Grinding and Polishing - Grits

Grinding and polishing grits and their effects

60 grit belts and disks provide a very aggressive grinding action. This grit takes large amounts of glass away very quickly. It makes shells and takes chips out of the glass with anything greater than light pressure. You need to create a small arris to avoid the shelling before grinding the face.  The metric size is 0.2337mm.

80 grit belts and disks provide a slightly less aggressive grind. But you must push lightly until you get the shape you want. On a new belt this is a remarkably fast process. Eighty grit belts can also take chips out of the glass, so be careful. Again an arris will help avoid the shelling.  The metric size is 0.1778mm.

100 grit belts and disks can also remove glass quickly with a new belt. Work at 100 grit until you get the shape or the big scratches are all gone from the 80 grit. As the belt gets worn, you may want to push harder to get the desired shape, but let the belt do the work. The metric size of this grit is 0.1397mm.

120 grit belts and disks remove scratches and still do some refining of shape.  The metric size is 0.1168mm.

200 grit belts and disks remove smaller scratches only. The shape of edge can still be adjusted, but only slightly. The metric size of this grit is 0.0737mm.

 400 grit belts and disks begin the polishing phase. Look for bigger scratches that you may have missed. The use of paint markers will help in this. Cover the the dry surface with the paint marker before beginning the polishing. This will show up any large scratches remaining after the first pass with the belt. If you find these, move back up to the level of grit that would remove any of the visible scratches, then work your way down again. The metric size of 400 grit is 0.037mm.

600 grit is a polishing phase. Take your time and move a little slower. At this stage, all the larger scratches should be gone and you are only polishing. The metric size of this grit is 0.020mm.

You can proceed to finer grits if you wish - such as 1200 (0.012mm) - but 600 is a practical grit at which to switch to cork and pumice, rouge or cerium oxide.

Cork is the final polishing phase before getting an optical finish with cerium oxide. The cork will grab the glass, so hold it securely. It is the friction between the cork and the glass that actually does the polishing. But do not let the glass overheat.

Grinding method You should not push hard with any of the grits. If you find that you want to get the work done more quickly, then it's time to put on a new belt or go to a coarser grit to remove the glass. You can use older belts as though it is a finer grit. The belts with finer grits will usually last a little longer than the coarser ones because the work is less agressive.

The grits of 100 and coarser are for shaping the piece.  The one you choose will be related to the amount of glass to be removed.  

After achieving the shape desired, it is usual to half the size of the grit (or in grit sizes - double the number) at each stage.  So after 100 grit, use 200, 400, and 600 one after the other.  

Of course you can do all this work without machines.  These grit sizes are available as loose powders.  The methods of working with a slurry of water and grit are described here.

Edge Treatment in Cold Working

Frequently people who are grinding the edges of bowls, aperture drops and other vessels that need to have a smooth rim find that they are getting small chips of glass coming from the edge of the ground part of the glass.

There is a way to prevent theses unwanted chips  

The long established practice of glass workers has been to give the glass an arris at the end of each grinding stage before they change to a finer grit.  This small area of angled glass, allows the continued smoothing of the glass without creating such a sharp edge that the glass there is not strong enough to resist the grinding action.  

You will notice on a bowl or other rounded vessel, that the chips are almost always on the outside. The inside of the rim normally has an oblique angle to the rim, and the outside an acute angle.  The explanation is held in the angle.  As the rim is ground down, the outer acute angle becomes very thin as well as sharp.  At some point the glass is thinner than the grit used to grind the surface.  This causes little chips of glass to break off the edge.

By creating an oblique angle at the edge of the grinding surface, the glass will remain thicker than the grit being used to grind the glass.  If you feel you are taking off a lot of glass, it is advisable to check that the arris is still in place.  If not, give it a light grind to maintain the arris while using that grit. 

At the end of each stage of grinding, you need to add an arris for the next stage.  The reason for doing it with the coarser grit rather than the one you are about to proceed to, is that it maintains all the grinding at the same stage, enabling the whole piece to be finished to the same level of polish.

