The secret of cutting glass

(Tutorial)

It's true, that watching an experienced glazier make a scratch on a piece of glass and then have the glass drop into two pieces in his hands, is a bit like watching a magician.

An experienced glazier can casually run a line down a piece of glass, cut around into a long curve and with a quick flick of the wrist, have the glass drop into exactly the design required, with no apparent effort at all.

How do they do it?

The answer is part practice, part knowledge, just like every other amazing trick.

What I am about to show you will not make you a glazier, but it will enable you to understand how it all works and give you enough information to enable you to be confident in glass cutting.

It is fun and very satisfying to grab a piece of glass, run a cutter over it and have it break down the line just like you wanted it to.

Glass types

The subject of glass is quite extensive and it is an amazing substance, however we will just talk about the types that you are likely to come across in your project.

Annealed glass

Most glass is produced as "annealed" glass. Also known as "plate" glass, "sheet" glass and other local names. When glass is in this state, it breaks into large chunks and slivers with razor sharp edges.

Main uses are in residential windows, aquariums, shop fittings and some shop fronts, and is easily cut as we will demonstrate soon.

Laminated glass

The first common treatment for annealed glass is to laminate it to another piece of the same thickness.

It is common to laminate that is, join together, 2 by 3mm pieces to get a thickness of 6.38mm (the extra .38 being the plastic interlayer).

The strength of this glass product is slightly less than the same thickness of a single thickness, but as it will not break into threatening pieces, it is considered a Grade A safety glass.

While it suffers slightly in the strength department, the advantage of this glass is that if it breaks, no large dangerous chucks are falling about to slash arteries and lop off limbs.

It is widely used in doors, low level glazing and anywhere that human impact could be a possibility, and is also easy to cut.

Toughened glass

Another common treatment of annealed glass is to subject it to heat followed by quickly cooling the surface. This has the effect of "toughening" or "tempering" the glass and its strength rates nearly 3 times its untreated condition.

Its also considered Grade A safety glass because when toughened glass breaks, it shatters into millions of pieces. Occasionally, it will simply do that without any help from a brick because its strength comes from the stresses set up during the rapid cooling.

This is satisfactory from a human impact point of view, however it is now impossible to cut.

Toughened glass has many uses, including door panels, windows, shop fronts, building walls, overhead glazing, heat-resistant viewing panels and thousands of others. When combined with other glass and plastics, it is a useful component in bullet resistant glass.

Other glass

Other types of common glass include mirrors, figured-rolled (where a pattern in rolled into the molten glass to make it opaque) and picture framing non-reflective glass.

The glass cutter

 While you can still buy the traditional glass cutter with it's wooden or steel handle, most glass cutters these days are oil-filled tubes with a small "pizza cutter" at the end.

 The wheel that does the "cutting" is actually just a steel wheel on an axle that effectively crushes a "trench" into the surface of the glass.

 It is not particularly sharp, and will not cut you if you run it over your arm or fingers, but it will put a mark on any hard surface, like glass, plastic or a ceramic floor tile.

 No doubt you have seen the movie version where the burglar "cuts" a hole in a window with a diamond and uses a glass sucker to pull out a neat circle of glass to gain entry.

 If you ever find anybody who can pull off that trick, let me know. After 30 years of cutting glass, I'd sure like to meet the person who could actually do it.

 It doesn't really matter which type of cutter you use, as you can get most satisfactory results from the wood or steel handled cutters. The main attraction of these cutters is that the initial cost is low and replacements can be purchased from hardware stores.

 The oil filled cutters on the other hand are generally only available from glaziers hardware suppliers and obviously they cost more. The reason for their popularity is in not having to dip the cutter in oil before every cut.

 The side benefit is not having the tray of oil soaked rag on the table, which can get in the way of turning or moving the glass.

 The decision on glass cutters depends largely on the amount of work to be done, but both types will do the job.

 There are variations on oil filled cutters that provide comfort for some specialised work like leadlighting. These included cutters with pistol grips, fine swivel heads, broad swivel heads amongst others.

 For most professionals one or two cutters designed for cutting either heavy plate up to 25mm thick and the other for general cutting is all that is required. You can be assured that your standard cutter is all you need for 99% of glass cutting jobs.

 The cutting part

 To understand "cutting" glass you need to understand that glass is not really a solid, like timber or steel, nor is it a liquid like water. Glass is actually part way between liquid and solid. It acts like a solid when it keeps out the rain, but it acts like a liquid when we try to "cut" it.

 The strange thing about glass is that we do not (usually) cut it. At least not in the traditional way of cutting things with a saw or knife.

 Glass is usually broken, not cut, which is why it is impossible to cut a very small amount off the side of a piece of glass.

 If you find that the piece of glass you just cut is slightly too big, you have two choices. You can either grind the glass down to the size you want or cut another piece.

