Rev. ... 2003-01-19, -21,-23; -02-24, -10-01, -12-11,
2005-06-26, 2007-06-12, -08-18
2008-01-06, -03-14, 2009-01-11, -12, -02-20 (layout)
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|Coldworking is what is done to glass after it has
been annealed (which may be in the middle of further hot
working, as with graal.) The purpose of this page is to provide
illustrations of some of the techniques. I am not strong on
coldworking, doing only the basic work listed below.
REPAIRING BROKEN, CHIPPED OR SCRATCHED
INTRO - Coldworking as a skill is completely different from hot glass furnace work. Where there is enough work, some artists will find a person who does cold working cleanup for hire. A studio doing enough output will hire a person just for coldworking.
All the operations that involve grinding, blasting, carving, etching, cut glass, or polishing are considered coldworking. This page also includes drilling and cutting flat glass, which are not usually considered coldworking, since they normally require only moderate skill and standard tools. Stained glass assembly is not normally considered coldworking, but is a separate discipline, as is beveling flat glass, although that involves most of the same skills and machinery as coldworking.
Non-decorative coldworking tasks include smoothing the edges of the punty mark, grinding the bottom flat for proper standing and signing the piece. Decorative methods include cutting, etching, blasting and polishing. Painting is not normally considered coldworking but is a separate operation. However, note that applying color with enamels may be intermixed with coldworking, graal involves painting a partly finished piece, carving away some of the paint, then heating and further encasing the work.
|1) A prism bit 2) a spear bit, 3) wire bit, and 4) a core
A prism bit is a chunk of hard steel with a pyramidal point (flat sides, sharp edges) that can drill small to medium holes. Normally only used in a drill machine. Search words "Prismatic Glass Drill"
|A spear bit looks like a flint spear with a fairly blunt end. Possible to use in hand drill, awkward to start - skitters. Search words "Spearpoint Glass Drill" Critical to using this bit is slow steady pressure as the tip grinds through the glass. It goes much faster once the sides are grinding the hole larger. Only one likely to be found in hardware stores.|
|A wire bit is a hard wire coated on the end with diamonds for use in a high speed (Dremel) tool. Search words "Diamond Plated Miniature Flat-Tip Drill"|
bit is a can shaped cutting surface mounted on a shaft.
The cutting may be done with diamonds imbedded in or
soldered to the edge of the can, or it may be grit poured
in the liquid around the bare metal (brass or steel)
cutting edge. Search words "PLATED DIAMOND DRILLS" Must be
used in a rigid tool, not in a hand drill.
Drills and bits are available from CRL
|Hi Mike, (E-mail 2008-03-14)
I ran across your site and thought I'd add my own tricks to the stew.
Back in the days when making your own helium-neon laser was all the rage, one of the requirements was that of cutting a precise miter in the ends of the laser body to receive the Brewster windows. I think the angle was pretty steep--about 27.5 degrees, but I'm not certain.
At any rate, the way we did it was with a hacksaw and a blade made of copper (no serrations, just sheet copper) and a slurry of automotive valve-grinding compound. It's not fast, but it results in a very smooth and accurate cut with no chipping.
An old workshop book that I have from the 30's mentions that one can drill a hole in plate glass using a standard HSS drill bit in a drill press and a slurry made of equal parts camphor and turpentine. You can't allow the drill bit to break through--you use a sharp triangular file to finish bringing the hole through to the other side.
At any rate, for whatever it's worth, you have my two cents.
| To drill glass, especially larger holes, it is vital to have a drill press,
even a small one, and a method of turning the bit fairly slowly, either a
speed control or belts on larger drill presses. Venders of bits will tell you
the recommended RPM. Around the site of the hole build a dam of plastic
clay, the oily kind used by kids that comes in colors - red, blue, green - not
will be put in the dam and the hole drilled through the small lake. The bit
should be withdrawn every once in a while to cool the leading edge and the
bottom of the cut (and reintroduce grit when using metal core bits without
diamonds.) The goal is to keep the glass from getting so hot it cracks from
thermal stress. If the back of the glass is accessible, the other side of the
cut should be covered with clay to keep the lake from draining through. If it
is not, then when the water does drain through, the bottle, hollow block or
shape should be filled with water. If the piece is small enough, it may be
possible to put in a bowl of water that covers it to the top. If the task is
to be done often enough, it may be worth building a tank to keep from fiddling
with the clay.