Wipe the surface dry and add marks with a paint marker.  Allow this to dry while you change grits.  The purpose of the marker is to assist you in determining when you have ground out all the previous marks, by the elimination of the paint.

Edge Working Options for Glass

There are a number of standard options for the worked shape of edges.  The simplest is to have a seamed edge, where just enough sanding is done to take the sharpness from the edge.

The next is to have an arris where more glass is removed, usually as a chamfer, but sometimes in a rounded, bullnose effect.  These are commonly used for glass that is to be toughened.

Flat chamfered and often polished edges are quite common also.

Bevelled glass is very common on mirrors as this reduces the reflection of the inside of the frame holding the glass.

As you can see from the attached illustration, there are a number of standard edge treatments, although some of them are uncommon.

The seamed, arrised and flat polished edges are easiest to create by hand grinding.  The other more fancy edges require machines.

Scum on Ground Edges

Almost without exception, ground edges show scum after fusing.  This scummy appearance is devitrification. This is caused by the powdered glass from grinding remaining in the pits caused by the action of refining the shape of the glass with a grinder. 

The suggestion that the glass should be placed in water immediately is of course a good precaution, although addition of vinegar is less efficacious than grinder lubricant added to the soak water.  This lubricant helps to keep the glass in suspension rather than settling into the scratches and pits of the grinding marks. The vinegar, which is often recommended, will etch the glass if left to soak and  smells up the place.  A better solution to soak the glass in is a 6% solution of tri-sodium citrate.

The glass needs to be made smoother than the standard grinding bit will achieve.  Normally, a 600 grit grinding bit will be sufficient to allow a good fire polish without any devitrification. Sometimes 400 grit will be enough. You will need to step down in grit from the standard (about 100) to fine (about 200) to at least super fine (about 400) grit.  If you can find a 600 grit bit, that can be your final smoothing before cleaning and placing on your piece for fusing.  Of course, this grinding can be done by hand with wet and dry sandpaper without any great labour.

There is, of course, a more simple solution - don't grind. I rarely grind any pieces for kiln forming.  Often, this is because I am working thicker than 6mm and know the gaps will fill during the forming.  If I need to make adjustments for 6mm pieces, and I often do, I groze the edges of the glass.  This gives a much cleaner break of the glass than grinding.  Of course, the edges are not as precise as when ground, but the glass remains absent of all the scratches that harbour the devitrification.  Often the fit does not need to be precise anyway. 

When the fit does need to be precise, the parts that do not fit perfectly can be filled with the appropriate colour of powder. This should be kept as near the gap as possible and piled up only a little over the gap to compensate for the lack of mass that powder has in comparison with sheet glass.  This powder technique, of course, does not work well on tack fused pieces.  There, the grinding and smoothing needs to be pursued.

Hand Finishing to a Flat Edge.

Hand finishing an edge does not require expensive electrical tools, although they do make the process quicker.  This is a note on how to get good-looking edges without expensive equipment. Only a few materials are required.

• ·         A thick sheet of float glass for the grinding plate
• ·         Aluminium oxide or silicon carbide grit in approximately 80, 180, 400, and 600 grits to act as the abrasive.
• ·         Wet and dry sandpaper of approximately 1200 and 2400 grits
• ·         Paint pens (white and gold work well)
• ·         Paper towels for drying
• ·         Water for rinsing
• ·         Large bucket or basin to collect the rinsing water

The thick flat float glass acts as the grinding plate.  It is flat and smooth, making the grinding and polishing flat. 

If you have a lot of glass to take off to get a straight edge use 60 or 80 grit.  If there is not much to take off, start with 120 or 180 grit.  There is no need to make deep scratches on the edge that will take time to eliminate, if a finer grit will do the job.

Put 80 (or finer) grit aluminium oxide or silicon carbide on the glass grinding plate and make slurry with water. 

Slurry mixed and the circular motions of grinding can  be seen
photo: hisglassworks

Move the edge firmly in circular or figure of eight motion over the grinding plate until flat. If the slurry becomes pasty, add more water as you do not want a thick grinding mix. Maintain the same angle of the glass piece to the grinding plate at all times so you have only one plane of glass to take to a polish.

