Fire polishing Bottles

“I've cut wine bottles horizontally and want to keep the boat shape but round the cut edges. [Will a tack fuse firing] do what I am wanting without changing the shape of the bottle? “

Temperatures

The softening point of bottle glass is around 720˚C. The temperature you have chosen for a fire polish is 730˚C. It will slump to some degree from about 700˚C.  That will not be high enough to fire polish the edges.  Reducing the soak time at 730˚C will reduce the slumping effect a little, but it will not polish or round the edges.

Ground edges

In addition, ground or sawn edges are so rough that fire polishing will not work well at any temperature, because the rough surface promotes devitrification.  To get a good fire polish, the edges should be ground to at least 400 grit, and 600 grit gives a more certain fire polishing result.

Cold Working

Fire polishing is not the most certain way to round and polish edges for a 3D object. Cold working with hand pads or grit is the low cost way to polish the edges.  The grinding will need to go through grits of 200, 400, and then smoothing pads and finally pumice or cerium oxide depending on the shine wanted on the edges.  This can be done by hand or by machine.  Paul Tarlow has an excellent eBook on cold working by hand, and there is some instruction in this blog. 

Spikes on Frit Castings

Credit: The Crucible.com

It is frequent to have castings from frit with spikes, needles, or prickles around the edges. 

Causes

These spikes result from the glass touching the edge of the mould or separator during the hottest part of the firing. The glass particles first begin to compact as the glass rises toward the fusing temperatures. As the temperature increases toward the casting temperature it begins to expand both horizontally and vertically from that compact mass. As it cools, the glass sinks down and retreats from the edge. This movement leaves some small bits of glass stuck to the sides. The glass contracts as it cools, leaving the spikes as it contracts from its hottest state. 

Avoidance

The usual recommendation is to mound frit in the middle and let it flow to the outside. Still, the glass flows to the outside of the mould at casting temperature and it touches the sides. Leaving the risk of creating spikes. Accurate measuring of the amount of glass to charge the mould with is important. With the right amount of glass, the mould will not be overfilled and so, reduce the spiking. 

Measuring the weight of glass for the mould is not difficult. In many cases, the manufacturer of the mould has done the work for you. If you need to calculate the weight of glass required for the mould, it is not difficult. A method is given here. In short, you use a dry fill of the mould. Measure the volume (using the metric system) and multiply by the specific gravity to get the weight in grams. 

Larger chunks of glass tend to produce fewer spikes than smaller frit. Usually longer soaks at top temperature are required to fully form the glass with smaller frit. It is also possible to drip glass into the mould from a pot suspended above the mould. Accurate measurement of the weight will still be important. But add 100gms/4oz. to the amount to allow for the glass that will stick to the pot.

My view is that with dams, it is better to use a straight sided shape with fibre cushioning around the outside. When annealed and cool, clean it well. Then fire polish with a slow ramp to 540°C/1000°F followed by a quick ramp to the fire polish temperature. This will polish the sides of the piece that were in contact with fibre paper.

Changing Coldworking Grit Sizes

 

One of the difficulties of coldworking is when to change grits. Checking the completeness of one grit grinding visually is difficult. It is difficult to see the effect while wet. Even when dry it can be difficult to see that all the previous marks have been ground out. This is the background to the recommendation that each successive grind should be right angles to the previous. It is easier to see marks that are in a different direction than marks that are just wider or deeper than others.

 However, there have been aids developed by cold workers in the past that are relevant today. Use a witness at each stage of grinding. The best at present are paint markers. These are the kinds used by metal workers to identify the pieces, their dimensions, etc.

 The paint needs to be dry, or it simply washes off the glass surface as soon as it is placed on the grinding surface. So, there is a process to ensure this witness works as it should.

 On a dry surface, run the marker at random across the surface to be ground. Let it dry before putting on the grinding surface. You can test by putting your finger on the paint. If some comes off on your finger, it is not yet dry. When dry, grind the surface. When all the colour has been ground away, it is time to change grits.

 Dry the glass surface again and paint it. While the paint is drying, change grits, or discs and do any other cleaning up between grits that is required. Test for dryness. Grind at right angles to the first grind. Make sure absolutely all pinpoints of colour are removed. If they are gone, dry, and paint. While letting it dry, change grits, clean up, and do the other miscellaneous tasks. Then test paint for dryness and grind.