 Naturally grinding down a piece of glass is time consuming and expensive as the cost of abrasive surfaces able to do the job is quite high. Glass companies like ours make decisions based on the size and cost of the glass and the time and machinery available to resize the piece.

 For example we might lift a huge shop front panel of laminated glass onto our Straight Line Edger to remove 5mm off one edge but we would not do the same for a small piece destined for a residential window. In the latter case it is cheaper to just cut another and put the first one back into stock.

 The lesson here is to make sure you double check your measurements before making the cut unless you happen to have a 5 ton, quarter million dollar, machine in your workshop to resize it for you.

 It's easy? 

The reason that you can cut glass so easily is because glass acts like a liquid, in that it is easy to pull apart, and like any other liquid, almost impossible to compress.

 Essentially, what you have to do is make a weak area on the surface (this is where the "pizza cutter" comes in) and then just pull it apart.

 This can be achieved in several ways and I will describe them now for you so you can choose which method is best for each situation, but first we need to describe the cut.

Yes, the ole "BUT FIRST"

Before we can make a cut lets cover the basic ergonomics and safety.

 Obviously you will need some place to work and a flat surface is essential. You may rarely see a glazier cut a piece of glass standing in the vertical but this is in exceptional circumstances and takes a good deal of skill.

 This leaves us with the table or the floor and we will assume you are using a table although everything that follows can be achieved on the less comfortable position on your hands and knees.

 You will need an old blanket or similar to use on the table surface. While it is possible to use a hard surface, inevitably you will scratch the glass and this is not a good look.

 I suggest an old blanket as your dearest will not take kindly to glass splinters inbedded in the blanket on a cold night.

 Tape the blanket into position to reduce slippage on the table top and it should cover at least the side closest to where you will stand.

 The next step is to set up the table with the equipment needed for the job.

 If you have an oil filled cutter, remove the knurled knob on the end and part-fill the cutter with oil. Do not tighten the nut after filling to allow some air to seep into the end of the cutter so that the oil will be released to lubricate the cutter wheel on the end. At the end of the session, you should tighten the nut to reduce seepage of oil from the cutter when not in use.

 If you are using the basic wood or steel handled cutter, you will need to obtain a small container lid and place a small piece of rag in it. Then pour a small quantity of oil into the lid and presto, you have a ready source of lubricant for your cutting wheel. You must remember to put the tip of your cutter into the oil before every cut.

 Next, you will need some sort of straight edge and that will depend on the length of straight cuts that you intend to try. Any piece of straight stiff material will do, but the most common is a flat piece of timber or aluminium.

 If you plan to do a few projects, a T-square will be required before you get really annoyed with the time it takes to make several cuts. Any art supplies or even stationery suppliers will have T-squares, but you don't need one to practice your cutting technique.

 SAFETY

 Now that you are about ready to start practicing your glass cutting skills, I won't insult your intelligence by going over obvious things like the fact that broken glass can cut you.

 Is glass cutting a safe thing to do? The answer is (for the record) "moderately".

 Can I guarantee that you will not cut yourself? For the record, "No I can't".

 I recommend that you use a pair of gardening gloves or similar, preferably with sleeves that cover your wrists. In addition you should wear glasses in the slight chance that a splinter could flick up into your eye. While this is remote, I don't want you to sue me because I didn't tell you.

 If you are still not terrified, let's get on with it.

The Rules

 Take my advice and do not start practicing until you read the three golden rules.

 The first rule is you only get ONE chance to mark the glass, so you must NEVER, EVER, go over a cut to make the score line deeper.

 This is without doubt the rule most ignored because first timers don't believe that in glass cutting deeper is not better.

 I won't even try to explain it, but take it from someone who has been doing this for 30 years.

NEVER GO OVER A CUT!!!!!!!

 Fine, you say, but what if I miss a bit of the score line? Good question and we will get to it in a moment.

The second rule is to keep the SAME PRESSURE on the cutter throughout the cut.

 This does need practice, although not as much as some might think. I guess it is obvious, but you should not practice on the glass you hope to transform into your masterpiece.

 It is fairly easy to find scrap glass, and by making cuts on the scrap glass you will soon find confidence in being able to keep an even pressure on the glass throughout the cut

The third rule is to make sure your glass cutter "drops off" the edge of the glass at the end of the cut.

Most beginners get to the edge, hesitate, and then stop, just before the edge of the glass. MAKE SURE your glass cutter firmly drops off the edge and onto the cover of your glass-cutting table.

Now we start the practice

 We start by taking the cutter as if was a pencil and dipping it into the oil cap. (Many years ago it was common to place the cutter between the index finger and the third finger so as to apply more pressure if required but that is not taught today.)