When the edge is flat, clean and dry the glass, and especially the ground surface to remove all traces of the coarser grit.  Set the piece aside to dry.

While the piece is drying, clean off the grinding plate. Scrape off the slurry into a pot set aside for that grit for further use, or into newspaper or other temporary container and then into a waste bin, not the drains.  It is a heavy material and will block drains. Rinse the plate off in a basin of water to ensure there are no coarse grains on the glass.  The residue will settle to the bottom and you can decant the water off once it clears. These grits are not very expensive so repeated use is not essential, just economical.

The next step is to paint the now dry glass edge with a white paint pen.  This will allow you to see when you are ready for the next step, by the disappearance of the paint from the scratches.  Of course, if you are grinding a white or other pale glass, a gold paint pen will be better to see those scratches.

While the paint is drying, make a slurry of the next finest grit. Then begin grinding. The first element in each grinding stage is to give an arris to the edge of the glass.  This prevents chipping the sharp edges.

 

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When the white paint is gone from the edge, you can progress to the next grit.

At each stage of grinding you can reduce the grit size by half (double the number). This is the generally accepted reduction of grit size to make the removal of the scratches of the previous grit least time consuming. You can reduce the grit size by more than half if you want. Most often reducing grit by large amounts means more time is spent at each stage.  Experience will show you how much you can reduce the grit sizes beyond the accepted intervals.

Stopping at 600 or 800 grit will enable an edge to be fire polished with ease and minimum heat.

At each stage you need to clean the glass and grinding plate as for the first change of the grit size. This repeated cleaning usually means that the artist either has separate grinding plates for each grit, or the grinding is saved up until there are a few pieces that need the same treatment.

A piece of wet and dry sandpaper fixed to a glass plate

After 800 grit, you may wish to progress to wet and dry sandpaper for the finer polishing, using 1200, 2400 and, if you want. 6000 grit. Fix the paper to a glass plate.  Often, simply folding two edges under the glass will be enough.  Add water and proceed as for loose grit.  Hand finishing to this level will eliminate the need for fire polishing. 

Of course, for smaller areas, you may wish to use diamond hand pads.  The need to use water and rinse between grits still applies.  The diamond hand pads are usually most suitable for short straight edges. The longer ones need the kind of treatment outlined above.

Glass Snagging on Grinder Surface

A number of people report difficulties in sliding the glass along the surface grid.  The glass catches on the grid squares and so does not move easily and smoothly when grinding.

 

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Some suggest sanding the grid to remove any rough places.  The difficulty with sanding the grid is that it will mean that you have to replace the grid before the grinder comes to the end of its life.  Whether you will be able to replace the grid is a risk you have to take if you do this.

It is better to give all sides of your glass a quick arris before beginning to grind. Although technically, an arris is the edge of a piece, it has come to mean the modification of the edge in glass work.

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 An arris on the glass edges can be made by hand with a pass of a grinding stone on the top and bottom edges.  

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It can also be done by a light pass of the glass along the grinding head. This arris protects your fingers too, as it removes the sharp edges of the glass.

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Make sure any points on the glass are slightly rounded, as they are most likely to get stuck in the grid.  You can nip the point with your grozers, or give a slight rounding of the point when making the arris.

It is important that you do not press down on the piece of glass. Press horizontally toward the grinder bit instead. The top is plastic and so deforms pretty easily.  With long pieces the bowing of the top means that the glass, which does not bow, will catch on the grid.  So, to keep the surface grid flat, hold long pieces at the ends.  This will remove any tendency to press down in the middle, as any downward pressure will be at the ends of the glass, allowing the grid to remain flat.

Stabilising Stringers

Stringers and rods never seem to stay where you put them.

• Glue them and they move after the glue has burned away.  
• Grinding a flat side to them seems a lot of work.  
• Easier, is to put them in the kiln and take them to a tack fuse to give a flat spot. But that takes a lot of kiln time.
• For stringers you can put a kink or curve in it by heating over a candle.  Rods require more heat than that. Of course, this is of no use for straight lines, and takes additional time.