 Repeat this process with each grit until finished. If on occasion you find you have gone to a finer grit earlier than you should have, go back to the coarser grit with you painted glass and repeat the progress back from the immediately previous grit through to the finer grits.

This process of using a witness works whether hand or machine cold working.

Grinding out Chipped Edges

Sometimes the edge of a drop vase is chipped during the polishing process, or more frequently, during use. What to do?

 Grind the edges down until the chip disappears. This seems like an obvious statement. But it is often overlooked.

 

 

The grinding can be at a slight angle to the length or parallel to the base of the piece. The angled grinding removes less glass but needs a jig of some sort to keep the angle consistent. The difficulties of obtaining a consistent angle grind, makes grinding a flat edge simpler.

 You can do this flat grinding and polishing by hand in only a half hour or so. Although a flat lap or belt sander will enable you to do it more quickly.

 You have to be careful while griding, especially when using the rough grits, to avoid small chips on the new edges. One trick I learned is to make a small bevel or chamfered arris the edge before doing the flat grinding.

 

Credit: www.pavingxpert

 If it is a large or deep chip you are grinding away, you will need to do it in stages.

 You do not want a large arris at any stage. It is possible to create such a large arris that you have to grind more glass away than the original chip would have demanded. When the grinding comes close to the end of the arrised edge, stop. Make a small arris on the edge again before continuing to grind the face. Repeat this as often as required until the chip is removed. 

 Make this arris at the start of every finer grit. The arris will not need to be so big as for the first, rough grinding. You are not taking off as much glass on the surface. But the arris will prevent tiny chips appearing at the edge of the polished surface.

 I give a final arris the polished edge. This gives a pleasant roundness to the edge. It also keeps the edge from being delicate and subject to further chips.

 

 

Grinding direction

There is a suggestion that the glass being ground should be passed along the in the direction opposing the spin of the grinding bit.

The grinder bit rotates in a clockwise direction as you look down on it.  This means the grinding surface is moving toward the left.  The recommendation is that you will get more efficient grinding by passing the glass to the right, against the rotating direction of the bit.

This is more theoretical than practical. In practice, the bit is spinning so rapidly that only a minor difference, if any, can be perceived.  Without a noticeable difference, there is no reason to be concerned. Move the glass you are grinding in the directions that you find convenient.  You will find a little more resistance when you move the glass from left to right, rather than the other way.

You are much more likely to damage the bit and grinder motor by excessive pressure against the bit than by any directional movement.

Needle Points

Often fused glass has prickles or needle points around the edges and especially at corners after firing.

This illustration is from Glass Fusing Made Easy

The nature of glass and its interaction with the separators is the cause.  As you heat glass it expands. Once the cooling starts, the glass contracts. Often a particle of the glass sticks to the separator while the rest continues to contract. This dragging of the glass along the separator results in the creation of little sharp points developing as the glass retreats to its final dimensions.

The best solution I have found to reducing the points at corners is to blunt any points or corners before assembly. Only a tiny amount of glass needs to be removed from the corners to reduce the possibility of these points being developed.

Small needle points can also develop along the sides of the glass too.  These are more difficult to avoid.  The most successful method for me is to use a loose separator.  This can be Thinfire, Papyros or a fine dusting of alumina hydrate or powdered kiln wash.  Although less widely available, talc can be used. Talc is known to be carcinogenetic with high exposure, so breathing protection is needed. All these powders provide enough lubrication to allow the runny glass to slide without sticking. 

Of course, you can use boron nitride, which is very slippery, but the cost of it makes it expensive in comparison to the other methods, including using fine diamond pads to remove the needles.

An additional consideration is the temperature you use.  The higher the temperature, the more the expansion.  Expansion rates are almost exponential above the brittle phase of the glass.  Reducing the temperature by 20C and doubling time or more means the glass does not expand so much and the additional time allows the desired profile to be achieved.  

Of course, paying attention to volume control - using 6mm or more thickness - will help to reduce the needle points.  A 3mm sheet both expands and becomes thicker at the edges by drawing more glass from the interior and the edge while attempting to reach 6mm.  This means there is an increase in the needling effect.  Although a 6mm piece retreats on cooling, it does not have the additional thickening effect of a 3mm piece.  Even a 9mm piece retreats on cooling, although the final piece has a larger area than at the start. 