The glass dust ground out by the drill is not good to breathe. While wet, it does not cause problems. Swirl the water to pick up the sludge and pour it through a scrap rag and throw the rag and glass out rather than letting it dry and pouring the dust around. 2003-01-19
| Having written that over four years
ago, I finally got around to drilling a pair of largish holes today
(2007-06-12) when I needed them. The project is an indoor fountain
using parts I have purchased over the years and put aside, often in
frustration. These four pictures show the results of drilling and the
assembly of the fountain.
The bowl was drilled upright, adding about an inch of water and a tablespoon of medium grit. The glass bowl was placed in another bowl, which happened to have wax in the bottom (see below for support) and about an inch of water was put in that. The shape of the bowl naturally caused the grit to settle in the cutting area and the slots in the bit whirled the water.
It was easy to hear when I had to raise the bit, every 10-20 seconds, to let in cooling water and more grit - the sound of grit cutting and brass turning without grit is distinctive. Because I was applying fair pressure, perhaps more than I should, there is shallow breaking away on the exit side, but not as much as the picture suggests as part of what you see around the hole is the flat that was ground on the bottom of the bowl long ago.
|Drilling the glass plate was a bit more exciting. The glass,
as you see, was roughly cut from a broken piece of 1/4" tabletop glass
found at the curb. I did not care to invest the time in making it
perfectly round and it wasn't quite big enough anyway. I also
found I needed a path for the cord so one flat was required.
A plastic wash basin was placed on the drill press platform, a piece of wood added to put water under the glass and protect the basis when the drill broke through and enough water was added to put a half inch across the glass. A heavy metal ring cut from fence post tubing and about an inch high was placed around the hole and the grit from the previous job poured inside.
Again the process was apply pressure, listen for the change and watch for binding, raise the bit to let the water in, go again. The ring could have been sealed, but I just held it with my hand. Enough heat was generated to make it distinctly warm. I think the shape of the wood support, a small block with a large hole drilled in it, was a mistake and a grid or other support that let more water in the bottom would have been better. I paused while drilling to let everything cool and relieve impatience. When it broke through again there was flat flaking away from the hole, perhaps 1/64" deep.
|Both holes were ground lightly with a wetted pointed grindstone.
The picture at right shows the first assembly. The small pump (with a frustratingly heavy cord) is suction cupped to the bottom, a short length of PVC pipe with notches for the cord and to let water back in is placed around the pipe. A short extension tube of plastic and brass brings the pumped water up. The glass plate goes over the tube, rests in the PVC and fits snugly in the bowl.
|Here the glass bowl has been placed over the tube, black river rocks
have been placed around the bowl on the glass plate, water added, and
the pump turned on. The pump has a lever to set flow rate.
The rate was annoyingly high the first time I tried so the water made
too much noise. I disassembled it all, cleaned some stuff
again, changed the setting and reassembled for a more satisfactory
burble. I also added some glass marbles and weight to the rocks.
The stainless bowl is a bit mechanical and the power cord is irritating. A pottery bowl would be more organic and taking steps to hide the cord would be nice. 2007-06-12 Added images of fountains -06-27
is done by putting a deep continuous scratch in the surface of the glass then starting a crack usually at one end of the scratch and working the crack along the scratch. Straight cuts may be worked by laying the scratch line on the edge of a surface and pressing down on the unsupported part or laying the glass at the cut on a dowel and pushing on both sides. More complicated cuts can be tapped to crack them; many older glass cutters have a ball on the end for the purpose. For more control, a dowel rod may be placed under the scratch and the glass pressed down on either side, so it can't be pushed further than the diameter of the dowel or dropped to the floor from the edge. This site http://www.fletcher-terry.com/hardware/fletcher/thick.shtml from a maker of cutting tools offers some excellent advice - they don't want their tools blamed for poor technique! Pliers exist with soft jaws that have a ridge on one jaw and valley on the other, so that when the valley is placed above the scratch and the ridge below and the plier handles are squeezed, a controlled cracking occurs.
Thick glass, 3/8"-3/4" requires a different cutting wheel and tapping to break usually starts in the middle with a heavy ball tapper. 2008-02-26, 2009-01-11
|The common glass cutter has a hard carbide cutting wheel on one end, slots for chipping glass edges on the side and a ball for running a crack on the other. Also sold without ball. About $4. Standard angle of cutter edge is 124° for thin glass. Also sold with sharper angles for thicker glass - 135°, 142° that really help. These cost more.||
|Cutters with a chip of diamond on the end range in price from $5 to $60. I have one at the lower price and one about $30 and they appear to work the same for me. Warnings were given that a diamond cutter is harder to control, but I have found that not true.|
|After observing a man cutting 1/2" plate at a specialty glass cutting and table top making place, I quickly realize that for thick glass a heavy tapper helps. The top one with a 3/4" ball costs over $58 from CRLaurance, so I cast one from lead and a couple from brass. Short handle length is to fit my modest glass cutting tools storage box and a longer heavier handle would improve balance. 2008-01-06, 2009-01-11||
After observing an expensive tool for controlled cracking
of thick glass that involved a heavy bar with two hold downs and a screw
drive to apply pressure from underneath so the glass does not have to be
turned over, I decided to make my own version for the thick plate glass I am
cutting for water features. Instead of the
I would turn over the glass and apply pressure with a
C-clamp. To focus the pressure, but do it in a soft way that would be
less likely to harm the glass, I would use large 9 gauge aluminum wire.