A simple method which can be used with a tiny amount of glue, or not, is to add clear fine frit around the stringers and rods. This is enough to keep them from moving once the glue is gone due to the heat.

Assembled panel by Kathleen Watson with the stringers surrounded with fine clear frit which can be seen as white

The frit should be put on the assembled panel once it has been moved to and placed in the kiln.  Any movement will disturb the frit and defeat the purpose of keeping the stringer or rod in place.

The fired result

This was fused to a rounded tack fuse and no signs of the clear supporting frit is visible.

This is a quick simple method to stabilise rod, stringer and other small items that may shift in the firing.

Straight Edges on Thick Pieces

As glass tends towards 7mm at full fuse, it is difficult to keep straight edges on thick pieces as the glass moves. If you want straight edges without dams, there are a few solutions:

1) Don't flat-fuse - apply less heat work so that the stack stays vertical instead of spreading. The degree of tack fuse required will be a subject of observation and experimentation.

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2) Plan on trimming the edges straight. You can use a saw or grinder and then either cold work the edges to polish, or fire polish.

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3) Add a couple of centimetres or so on each side of your base glass, so that a 20x20cm piece becomes 24x24cm, and flat-fuse as normal. The volume change will (mostly) be absorbed by the extra glass, so that you can simply trim it back to the right size and cold work the cut edges.

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Organic Burnout Marks

Occasionally there is a haze at the centre of the back of large pieces of fired glass. This seems to happen when a large piece of glass is placed over fibre paper (of whatever thickness) that has not been pre-fired. 

 This is based on my experience of doing large pieces on thinfire or other fibre paper with a relatively fast rate of advance. What seems to happen is that the edges of the glass soften enough and early enough that not all the binder in the fibre papers can burn out and the combustion gasses escape from under the glass. The resulting haze is the remnants of the combustion product fired to the surface of the glass.

I have found that flipping the piece over and taking the glass to a low temperature fire polish is enough to return the glass to its usual appearance. You can, for extra insurance, apply a devitrification spray, although I have not found it necessary.

You could, of course, work the back of the glass with pumice and cerium oxide to bring back the original shine without firing. But my impression is that the areas with haze are fractionally depressed into the back surface. This means that a lot of glass has to be removed to reach and polish the hazy areas.

Grinder maintenance

There are several elements in maintaining one of the work horses of many glass studios.

Water

Ensure there is enough water to supply the pump or sponge that wets the grinding bit before starting any grinding. Too little water reaching the bit, fails to lubricate the diamonds and keep the glass cool. If you are getting a white paste or a powder on or near the glass, you need to increase the water supply.

Empty the reservoir daily. This keeps the water from producing a smell, and allows you to clear the glass residue from around the grinding bit. 

If you are changing to a finer grit, it is important to change the water, clean the resevoir, and thoroughly clean the sponge each time you make that change. Otherwise, you risk bringing coarser grit to scratch the finer grinding surface.

You can also buy a additive for the water – often called a diamond coolant – which is intended to provide a kind of lubrication for the diamonds. This may extend the life of the bit a little.

Bit maintenance

Periodic removal of the bit and lubrication of the shaft should be part of the regular maintenance of the grinder. You should make sure that the socket for the grub screw is clear of glass residues before attempting to turn it. I do this by using a needle or other thin sharp object to clear out all the glass powder. When the socket is cleaned, I push the key into the socket very firmly and hold it there while turning. Prevention maintenance is to fill the socket with vaseline or thick grease after tightening the screw.

Inspect your bit carefully for smooth areas showing that the diamonds have been worn away. Also look for dents, and other irregularities on the surface, indicating that the bit is damaged. In these cases, the bit should be replaced.

Before putting the old or new grinder bit back, ensure the shaft is smooth and without corrosion. Then coat the shaft with Vaseline or a proprietary anti seize-compound. This will ease the removal of the bit later. If the shaft is corroded, use a strip of fine wet and dry sandpaper to shine the shaft.