- - - -

There are various preventive measures that can be taken to avoid needle points on fused glass.  These range from altering the edges of the glass, using fibre papers that turn to powder, using refractory powders, or boron nitride. Post firing solutions relate to cold working.

Smooth Surfaces on Drop Vessels

It is widely recognised that the usual results of kiln forming are one textured side and a smooth upper side. The common methods of having upper and lower surfaces both smooth is to blow the glass, avoid allowing the glass to touch the mould, and cold working the textured side to smooth.

The question arises about the possibility of getting smooth surfaces on the inside and outside of a drop vessel.  As the glass in a drop only touches the mould at the collar and edge, shouldn’t the glass be smooth on both sides?  The answer to that is in the temperatures and time used.

The temperatures used in a drop are not high enough to be certain of smoothing the outer surface.  But the soak times at drop temperatures are enough to create a fire polish on the upper/inside surface.  This indicates the blank in a drop should be placed with the texture up, facing the heating elements.  The smoother side facing the floor will be stretched and will remain smooth. 

The smoothing effect of firing with rough side up does depend a little on the depth of the drop.  Shallow drops will not have the same heat exposure that deeper drops do, assuming that a moderate heat is being used over three to four hours.

This implies that the design to show on the inside of the drop should be in contact with the separator when fusing the blank.

Creating Flat Bottoms by Hand

No jokes please!

Often the moulds we use do not have a suitably flat bottom to them, making the resulting item wobble when set on a flat surface.  There are several ways to create a flat spot in the mould, reaching in to re-set the glass while firing, putting the glass in at a complimentary angle for a second firing - but they are not always successful.  

Of course, if you have the money you can use a flat lap or a linisher with a back plate to grind a flat spot on these bowls and other unstable pieces.

But,

You can still make a flat spot on your piece without machine tools.  Use a piece of float glass larger than your piece as your grinding base.  Put a slurry of 100 grit sand on the base and put your piece over.  Holding it level, make circular motions with firm downward pressure.  In only a few minutes you will have produced a large enough flat spot to stabilise your piece.

If you do not like the mess of the slurry, fasten a 100-grit sandpaper onto float glass, add water and do the same as you would with a slurry of grit.

An excellent video by HIS Glassworks comparing hand and machine coldworking is available here.

Pin holes in melts

Pin holes in screen and pot melts are the result of very small bubbles rising to the surface.  These bubbles are sometimes within the glass melted, but more often come from small amounts of air trapped within the flowing glass.  These are perceived to be unsightly, or make it impractical to make a functional piece from the melt.

There are ways to minimise bubble formation or to deal with the formed bubbles.

Bubble Formation

In pot and screen melts, the glass spirals as it touches down onto the shelf. This spiralling action can trap small amounts of air as each successive spiral forms beside the previous one. Efforts at prevention of tiny bubbles in the final piece need to concentrate on this fact.

Bubble Squeeze

A preliminary element in bubble prevention is to have a long bubble squeeze to allow the glass to settle in the pot or on the screen so that the rest of the process can proceed with a minimum amount of air trapped within the flowing glass.

Two-Stage Drop

In some cases. it is possible to have the glass flow from the pot onto an angled shelf where the spiralling glass has to flow from the initial touch down to the edge and then flow onto the shelf.  This allows any tiny bubbles initially trapped to escape before the final drop onto the shelf.  This provides two mixing processes and means that a lot of clear glass needs to be included to avoid a complete mix of the colours.  It requires careful selection of the original colours to avoid a brown or black result.  It also requires a big kiln with sufficient height for a two stage drop.

This two-stage drop is of course, not suitable for a screen melt where you wish the glass strands to remain.  Nor is it suitable when you wish to have many “pools” of colour mix in the final piece.

Where the two-stage drop is not practical or suitable other methods can be used.  These relate to scheduling, cold working the surface and re-firing the piece.

Schedules

Scheduling relates to using a soak at full fuse temperature before proceeding to the anneal.  The melt will occur at 850°C to 950°C.  You can cool as fast as possible to a full fuse temperature of about 810°C and soak there for an hour or more.  This allows the small bubbles to surface, break and heal.  Schedule the rapid cool to the annealing soak, once the high temperature soak is complete.  This will eliminate lots of the bubbles, but not all.