 Now simply start near the top of your scrap glass and "draw" a line down the glass until it drops off the edge onto the table. Wipe away the oil smear from the glass to see the score line. If you cannot see it, you need to apply more pressure.

 Try again at different places on the glass until you can see an unbroken line from where you started to where it dropped onto the table top.

 Assuming you can now see an unbroken line you can break the glass by placing a pencil under the last half inch of the cut and pressing down on either side, like in the drawing.

 We do this by placing a wedge under the weakened area and pressing down on either side.

 This, I call, the "Press Snap" method and it is probably a good idea to restrict yourself to this simple break technique until you have a fair amount of practice

 If your cut is almost right, but there is a small gap in the line, carefully join up the line to make it continuous. Generally this will work, especially if you use the "Drop Snap" breaking method which will be explained shortly.

 What I can guarantee is that it will work better than trying to break the glass without joining up the line.

 Before you move on to cutting up glass for your project, you should do many practice cuts, always making sure the cutter is well oiled.

 You don't even have to draw a straight line on the glass. You could weave it about all over the place and the break would follow your mark (we hope).

 This is why we can "cut" circles and curves.

 Now that you know how it works, you can cut glass just like a professional, but there are a few other things you need to know.

Next we will look at several breaking techniques before starting on cutting out your project.

 The Break

Over the last 30 years I do not recall there being any common names for the various techniques but I will give them some now to help you distinguish between them and measure your progress as you master them.

 The Press snap

For people new to glass cutting, the easiest and most common break method is to place a small piece of timber under the end of the cut line and press down either side. This can be a variety of shapes and sizes, including a common lead pencil, a square short piece with 2 rounded corners, or even just a flat piece of plastic.

 It matters little, because what you are trying to achieve is the "stretch" or "pull apart" the top surface of the glass and we normally do this by placing a wedge under the weakened area and pressing down on either side.

 The Grip Break

The Grip Break is commonly used by glaziers for speeding up the work and is achieved by picking up the edge of the glass and placing thumbs either side of the cut with index fingers curled under the bottom surface of the glass either side of the cut. (Of course as the bottom side has not been cut, I mean either side of the vertical distance under the cut, but you guessed that didn't you?"

Then all that is required is to "pull the glass apart" by pressing upwards with the fingers while levering the glass apart with the thumbs.

Experienced glaziers can break quite long pieces with this method, sometimes lifting the glass up to chest height to achieve the "run." I do not recommend this method until you gain confidence, as there is a greater risk of cutting yourself.

The Drop Snap

 This is the "snap" over the edge of the table most often used in factories for runs up to a couple of metres long. While it is unlikely you will be cutting glass that long it is a legitimate method for shorter cuts too and it is not as difficult or as scary as it sounds.

 The technique demands a certain degree of confidence, but you can do it soon after mastering the cutting process.

 After making a long straight cut, turn the glass through 90 degrees so that the cut line parallels the table edge but still on the table. The cut should stay about 50mm or 2 inches on the table and you are holding what will be the smaller of the two pieces after the break.

 The technique now requires that you lift the glass about 6 inches and, holding firmly onto your part, "drop" or "snap" the glass downwards. The smaller piece should now be in your hands with the other edge still on the table. Don't push it back or the collision between the two pieces will chip the edges of both panes of glass.

Breaking Pliers

 These are essentially like the pliers used by many tradesmen, and the main difference is that the jaws are not flat, but concave. When they are applied either side of the cut, the curved upper and lower jaws put pressure on the flat surface of the glass and bend it down away from the cut line, just like the pencil under the glass when you push down.

 They are mainly used when cutting thicker glass in relatively small pieces when the glazier's fingers cannot apply enough pressure to effect the break.

Glaziers pliers

 These certainly look very much like pliers used by many tradesmen but they have no "tread" or grooves inside the jaws. Most common use is to grip the edge of a small off cut in order to get enough leverage to break it off the mother sheet.

They also have a knob on the back of one of the jaws especially useful for giving the glass a thump to get the "run" started.

The Start Break

 This technique is used on thicker glass and the concept is to make a "start" or "run" in the glass in order to get the cut moving. After making his cut, the glazier will turn the glass so that the end of the cut hangs over the edge and taking his pliers, (or any hard object) sharply hit the glass from underneath directly below the cut. This produces a "start" in the glass and the break will be completed by one of the other methods already mentioned.

 Laminated glass

 All of these techniques are also applicable to laminated glass, and essentially you act the same way. After making the cut and break in the top piece, turn the sheet over and make an identical cut and break to the other side.

After both sides have been cut you can gently stretch the interlayer by holding the off-cut while letting it sag towards the floor and running a sharp blade along between the two pieces.

 The more common way is to tip methylated spirits onto the break and allow it to melt the interlayer while stretching as described.