Sometimes bits need to be dressed – removing protruding diamonds, or cleaning and exposing new ones on a worn bit. To dress the bit you can grind some scrap glass, brick, or use a dressing stone to lightly grind some of the abrasive material away. This can extend the life of the bit.

Adjustment of height

If your grinder bit is too low or too high the diamond surface will not grind the whole of the glass edge. This can lead to chipping of the surface of the glass at the edges.

A good practice is to start with the bit as high as possible to allow for differing thicknesses of glass. As high as possible is with the bottom of the diamonds just below the platform of the grinder. This will ensure that you can deal with varying thicknesses of glass without immediate adjustment. You can then reduce the height of the bit as it wears.

Finger protection

Grinding lots of glass pieces often leads to a number of small cuts on the tips of your fingers. There are several things that can be done to reduce these cuts and the tenderness that comes from lots of grinding.

The first thing is to take the sharp edges off the glass. You can do a simple, light grind all the way around the piece. This removes the extra sharp edges that often remain after breaking the glass.

You can go a step further and do a light arris around the piece. This is just lightly holding the glass at about 45degrees to the grinding bit and going all the way around on all sides. This does not take off the shape of the piece, but gives a more rounded feel to the piece.

Illustration of the effect of holding the glass at an angle to the grinding head - not so much needs to be taken away as in the illustration to get the effect

It is not generally recommended that you wear gloves around rotating machinery. There is too much risk of injury, even on a small grinding machine. Some of the alternatives to gloves include plasters (band aids), masking tape, electrical tape.

Other purpose-made things you can buy include rubber finger protectors, finger tip pads, finger caps (as used in counting money).

Other tools are made to hold the glass such as the grinder cookie

and Nick's Grinder's Mate 

Fire Polishing Jewellery

Sometimes jewellery pieces come out of the kiln with imperfections that need to be ground away.

You can do a quick shaping on the conventional grinder and then fire polish. However, you will not get a fire polish at a low enough temperature to avoid distorting the piece. The way to get a fire polished surface or edge is to make the edge less rough before putting the glass into the kiln.

After you shape the pieces on the grinder, make sure you scrub them well to remove any particles created by the grinding process. Dry them and then cover the rough areas with a white paint marker. This will indicate when you have removed the scratches caused by the previous rougher grinding.

Get out your wet and dry sandpapers. Start with the most coarse (about 200) and work the ground area until the paint has been removed. Keep the sandpaper and the area being worked damp. When all the white paint has been sanded away, wash, dry, paint and go to the the next finer grit. Repeat this with progressively finer grits – normally use the grit number twice the previous grit (larger numbers indicate finer grits).

Normally, going down to 400 or 600 grit will be enough to enable a fire polish at a low enough temperature to avoid distortion of your piece. This heat range will be at a low temperature tack fuse.

This smoothing process does not take very long and is much cheaper than buying several finer grinding bits - most grinder bits are 60 to 80 grit, although it is possible to get bits up to 600 grit.

Grinder Bit Height

If your grinder bit is too low or too high the diamond surface will not grind the whole of the glass edge. This can lead to chipping of the surface of the glass at the edges.

Example of top of bit almost too low for the glass

A good practice is to start with the bit as high as possible to allow for differing thicknesses of glass. As high as possible is with the bottom of the diamonds just below the platform of the grinder. This will ensure that you can deal with varying thicknesses of glass without immediate adjustment. You can then lower the bit as it wears.

Example of nicely adjusted bit

Of course, you need to ensure there is adequate water reaching the grinding bit to avoid overheating the glass, and to keep the dust from grinding from getting into the air.

Grinder Chipping Glass

There are a number of reasons that may cause the grinder to chip the glass surface. Some of the things to check are:

Too much pressure

It may be that you are pressing the glass into the grinder head too hard. The grinder head should do the work. Firm rather than hard pressure should be applied. If the grinder slows, it is an indication that far too much pressure is being applied.

Insufficient water supply

There may be too little water reaching the head to lubricate the diamonds and keep the glass cool. If you are getting a white paste or a powder on or near the glass, you need to increase the water supply.

