Drilling Glass, 5

Drilling holes with copper tube and grit

You can drill holes by using loose grit and a copper tube of the correct diameter. It can take quite a while. You will need to have a chuck big enough to take the tube, or have a means to reduce the tube diameter to the chuck size. Alternatively, use core drills that have had the diamonds worn away.

Prepare the glass as for a drill press without a flushing head, so the water and grit are confined. The dam can be putty, plasticine, clay, or other mouldable material put around the area to be drilled.

The grit can be sandblast grit or other abrasive of about 100 to 200 grit.

Drill as normal.

If the core gets stuck in the bit, knock it out with some stiff wire or a nail. Always remove each core right after drilling. They are very difficult to remove if there is more than one in the core of the bit.

For other tips on glass drilling see:
Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Drilling Glass 6

There are a variety of tools that can be used to power glass drills.

Dremmel and similar craft motorsThese are light duty high-speed drills. Those with variable speed controls are especially useful. They work best for small diameter holes. They must have the speed turned down for drilling, especially for larger holes.

These can be combined with a flexible drive shaft for lighter weight.

Drill pressHowever, the most important thing to have when drilling glass is a drill press. Doing it by hand is very difficult and wears out diamond bits very fast. Dremmel and others make drill presses for their tools.

Drilling machinesPurpose made glass-drilling machines are important for larger holes and production work. Flushing head adapters are available from suppliers that will convert a standard drill press into a glass-drilling machine. These tend to be much slower than the Dremmel style motors, but are very steady. The important thing about these is that they use hollow core drill bits, allowing the water to be fed through the drill bit directly to the glass-drilling site.

Drill bits
The other tool needed is drill bits. The recommended type depends on the size of hole to be drilled.

Small diameter holes, up to and including 3mm require solid bits. These can be spade (unusual) or solid diamond-tipped bits. A number of manufacturers make solid drill bits from 2-6mm and some (especially lapidary suppliers) make the very small diameter bits from less than 1mm to 2mm.

Larger diameter holes are best drilled with hollow core bits, as less glass needs to be removed to achieve the hole. These can be used with a flushing head or simply by directing water to the drill bit, with a dam to hold the water around the site.

The bits will last longer if you use a drill press. The press keeps the bit wobble to a minimum and maintains the vertical, both helping to reduce the wear on the bit.

Hollow core bits
Hollow core diamond bits are of two types:

One -where a heating process attaches the diamond - is called sintered in Europe and a number of other countries.

The second – where the diamond is bound to the metal with resins and other chemical attractions – is called bonded in Europe.

Bits of the first type are longer lasting and more expensive. These can be dressed with an aluminium oxide dressing stick to maintain their effectiveness.

Bits of the second type wear quickly and should not be dressed.

In general a diamond core drill breaks out much less glass at the bottom of the hole than a solid drill bit.

Water pumpA further tool that is useful to have is a re-circulating pump. This can be a small fountain pump with a flexible spout to aim the water on the drilling site. A foot switch can control the water flow. A large tub is required to act as the catch basin for the water that comes off the drill and as the reservoir for the pump.


For other tips on glass drilling see:


Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling Speeds for diamond bits

Drilling Glass, 7

Drilling glass without a drill press

It is best to have a drill press for drilling holes in glass, but there are ways of doing it with a hand drill.

Make a ring of modeling clay, plasticine, putty or other mouldable material about 5cm/2" in diameter and press it around the drill site. Fill the ring with water to cool the drill site and surrounding glass as well lubricate the drill bit.  Buying sinered (more expensive) bits is worthwhile as they work much better than the cheaper resin bonded ones.

Use a paint pen to mark the spot where the hole is to be. Without a drill press, starting at an angle with a slow drill speed will stop the bit from sliding around as you establish the drilling point. As the glass surface is roughened, bring the drill to vertical. Move the drill up and down a little as you drill to allow the water into the hole. If you are using a solid or spade drill, a little oscillation keeps the bit from jamming in the hole. Do not do this with a core drill.