A sample friring schedule from bubble squeeze upwards and then down to a high temperature bubble reduction soak

Cold Work

Cold working the melt is about abrading the surface to open the bubbles that are just emerging to form a small dome at the surface.  Sand blasting is a common form, as usually kiln wash or fibre needs to be removed from the bottom of the melt, and some devitrification from the surface.  It would be possible to continue to grind the surface of the glass to eliminate the small depression in the glass caused by the now opened bubble, but this is likely to expose more bubbles that were at a slightly deeper level. What next?

Fire Polish

As you will need to do a fire polish firing after blasting or grinding the surface, you can use a full fuse temperature to allow the surface to become plastic enough to fill the bubble holes.  Remember to schedule the firing as though the piece were at least 12mm thick.  You may find that more bubbles are exposed in addition to the ones healed at the conclusion of this second firing.

Flip and Fire

An alternative is to fire upside down.  You will have noted that there are no bubbles on the bottom of the melt.  This is because the bubbles have risen through the heated glass.  This physical fact can be used in the second firing.  Fire with the melt surface to the shelf.  It is best to have a clean and newly kiln washed shelf, or fibre paper (not Thinfire or Papyros) under the glass. Fire the glass to a full fuse or high temperature tack fuse with a significant length of soak to allow the bubbles near the original surface to move toward the interior of the glass.  After firing, the glass will need thorough cleaning before being fire polished. This should leave you with a pin hole free piece.

Conclusion

Achieving a pin hole free pot or screen melt requires several stages of coldworking and firing.  This makes melts inexpensive in materials (it is scrap of course) but expensive in time and firings.

Fire Polishing

Polishing of glass can be done in the flame, in the kiln, by acids or by grinding with successively fine abrasives depending on the nature of the piece and the equipment available.  Fire polishing in the kiln is widely popular as it utilises existing equipment, avoiding purchasing additional cold working equipment. This post indicates some elements about fire polishing in the kiln. 


Fire polishing is the technique most often available to kiln formers. This is the process of heating the glass to less than a full fuse to achieve a smoother texture on the glass. It is often used after sandblasting or hand sanding a piece in order to give a smooth shiny surface to the glass without extensive cold working with successively finer grits to get a polish. It also can be used to give a variety of textures from a sealed but almost unchanged sandblasted surface, through a satin-like finish to a very subtle difference between full polish and slightly textured surfaces in the same piece.


Fire polishing range

The temperature range that this occurs between slumping and tack fusing. The normal range is 650C to 750C depending on the glass, the soak time and the speed of advance.  The purpose of this kind of firing is to get the surface of the glass hot enough to form the desired surface without soaking long at higher temperatures, as this is also the devitrification range (700C - 760C).  Normally there would be a minimal or no soak at the top of the temperature range.

When to fire polish As this temperature range is above the slumping temperature, fire polishing should be done after fusing and before slumping. As this will be the last operation before forming, you also should do any work to shape the edges and deal with any other imperfections, before fire polishing. After doing any grinding or other work on the edges or surface of the piece, thoroughly wash and polish the piece dry.

Methods
You can take the fused piece that has been treated to remove the devitrification up at the same rate as for slumping the piece to the tack fuse temperature.  The higher you go, the less soak time is required. Of course, the higher you go, the longer you are in the devitrification zone.  

Some people advocate a quick fire polish.  This is achieved by firing at a relatively slow rate until a low slump temperature is achieved.  Then fire very quickly to the tack fuse temperature with no soak and return to annealing temperature as quickly as possible.

The quick fire polish does achieve a minimum of time in the devitrification zone, but it eliminates all subtely in the surface.  A long soak of up to 90 minutes at a moderate slumping temperature will give a satin appearance to an abraded or sandblasted surface.  A shorter soak will seal sandblasted work without eliminating the texture of the sandblasted image.

In all the cases of fire polishing you need to peek at intervals to determine when the desired surface has been achieved.  This requires careful placement in relation to the place from which you will be able to peek at the surface.  For a fully polished piece, you will see the reflections of the elements.  For more subtle textures, you need to think about what you want to see, peek, close the lid or observation port and think about what you saw.  If it is not yet what you want, peek at another interval in the same way, until you observe the surface you want.