Worn or damaged grinder bit/head

Inspect your bit carefully for smooth areas showing that the diamonds have been worn away. Also look for dents, and other irregularities on the surface, indicating that the bit is damaged. Any dents or smooth places on the bit cause a vibration that is similar to a tiny hammer tapping the edge of the glass.

Grit size

It is possible that you may be using too coarse a grit on the grinder bit/head. The more coarse the grit is the larger the chips will be taken off the edge surfaces. Smaller grits take smaller chips off the edges, and so are less obvious.

New bits

Examples of the range and grit differences in grinding bits

If it is a new bit that is causing the chipping, consider dressing it. New bits often need to be dressed – removing protruding diamonds, or cleaning and exposing new ones on a worn bit. To dress the bit you can grind some scrap glass, brick, or use a dressing stone to lightly grind some of the abrasive material away. This most often settles the bit and avoids chipping.

Paint and Cold Working

One of the difficult things in cold working is determining when it is time to move on to the next grit size.  You really cannot tell while the piece is wet, because the water disguises the minute scratches put into the glass by the grinding process.

After you have dried the piece, you can coat it with a white paint pen. It is not necessary to cover the piece completely in white pen.  The white marks will be taken away by the grinding to give evidence of where you have already ground the piece.

Piece prepared for the next stage of grinding

You must make sure the paint has completely dried, or it will wash off with the water used in the grinding.

I find I get best results from this "witness" by making the paint pen marks at right angles to the grinding direction.  When all traces of the paint have disappeared, you have thoroughly covered the piece with that grit size.  Being ever cautious, I tend to dry, paint and do a second pass at the same grit before going to the next finer grit.

Cold Working Holes

If you have, or expose, holes while cold working a piece, you need to keep the glass waste from settling into them. Dry the piece and with a bar of hard soap rub over the area until the holes are filled with soap. Then polish off the excess with a cloth or paper towel. 

The filled hole in this piece is the white dot half way up on the right side

When finished cold working, you only need to wash out the soap, which is much easier than trying to get rid of powdered glass.

Based on comments from Cynthia Morgan, posting as Morganica

Glass Stains

A lot of people want to use old window glass – for recycling reasons, for the character of the older glass or because it is cheap or free. This glass often has stains or the appearance of corrosion or etching from age or storage conditions. To remove these blemishes you should start from the least aggressive method and work your way through the more aggressive methods until you find one that achieves the cleanliness you desire.


When using these methods appropriate breathing and hand protection are a requirement. 

Surface deposits

The first method is to wash the glass in water with a bit of crème cleaner. Place the glass flat on a work bench and scrub it with a brush containing the water and cleaner. Rinse and dry. This may be all that is needed.

More persistent stains require chemicals. Use rubber gloves for this kind of process. Soak the glass in a dilute (5-10%) solution of lye. This is also known as caustic soda. Chemically it is sodium hydroxide (NaOH). After the soak wipe with a disposable towel and dip it into a dilute solution of hydrochloric acid (5-10% again) or vinegar to neutralise the caustic soda. Wipe the glass with a towel and rinse with plain water and polish dry.

Metallic stains or iridescence and etched surfaces

If the blemishes are more than surface deposits, more aggressive methods are required. The metallic-like stains and iridescence are usually evidence of the corrosion of the surface of the glass. These and any etching require grinding and polishing.

You should start with a polishing process to determine if that will be sufficient to bring clarity back to the glass. The use of an optical polishing material such as cerium oxide paste and a felt polishing head is often enough. Ensure that you do not let the cerium oxide get any drier than a paste to avoid localised heating and therefore breakage of the glass. Rinse the glass in clean water and polish dry.

If this does not remove enough of the surface to provide the desired clarity of the glass you can try either using pumice or jeweller's rouge, or dilute hydrofluoric acid.

Hydrofluoric acid used in a 10% or less solution will remove the surface of the glass, so eliminating the evidence of corrosion. It will even out, but not eliminate, the evidence of any etching. The disadvantage to this method is the risks associated with such a dangerous chemical. There is within this note on acid etching a door panel some advice on safety.