I cut a short length of hard flat angle iron (brown in the picture) and
Marine Goop glued two carefully straighten aluminum wires to the top about
2" apart. I then took a longer piece of wire and Gooped it to the
swivel pad on the clamp (cleaning all surfaces before gluing.) It
would be easier to handle in several ways if the wire on the clamp were
glued to the fixed jaw, but I quickly found that some breaks at angles to
the edge could not be accommodated that way.
In use, the angle iron is positioned so the wires straddle the scratched line and the C-clamp is positioned and snugged down. The long "handle" on the clamp wire is aligned with the scratch (not shown in photo.) Pressure is then applied by turning the C-clamp handle until the crack runs, usually explosively. With only two days experience, I am running much straighter lines and being able to do breaks within an inch of the edge of the glass where gripping was awkward before. I put the wires 2" apart to handle splitting pieces that narrow. I believe that wires further apart (3-4") would allow more control, bending the glass near the edge to start a crack without running it across the glass in a pop. 2007-09-22
This list copyright Mike Firth 2000 and of copyright holders quoted.
Glass Cold working DRAFT 10/28/96
COLDWORKING Rev. 12/15/96
The most basic equipment is a flat wheel for grinding and two thick buffing wheels for polishing, although it is possible to grind on a flat steel or glass plate. A flat wheel grinder is not difficult to build and since similar equipment is used in lapidary work, is available from several sources. Plans and a list of sources are at the end of this section.
In a better equipped shop, there will be several grinding wheels so that course grit does not mix with finer and cleaning of wheels is unnecessary and so that a wheel is available for each of the steps in polishing. An alternative to wheels is a belt grinder, which is said to remove glass quickly while staying cool. Unfortunately, building one is difficult and sources are limited. Belt sanders from other sources (e.g. woodworking) are difficult to modify because they almost always have direct drive motors at the bottom where they will get wet. Belt sanders ready to go from lapidary sources will start at over $475 for a small unit (about 3" x 9" grinding surface.)
The latest change in equipment is the use of diamond grinding wheels and diamond plates which mount on wheels. Diamond grinding is said to remove glass more quickly than other grinding methods while leaving a smoother finish that may need less or no polishing. Diamond polishing is done at a higher speed than other forms, but a good flow of water is vital, not only because heat will build up faster in the glass at the higher cutting speed, but if the plate gets too hot the material holding the diamonds to the plate gives way and the diamonds are lost.
Technique - In summary, a coarse grinder will be used to remove the bulk of the glass as quickly as possible without overheating the glass. This is followed by any needed detail grinding, rounding edges, removing bumps, etc. One or more finer grinds will take out the large scratches and prepare the piece for polishing. A succession of finer polishing compounds will be used to produce an optically clear surface.
First Grind - If you have never worked glass before or if you are working a wheel that you have never worked before, always test the wheel with a well annealed fairly heavy scrap piece before doing any serious work. An abandoned paperweight may be a good choice. The first thing to check is whether if the machine is stable and doesn't tip from leg to leg. Check power connections for safe wiring, working with water and random electricity can be a nasty combination. Power up and check that the wheel seems in balance and the motor and wheel are moving smoothly. Turn on the water and check for good grit pattern and water flow. Begin your test by touching the chunk of glass to the wheel and listening to how it sounds. Faster than anything you can see, you will be able to hear changes in speed and grinding. Move from the rim to the center. Try various locations to see whether pressure on the wheel tends to stall the motor or make the belts slip. If the latter, see if an adjustment can be made.
Make a serious cut by applying pressure about midway center to
rim and learn how much glass the wheel takes off. Try cuts nearer
the center and at the rim. Get a feel for how much cutting is
being done along with how the wheel sounds and how the glass
vibrates and pulls while being cut.
Remove bulk of glass - Two great risks occur when removing the
bulk of glass:
Contact Mike Firth