Combining fire polish and slumping It is sometimes possible to fire polish and slump at the same time, but this is a risky technique often leading to changes in shape or an uprising of the glass at the bottom of the mould. It is possible to fire polish glass as low as 630 with a long soak – 60 minutes or more. If you are determined to fire polish and slump at the same time, it's essential that you watch the piece very carefully to prevent over-firing.

Fire polishing already slumped items Similarly, re-firing already slumped items to a fire polish rarely succeeds. Distortion of the piece is more likely than achieving a fire polish on an already slumped item.

Again, in these more difficult circumstances, you must observe at intervals to ensure you do not over fire and distort your piece.

Schedules
The reason that no indicative schedules are given is that different glasses, and different lay ups require different firing conditions.  These are dealt with elsewhere in the blog.

Alternatives Alternatives to fire polishing include acid polishing, which can present a health hazard, and is normally an industrial process. The other common method of polishing is to cold work the piece. This often requires specialized equipment, but can be done by hand if you have the time.

Single Layer Slumping

Almost all glass can be slumped as a single layer, whether produced for kiln working or not.  A few are extra sensitive at even slumping temperature and change character at around 630C-650°C, but all others can be slumped.  This posts concentrates on slumping of single layers of non-fusing compatible glass, but most of these elements can be applied to fusing compatible glass too.

The things you need to take care about are:

• Temperature
• Soak Times
• Edges
• Devitrification
• Annealing
• Testing

It certainly is possible to slump single layers. The resulting glass will be slightly less robust than two or more layers of glass, but simply because it is thinner.

Temperature

The temperature that you use needs to be high enough to allow the glass to take the shape of the mould, but low enough that the glass does not distort or stretch and thin.  This is a balance that you can achieve through observation of the firing. 

It most often is best to use the lowest practical forming temperature that you can.  Practicality here is about how long you want to wait for the glass to conform to the mould.  It is possible to take the glass to about 580°C and soak for multiple hours, but not very practical.  It does depend on the glass as to the temperature to be used for the slump.  There are two sources here that can help: the slump point test  and this table of glass characteristics. 

Soak times

A practical soak time will be 30 – 90 minutes, which will avoid marking the underside of the glass.  This means that the temperature will need to be lower than the softening (or slump) point of the glass. Your slump point test will tell you the temperature at which the glass begins to deform.  That is the best temperature to use.  If it is taking too long, advance the temperature by about 10°C.  If you used the table of glass characteristics to find a softening point, reduce that temperature by about 30°C as a starting point.

Edges

The temperature that you will choose to use is not high enough to allow the edges to change as they would in a fuse.   This means that you need to have the edges exactly as you want them in the finished project.  This will require cold working by hand or machine.  Neither will take a long time, but require the correct tools. This post gives you the comparison of fused and cold working methods.

Devitrification

While most glass can be slumped you need to be careful with opalescent glass, as it can devitrify easily.  Most wispy glasses are fine, but the more opalescent wisps they have, the more difficult there may be.  Streaky and single colour glasses are usually fine. 

Annealing

Another element in slumping glass not formulated for kiln working is the annealing of the glass after the slumping.  The annealing temperature can be estimated as 40C below a low temperature slump of a 280mm span of glass. The slump point test mentioned earlier will help determine the annealing point. You need to soak for a time - maybe 30 minutes - at the estimated annealing temperature and then cool slowly in case you have miscalculated on the annealing temperature.  In any case, a long slow anneal cool will pay dividends in a more robust glass.

Testing

You will find some manufacturers’ glasses are less adaptable to kiln forming than others.  So, it is best to run tests on the glass before committing to larger projects.

Remember TADSET - temperature, annealing, devitrification, soak, edges, test.

Vinegar for Cleaning

Cleaning glass with acids causes corrosion of the surface of the glass.

So many people mention using vinegar to help clean the ground edges. I can't resist commenting. Vinegar is acidic. Glass is alkaline. Leave the glass in the vinegar too long and it will affect the surface of the glass.

Sometimes it dulls. Sometimes it corrodes to give a mild iridised appearance. The acid removes the alkaline materials – potash, lime, etc. – leaving a pitted surface at the microscopic level.  Left long enough – hours rather than days – the surface will begin to appear dull due to the pittiing. It is at this stage that it is easy to introduce contaminants which may later form nucleation sites for devitrification.