A Dremel running at top speed is way too fast. Slow it down with the speed control.
Every diameter drill bit has an optimum drill speed. The smaller the bit is, the faster the speed required.


For other tips on glass drilling see:


Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Drilling glass, 8

Hole Placement

The general rule on drilling holes in glass is that the edge of the hole should be further away from the edge than the thickness of the glass. This means that the edge of the hole on a 6mm thick piece of glass must be more than 6mm from the edge of the glass.

The calculations are simple arithmetic. 

• calculate the centre point of the hole by adding the radius of the hole to the thickness of the glass plus at least 1mm. 
• For example, to drill a 10mm hole in 6mm glass, you add 5mm (radius of hole) to 6mm (thickness of the glass) plus 1mm = 12mm as the minimum distance from the edge of the glass to the centre of the hole.

Remember this is the minimum distance. For safety and durability in architectural or heavy circumstances, an additional margin must be added.


For other tips on glass drilling see:
Centering the drill bit
Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Drilling Glass, 9

Drilling speeds for diamond bits in glass

Diameter 3-4mm
Speed 6000 rpm

Diameter 5-8mm
Speed 4500 rpm

Diameter 9-12mm
Speed 3000 rpm

Diameter 13-16mm
Speed 2500 rpm

Diameter 17-25mm
Speed 2000 rpm

Diameter 26-28mm
Speed 1800 rpm

Diameter 29-44mm
Speed 1500 rpm

Diameter 45-64mm
Speed 1200 rpm

Diameter 65-89mm
Speed 900 rpm

Diameter 90-120mm
Speed 800 rpm

[Based on CR Lawrence and Amazing Glazing recommendations]


For other tips on glass drilling see:

Keeping things wet
Using a drill press
Drilling with a Flushing Head
Avoiding chipping
Drilling holes with copper tube and grit
Drilling tools
Drilling glass without a drill press
Hole Placement
Drilling speeds for diamond bits in glass

Kiln Shelf Breakage

Placing moulds directly on shelves can cause breakage as my kiln keeps reminding me. If you put a mould directly onto the shelf, it apparently keeps the kiln from evenly heating the shelf on the way up and the mould keeps the shelf hotter than the edges on the way down. I don't know whether the shelf breaks on the way up -although I think that is so - or the way down. It doesn't happen every time, and that's why I forget.

It seems there is a critical relationship between the size of the shelf and the size of the piece covering the mould. The greater the proportion -up to some maximum, maybe 90% - the greater the likelihood of breakage it seems. A fully covered shelf would heat and cool along with the mould. When the mould is small in relation to the shelf, the heat can travel under the mould well enough to avoid breaking, it appears. It is the large range in between that causes the trouble.

A preventative is to fire without a shelf. But failing that possibility, raise the mould a little from the shelf with kiln furniture or pieces of thick fibre paper. Also keep the shelf elevated a little from the floor of the kiln.

Grinding to Shape

There are lots of ways people use to keep marks on the glass while grinding.

Paint markers will stand up to a lot of water if allowed to dry before being taken to the grinder.

Covering the marker line with Vaseline or lip salve will preserve the line longer.

Sticking down a water proof pattern piece on the glass will allow grinding up to the edges of the pattern piece without it breaking down. But of course, it can be ground away or pushed aside by the grinding head.


All these methods assume that there is a lot of grinding needed.

If you cut accurately, only a small amount of grinding will be needed and permanent felt tip/marker on glass lasts long enough to do the job.

Rapid Heat Rises and Their Effects on Firings

Based on a communication from Phil Hoppes

A word of caution. Never use 9999 for a ramp up. Note: 9999 just means on an up ramp the elements are full on, no cycling. On the down ramp the power is completely off until the desired temperature is reached. Your kiln will rise in temperature limited by 2 things - the type of insulation and the number of elements. This can be anywhere from 300 – 450C/hr. to as high as 1600C/hr.)