If you decide to avoid the hydrofluoric acid method, you can use mild abrasives such as jeweller's rouge or pumice with water and a felt polishing head. Once you have ground the whole of the surface, you need to wash it very thoroughly in clean water. Then change the felt head and go to cerium oxide to provide the optical finish.

Note:

All the dilutions and solutions suggested here are with water.

This note assumes the glass has had any paint already removed. For a method of removing house paint see this tip.

Negative assembly

To get a crisp design finish to a fused piece especially with thin elements like stringers it is often recommended to fire upside-down. This means that the bubbles move toward the final bottom of the piece rather than rising and disturbing the design on the top.

Assemble the piece on your workbench as usual on top, but use a toothpick or small brush to dab some GlasTac or other glue near the two ends of each piece. Let it dry overnight and then carefully flip the whole arrangement upside down onto the kiln shelf.

If you don't like the glue and flip over technique you can try another. Draw your design onto Thinfire shelf paper in negative with a graphite pencil. If you have a strong light source you can draw on the reverse (printed) side and trace the negative onto the upper face of the Thinfire. Then assemble your pieces upside-down on the shelf. You can assemble the whole in the kiln and there is no need for glue.

When fired, clean the piece thoroughly and decide whether you want that texture or a smoother surface. If you want the smoother surface, put in the kiln for a fire polish. This is often known as “flip and fire.”

Aperture Drops Finishing

After the piece has cooled and been removed from its ring, you can consider how to finish the piece.  The first decision is whether to retain or remove the rim from the vessel. In some cases, the rim can be retained as an integral part of the piece and there is little work needed to finish the piece.  Possibly only tidying up the edge of the rim and cleaning the bottom.

Removing the rim

But for most aperture drops and for most people, it is desirable to remove the rim. To have successful drops without rims, you most often need to have access to cutting and polishing equipment.  There are several ways to do this. 

The method that uses least equipment is to score around the upside down drop just above the rim.  When scored, tap the rim with a soft hammer to release it.  This is not always an even break and sometimes runs into the length of the drop.

A low tech way of cutting is to put a diamond cutting blade on a Dremel-like battery powered tool and with a flow of water grind through the side of the drop.  It is best to have a small flow of water directed at the cutting area, rather than immersing the rim in a bath of water.  This helps avoid electrical shock.

The rim can be cut off in portions with a tile saw, cutting quarters, eighths, sixteenths off the rim, approaching the edge of the drop.  Those with adjustable height wet saws can cut through small portions at a time of the rim, and support both the rim and the drop, especially when nearing the completion of the cuts.

There are also specialised versions of the wet angle grinder that make cutting of the rim easy and much more certain of a good result.

Finishing

After any of these methods of removing the rim, the drop edge, and possibly bottom, needs to be ground and polished.  Fire polishing is not possible as the drop would collapse long before the rim was smooth.

Because the rim will be relatively thin, it is possible to grind and polish with hand pads.  However, it is quicker to use a flat lap or linisher with a succession of finer grits to grind and polish an edge.  HIS Glassworks has a series of videos and this one gives good information on the methods and progression of grits to get to a polished edge whether by machine or by hand.

Grinder Head Grub Screw

Need help! The small screw that secures the grinder bit to the shaft was stuck and my efforts to loosen it resulted in stripping it. I've tried spraying it with lubricant -- still no luck. What can I do?

There is a tool that many mechanics and tool shops have. It is normally square or triangular. It is used by drilling into the broken off bolt, or in this case, the grub screw. The tool is hammered into the hole and then with a wrench/spanner loosened.

However, you should make sure that the socket for the allan key is clear of glass residues. I do this by using a needle or other thin sharp object to clear out all the glass powder. I am sure there are other things to clean out the hole too. When the socket is cleaned, I push the key into the socket very firmly and hold it there while turning. This has worked for me in the past.

Once the grub screw is out, you need to get a replacement, so the same problem does not re-occur. I keep the screws from old heads in my box of grinder parts for this eventuality.

So the maintenance is not only on the shaft but also on the fixings. Putting a dab of Vaseline or thick grease into the socket will help keep it clear of the glass residue.