If you must use vinegar, rinse with it. Do not soak your glass in a vinegar solution.

Alkaline cleaners

Two alkaline substances that are used to clean glass are baking soda and ammonia.  Both are effective cleaners and do not have a reaction with the glass as they both are alkaline. The glass can be left to soak for a brief time in a solution of these chemicals, although I would not be happy with an open bath of ammonia.

But the effective part of what people are doing to clean the edges is the scrubbing. Scrubbing the glass powder out of the pits left by the grinder is what really works.  When leaving the glass in a bath of even plain water, you are giving the powdered glass the opportunity to settle into these pits.  Once settled into the pits, the powdered glass can become like cement to remove.

There is a much better cleaner, especially for removing kiln wash and investment mould material.  It is a neutralised acid - tri-sodium citrate.  It is often sold in the dihydrate form.  It is used in the food industry and so is widely available. Glass can be left in the 6% solution for days without being etched.  The unwanted material is chelated from the surface during this soak.  Occasional agitation may speed the effect by removing the loose material on top.

Mechanical cleaning

You could have a much better effect if you scrubbed under clean water before placing in a bath of water with grinder lubricant.  This material promotes a gel like glass residue. This gel prevents the glass becoming caked like cement.

A final scrub to thoroughly clean before assembly is a good idea. Each piece should be polished dry with lint free cloths or uncoloured absorbent paper.  If any particles cloth or paper are left behind, they will burn away long before devitrification can begin to form.

Of course, the best solution is to grind with 400 or 600 grit.  This is fine enough that there is not enough powder left to promote devitrification.

Devitrification on Ground Edges

The first element in preventing devitrification is cleaning.  Making sure all the edges of the glass are clean will help.  OK, you have cleaned the edges well after grinding. You still get detrification, so you want to know

Why do ground edges get devitrification? 

To answer this question, you need to think about how glass behaves in the kiln. As it heats up the glass expands, pushing the cut edges into the separator on the shelf. The pits caused by the grinding have not yet become fire polished.

When the glass retreats on cooling the pits in the edges of the glass, although very small, pick up some of the separator. These small particles act as the nucleation points for the crystallisation of the glass which is generally called devitrification.

The glass of a single 3mm layer retreats further on a single piece than on a 6mm piece. This rolls the devitrified glass upward onto the upper edge of the piece.

Prevention of devitrification of the ground edge is to have the pits in the glass edge finer than the particles of the separator. This is more than just washing the glass immediately after grinding to remove the glass powder from the grinding scratches.  Yes, this will reduce the chance for devitrification, but not totally prevent it.  As noted above, the pits in the glass will pick up particles of separator on expansion, giving nucleation points for the devitrification.

Further coldworking beyond the initial grinding is required to reduce the devitrification possibilities.  This involves using finer grinding bits or smoothing by hand with finer grits.  This does not have to take long, as the shape has been achieved by the grinder.

The logic of prevention is to have the glass edge smoother than the particle size of the separator, so the finer and smoother the separator, the smoother the surface of the glass edge must be.  

But my devitrified edge was on top of other glass

The follow-on question is about why devitrification occurs on ground edges that are not near the kiln shelf.  There are two elements to consider.

It is claimed that the fumes of the binder burning off can settle in the pits of the ground glass, providing those nucleation points for the glass crystalisation. The suggested solution is to vent the kiln to about 400C to allow the combustion fumes out of the kiln rather than keeping them inside the kiln.

The second and more certain element is that the grinding creates microscopic pits and fractures in the glass where the powder from grinding settles.  Almost no amount of cleaning will completely remove this residue from the tiny pits and fractures resulting from grinding. 

There are at least two solutions to this cleaning problem. Don't grind unless absolutely necessary - groze instead.  The second is to lightly cover any ground edges with clear powder frit.  You could of course consider ultrasonic cleaning or power washing, either with a dishwasher, or outdoor power washer.  Both these seem to be so completely out of proportion to the problem, that I have never used them.

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.