If the time it takes to go from your lower temp to your upper temp is less than 40 minutes, your controller will be unable to accurately control the top temperature. For example, if you want to ramp from room temperature (20C) to 300C and for your kiln 9999 on an up ramp is 850C/hr., the temperature rise you are looking to accomplish is 280C and your kiln will reach 300C in just under 20 minutes. The problem is that most controllers need around 40 minutes in any ramp cycle to "learn" how the kiln is responding to the inputs that are given to it by the controller. Slower ramps need less “learning” time, faster ramps need more time.

What will happen if you programme a ramp shorter than your controller will respond to is that the temperature in your kiln will not stop nicely at the programmed 300C. The controller has not learned how to stop your kiln from rising in temperature yet and the temperature will rise much higher than your programmed value.

Depending on your kiln and your controller this can be quite significant. Most controllers have a peak shut off value, somewhere between 55C and 85C above your programmed amount. Some controllers allow you to program this value also. If the temperature in your kiln overshoots the value it was programmed to stop and the amount of overshoot exceeds the programmed shutoff temp your controller will shut down. This is a safety feature and the controller is doing what it is suppose to do. If you have something in your kiln however and this happens it will not be annealed properly and you will have to very carefully re-fire to remove the stress or it will break into pieces.

It is a good idea to know just what your kiln will do. You can do this by taking an empty kiln, program 9999 in an up ramp from room temp to 815C. This is the typical peak you would use in a full fuse. See how long it takes for your kiln to reach this temp. This will give you the maximum up ramp rate of your kiln. You can use this rate to calculate if you violate the “learning” margin of the controller. 

It is advisable not to exceed 350C/hr up ramp unless overshooting the top temperature does not matter.

The 9999 ramp in almost all cases will be used to go from the top temperature to the start of the annealing cycle.

Prevention of Needling in Dammed/ Box Cast Work

To avoid needling in box cast or dammed work you need to provide a space for the glass to flow into.

This is done by using 3mm thick fibre paper to line the damming materials. The fibre paper is cut to 3mm less than the finished height of the fired piece.

Fire the glass with a long bubble soak. This allows the glass to almost achieve its final height before it becomes less viscous. It will still be higher than the fibre paper and as the glass continues to be more “soft” it will round as it reaches full fusing temperature. There is not enough glass above the fibre – only 3mm – for the glass to run over the fibre, as the surface tension holds it in until 6 or 7mm above the fibre. The top edge of the glass does not touch the fibre or dam, so there are no needles.

Another way to avoid needles in this kind of work is to make the dams larger than the glass being contained. That is, place the dams a short distance away from the glass. The glass will then flow out to meet the dams. Since the glass is not contracting it will not have needles. This is a good solution when the thickness of the glass is not critical. You control the area of the piece by the placing of the dams.

Charging the Pot for a Melt

The way you charge (load the glass into) the pot makes a difference to the resulting piece.

A good way to get strong colour separation is to put two colours on opposite sides and a third colour or clear between them. The two side colours will have best separation if they are not more than 1/3 each. As the glass begins to flow out of the pot, all three colours will come out at once and form concentric circles (assuming a circular hole in the pot).

Vertical stacking of multiple colours

You can manipulate and alter the results with a fair amount of predictability by changing the diameter and shape of the hole, charging the pot with more than three colours or less, rearranging the orientation into a sunburst orientation or whatever comes to mind. Be sure to keep notes on what you did and what the results were in case you want to reproduce the effect.

Think about how the glass will flow out of the pot when you charge it with glass. If you layer colours horizontally from C (on top) to A (on bottom), it will initially flow out in colour A, then B, then C. After that initial flow, which will be on the outside of the finished piece, the main flow will be from the top (C), then the middle (B) and finally the bottom (A). This is because after the initial flow, the rest of the glass comes out in a funnel shape pulling the top and small portions of the underlying glass.

This means that layering is the best way of mixing colours. You need to think about colour combinations too. For example yellow and red become brown; yellow and blue a dark green, etc.

The proportion of dark colours is important, for example, as little as 2% of black can make the whole piece very dark. If you have dark colours, you need to add a large proportion of clear or very light opalescent glass.

If you use frit, large pieces are better than smaller ones. Even so, you need to be careful about the colours you use so the whole does not become muddy.