a 36 mesh screen and were trapped on a 50 mesh screen.  This means that scores gouged by a particular grit size will be of fairly uniform depth and the glass will have a uniform appearance.
Industrial diamond is also available in a range of grit sizes, as fine as 1800 grit.

Manufactured forms

Most common forms are:

• Abrasive cloths and papers
• Hand held stones
• Grinding wheels and discs
• Mounted points
• Coatings on metal shapes.

Abrasive papers and cloths

A common term is 'sandpaper', because 'once upon a time' that's about all that people had to use. Sheets of fairly ordinary paper were coated with glue and sprinkled with sand. Neither the paper nor the glue were waterproof, so as soon as it got wet the sand fell off. Then came water resistant papers and waterproof adhesives and sheets could be used either' wet or dry'. This rapidly became known as "wet and Dry".

Sand was replaced with tougher abrasives, but we still 'sand' something down. 'Aluminium oxideing' something down doesn't have the same ring about it.

Abrasive grit of a particular size range is bonded onto cloth or paper in a uniform layer using a resin adhesive. The particles can be tightly packed or spread further apart, (closed coat or open coat) depending on the purpose for which the sheet is designed. For a specific application a particular spacing of particles may give superior performance, with clogging or a shorter life occurring with different spacings.

General purpose products sold in suburban hardware outlets will do a reasonable job on most materials.

The particles are only one layer deep, so when that layer has dulled or cleaved off to the extent possible the item is worn out.

As with all abrasives, there is a use point beyond which they are no longer doing effective work, but just producing heat. Experience will tell when this is so.

Sanding belts

When used as belts on sanding machines, the cloth product will have a recommended surface speed stated in feet or metres per second. They will work best when used close to that speed. Many machines have a spring loaded tensioner to keep the correct tension on the belt. Too much pressure between workpiece and belt can cause it to slip. If that is the only way to get it to cut, then it’s likely that the belt is worn out.

The grades supplied by the makers of glass working equipment will usually be the most appropriate for that work.

Direction of running

Sanding belts have a spliced join where one end is cemented over the top of the other. The ends are cut at a 45 degree angle, the abrasive and binder is ground off one end and the other end is cemented over the top of the cleaned end; cloth to cloth. 

The belts should have an arrow on their inside face showing the correct direction of running. If not, it is with the outer surface of the overlap trailing; so that the end of the cloth will not snag and tear.

Hand held stones

These consist of a mix of abrasive grit and a binder to hold them together.

They are most commonly made by mixing the grit with clays and other refractory matter and firing them as a ceramic, after which they are ground to shape.

Aluminium oxide (alumina) will be white, but silicon carbide will be some shade of grey, depending on the size of grit and on the proportions of grit and clay binder.The binder is weaker than the grit, so a paler grey will generally indicate a weaker bond or a more open grit structure.

Hand stones are made in a range of shapes grits and sizes. They can be rectangular for sharpening chisels and knives, round with flat faces for sharpening axes, very small for keeping in a pocket or purse, whatever.

Unlike the single layer coated abrasives, here there are multiple layers of grit, stacked layer on layer. As particles in one layer become blunt they will either cleave or be pulled completely out when the force exerted by friction breaks the bond holding them in place. A new and refreshed face will be presented. A stone will not wear evenly, so over time the surface of a flat stone will become uneven, but this can be minimised by using all areas of the surface.

Grinding wheels and discs

These are made in a manner similar to hand stones and are designed to be mounted on power tools and rotated at high speeds. They are machined true and balanced during manufacture.There are special requirements for their mounting and for their maximum speed.

When mounted on spindles, such as on a bench grinder or angle grinder, they should always be clamped between two cheeks of equal diameter. They will usually have soft paper discs stuck on both sides concentric with the mounting hole, These are known as ‘blotters’ and even out the pressure of the cheeks, to avoid overly high point pressure cracking the wheel or disc.

The cheeks should be marginally smaller in diameter than the blotters.

• Do not use wheels or discs on bench or hand grinders without blotters or with uneven size cheeks.
• Do not use them with chipped edges or with cracks. They can shatter and fly apart.

Wheels and discs are designed to be used at high speed. They cut best and will last longest when operated close to their rated speed. They wear most rapidly when running at low speed. Most importantly, do not place a load on them to slow them down, or as they are slowing down, as the wear rate will increase alarmingly.

Mounted points

These are variously shaped blobs of abrasive mounted on steel shafts for grasping in chucks or collars. They are available in a wide range of shapes and can be used for carving or engraving, as well as working in recesses and other difficult places.

Coatings on metal shapes

For glass work, this includes;

• Grinding heads for vertical spindle glass grinders.
• Edge coated discs for use in cut-off saws
• Band and ring saws of various shapes

These will usually be coated with industrial diamond which can be attached either by adhesives or by brazing or electroplating.

Diamond impregnated shapes

To be continued.