Dog boning in Slumping

Often even in shallow rectangular moulds the sides pull in during the slump.  To know what things to try to correct this effect, you need to understand why this effect is occurring.  These two pieces show the effect in different ways.

ebay 0916_slump_01

 This slump shows that even with thick glass the sides curve inwards even on shallow slumps.

theglassundergroundnj.org

This slump shows the interesting effect that the further up the piece you look, the greater the curvature. This relates to the greater amount of movement required by the glass to conform to the mould at the outer edges.

Why

During the slump of a rectangle or square the whole shape of the glass sheet is changing.  It is slightly stretching to form into the “hollow” of the mould, but it cannot stretch evenly all over, especially at the corners.  If you think of the analogy of Draping a piece of cloth into a rectangular depression, you will find it wrinkles up at the corners if you smooth it at the sides. This indicates the material is attempting to overlap there as it does not have a dart to take up the excess cloth.

This similar to what is happening to the glass sheet.  It is relatively thicker at the corners than along the sides.  Therefore, it does not slide down the mould at the corners as on the sides. It is simply thicker and is compressed by the movement of the glass at the sides.

Prevention

The question is how to use that knowledge to avoid or minimise the dog boning during the slump.  There are probably lots of methods, but three have occurred to me and others.

Add more material along the sides.  This involves fusing a piece with shallow arcs rather than straight sides.  This gives more material to counteract the dog boning effect when slumping a rectangle.  The difficulty is getting the proportions of the arc correct in relation to the length of the sides. You also need to ensure the arcs on the sides are not so much larger than the mould that they slump over the edge.  This means the whole piece will need to be cut smaller than the mould.

Remove material at the corners.  This takes the opposite approach.  To avoid the increased amount of glass at the corners, you remove some of it.  That is, you round the corners of the pieces to be fused. How much you will need to round the corners is a matter of experience, but is a shorter learning curve than cutting the edges in an arc.

Reduce the temp and increase soak time.  This approach requires less skill in cutting a shape.  It relies on giving the glass time to relax into mould with a minimum of stretch.  You need to find the lowest practical temperature at which to slump.  This will be the temperature at which you can first see the deformation of the glass in the mould.  Hold the temperature there for as long as it takes – possibly one or two hours. It is likely that you will still need some rounding of the corners of the glass, but only your experience will determine that, and if so how much.

Cold work the edges until straight.  This can be done by hand or by machine.

Further information is available in the ebook: Low Temperature Kiln Forming.

Dog Boning Causes

I fired a one-layer piece of glass and it shrank. What did I do wrong?

Cause

This result relates to the thickness that glass, under kiln forming circumstances achieves.  The combination of gravity and viscosity lead to this effect.  As the glass becomes less viscous (more runny), the surface tension is greater than gravity and so it becomes thicker at the edges.  This additional glass is supplied from the edges and to some extent from the interior. The glass in the middle becomes thinner, allowing in certain circumstances bubbles or holes to appear.

This illustration from Fusedglass.org shows the effects of gravity, which is related to mass, and viscosity.  The lack of mass means the surface tension allows the glass to draw up to be come thicker, forming the classic dog boning appearance.

Prevention

Knowing why this occurs allows you to take come precautions, when firing single layer pieces, to help prevent the shrinkage, often known as dog boning.

Fire larger

You can cut the glass larger than the final piece will be.  After firing, you cut it down to the size you want.  You may have to do a bit of cold working to get a rounded edge to the glass before any further processing.

Fire lower  

You can fire at a lower temperature for a longer time.  You will need to observe to determine when the glass begins to shrink. Either stop the temperature rise and soak there for a time, or reduce the temperature a little and soak for as long as needed to get the surface texture wanted.

Fire oval or circular pieces.  

With these shapes the shrinking is not so obvious, as it occurs all the way around.  With rectangular pieces, as the glass shrinks, the corners become thick more quickly and so do not shrink as much, giving that dog bone appearance.  Rounded pieces become thicker all the way around more evenly and the shrinkage is not so obvious.  However, you still get thinning in the interior which can lead to holes or bubbles, so observation is still necessary to prevent excessive thinning and bubble formation.

Fire thicker

The real prevention is to fire two layer pieces as that is the thickness at which viscosity, surface tension and gravity are in balance.  So the glass does not change size at kiln forming temperatures.

Cold work

Alternatively, you can cold work the edges back to straight parallel edges.  This can be done by hand grinding or by machine.

Further information is available in the e-book: Low Temperature Kilnforming.