Other breaks

 While there are other methods including the spectacular Blanket Flick for very fast cutting of thin glass (like making smoke signals) and Table Breaker Bars (for sheets up to 5 metres or 15 feet across), these are not applicable for day to day cutting.

Your Project

As by now you have had plenty of practice on the scrap glass, we can confidently start cutting the good stuff, can't we?

 We start by standing square-on to the glass and measuring to the point where we want the cut to start. You can mark this spot with either a felt pen, or as we sometimes do, make a very small scratch with the glasscutter. Probably not what I would recommend until you gain confidence with the glasscutter.

 Mark the point where the cut will end and then place a straight edge (if the cut it to be straight obviously) just wide of the marks. The reason for this, of course, is to allow for the thickness of the head of the cutter.

Starting at the top edge, and by that I mean VERY close to the top edge but still on the surface of the glass, pull the cutter towards you at a moderate pace and with an even pressure until it drops off the glass onto the table top.

 Hopefully you heard a continuous crinkling sound that indicates a good clean cut. Now, if you feel that you missed a bit, wipe away the oil line and the cut line should be obvious.

 (Don't forget the oil before every cut, or if you have the oil filled type, the nut is not too tight.)

 You should listen to the sound the cutter makes as it travels across the glass to hear if you have missed a small part of the cut. If you do miss a bit, FIND IT and just mark the bit you missed.

 DO NOT go back over the parts you have already marked. If you miss just one tiny part and the score line is not continuous, the glass will not follow the line and the glass will not break the way you want it to.

 Now apply one of the breaking techniques outlined earlier and you should have an accurately cut piece of glass ready to be turned and cut to the final size.

 If your project is, say, replacing glass in a window and all the edges are covered, you need do little more than fit it, however if you are building, say, an aquarium, we need to look at edgework.

Edgework

 All exposed glass edges need some work for the sake of safety. The bonus is that edgework will actually make the glass less likely to break under stress.

 If you could see the edge of the glass under a microscope you would see small cracks and "starts", very much like the "start" you make when breaking thick glass.

 The act of arrissing the edges with an abrasive like sand paper or a diamond edging tool, smooths-over the corner in the same way it does in timber. This rubbing down takes out the small "starts" and helps to resist accidental breakages.

 If you are going to use the glass for a project with exposed edges, now is the time to take the sandpaper to the glass and, being careful not to scratch the surface, get rid of the sharp edges.

 You should make the arrissing movements away from surface area, and avoid "peeling back" the sharp edge.

A diamond arrissing tool can simply be run along the length to be treated.

 Photo courtesy Nico Tool Co,

 With enough energy and changing to different grit papers and polishing grades, you can polish the edge to perfection, alothough this is usually achieved with belt arrissing machines.

To be more comprehensive, you can attack the edge with confidence using a variety of implements, including the traditional stone providing you use plenty of water in the process.

To use sand paper, the only restriction is the strength of the backing, ie paper or cloth for example.

Easiest way to do the job is to use a 120 grit sand paper attached to a timber block. For a finer edge you could go to 240 or even 320 particles per square inch.

If you want a bigger arris on the edge you could start with something as rough as an 80 grit to remove more material and then when you have the desired "bevel" work you way through the finer grits until satisfied.

For a polished edge, we use a cork belt on our machine after working up to the finest grit belt we have available, usually the 240 grit. The drawback for going straight from a rough belt or paper to the polishing is the wear on the expensive cork belt if the glass edge is too rough to polish.

Curved line cutting

 If you are planning a curved line cut, there is a slight variation in the way you use and hold the cutter.

 Take the cutter as usual and place your other hand around the fist holding the cutter. This 2 handed position gives you much better control of both direction and pressure on the cutter head.

 Now, after making sure the cutter is oiled, start from the bottom of the cut and push the cutter away from your body. This technique will also provide the best view of the line and you can stop part way through the cut to adjust the work to suit your position. As with any glass cutting, you must have an unbroken line and you must not go over the cut.

 You can now apply the "Start Break" technique, this time from the beginning of the cut, and following the break along until you have "tapped" the line all the way to the end.

If the curved line is broad sweep with gentle curves, the "Grip Break" where you pick up the edge and pull it apart in your hands will work just as well. Sometimes this is used in conjunction with a tap underneath to start the "run".

The main restriction on curved line cutting is the inside curve must not be too sharp. For example, if you wanted to cut out something the size of a power point from the side of a mirror, you could make the cut, and you could "Start Break" the cut all the way around, but removing the piece is not so easy.

 There are techniques for side cutouts and holes in glass but as they are advanced glazing work, they are beyond the scope of this tutorial.

 Even without these advanced skills, the knowledge you have already acquired sets you apart from other skilled tradesmen.

 Congratulations and good luck with your projects

Dan Hughes