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. 

Mending a crack

 I had a piece crack due to an annealing oops. I put powder on it and put it back in at a higher temp with a much longer anneal time. It looks great on the front, but I can still see where the crack was on the back. Is it supposed to be like that? I didn't think to put powder on that side.

If you think about why you get crisp lines at the bottom of a strip construction and a more fluid appearance on the top, you will be near the answer of why a repair looks ok on top but shows the crack on the bottom. The temperature on the bottom of the glass is less than on the top at the working temperature. And less again than the air temperature which we measure. This means that the bottom part of the glass has less chance to fully recombine. This, combined with the resistance to movement of the glass along the shelf, results in evidence of the crack being maintained.

Credit: Clearwater Glass Studio

There are some things that can be done to minimise the evidence of the crack. Make sure you know why your piece cracked before you try to mend it. An annealing crack will need different treatment than a thermal shock crack or a compatibility crack. Simply refiring the piece may only make the problem worse.

One approach is to place a sheet underneath. Make sure the broken glass is well cleaned and firmly pushed together. Dams may be useful to keep the glass compressed together. Glass expands both horizontally and vertically during the fusing process. Confining the glass will transfer most of the expansion in a vertical direction. This additional (small) vertical movement may help in forming the glass seamlessly. The broken glass now being supported by an unbroken sheet will enable the movement required to “heal” the crack.

If you do not want to change the surface, you can fire upside down. To do this you need to have a loose bed of powdered kiln wash, or whiting (a form of chalk) that is thick enough to press the textured side fully into the separator. Make sure the glass is pressed together without any separator getting into the crack. One way to ensure the crack does not open is to use a small amount of cyanoacrylate (super) glue which will burn away during the firing.  Put a sheet of clear glass over and fire. Thoroughly clean the face after this repair firing. The ultimate top needs to be fire polished to remove the evidence of the crack, and if it has picked up any marks from the powder.

You could, of course, fire upside down in this way but without the additional sheet, to avoid making the piece any thicker. This may or may not work well. If the base layer is one layer thick, it may pull in at the sides and pull apart at the crack where it is one layer thick.  It is also possible that bubbles will develop in the thin parts of tack glass because of the uneven thicknesses.

A final note. Placing powder on the back will not improve things. The powder will not fully incorporate with the glass and so leave a rough surface without concealing the crack.

Avoiding breaks

To repair or not

The process of repairing

Fire Polishing of Frit Castings

Image credit: Obsession Glass Studio

 Fire polishing castings is relatively difficult.  Even though people may suggest temperatures for this kind of fire polish for castings from frit:

• ·        They are relevant to particular kilns. 
• ·        They are also dependent on the ramp rate. 
• ·        The presence or absence of a bubble squeeze is important. 
• ·        The size of the casting is relevant.

 The objective is to get a fire polish without distorting the shape of the piece.  The general procedure is to fire slowly to the softening point. This is to ensure the casting is of similar temperature throughout. The softening point for fusing glass is around 540°C/1000°F. You should soak at that point for a time to ensure the glass is all at that boundary between brittle and plastic.

 You may prefer to use a bubble squeeze soak to achieve the same thing.  This has a slightly higher risk of distorting the piece.  If you do use the bubble squeeze, it should be done at the lower end of the bubble squeeze after a slow rise.  The casting will not be subject to much change at 600°C to 620°C/1110°F to 1150°F, if the soak is short.

 The rates to be used are dependent on the size and thickness of the piece.  Larger and thicker pieces need slower rates than thin ones.  Fire at an initial ramp rate for twice the thickness to be sure of heating thoroughly.

 When the softening point is reached, or the slump soak is complete, proceed at a rapid rate to the tack fusing temperature. To get the result you want you will need to observe.  Peek at frequent intervals. Be prepared to advance to the next segment when the gloss appears on the surface.  Your controller manual will tell you how this is done.

 

Annealing Previously Fired Items

“Double the annealing soak time for each firing” and “Slow the rate of advance each time you fire” are common responses as a diagnosis when a piece breaks in the slumping process.  It may come from the fact that once fired, It is now a single piece that needs a slower rate of advance on the second firing.  I’m not sure where the idea of doubling the annealing process originates.

You need to think about why you would slow the rate of advance and double the anneal for each subsequent firing of the piece.  This is an investigation of the proposals.

Thickness determines ramp rates and annealing

Annealing soak lengths and cooling rates are related to thickness and complexity.  If no additions or complications are added between the previous and the current firing, there is no reason to extend the soak or decrease the rate of cooling.

You of course, need to consider what lay-up and process you are using in the additional firing.  Have you added any complexity to the piece in the previous or the current firing?  If so, you do need to consider how those changes will affect the firing requirements.

Fire polishing

The question to be asked is, “if the piece was properly annealed in the first firing and shows no significant stress, why do I need to change the firing?”

The answer is, “you only need to slow the heat up because it is a single piece now.”  You do need to know that the existing stress is minimal, of course. A note on stress testing is here.  If there is little or no stress from the previous firing, the annealing and cooling can be the same as the previous firing.  Nothing has changed. You are only softening the surface to a shine.  The anneal was adequate on the first firing, and it will be on the second.

If you are firing a pot or screen melt, you have added a complexity into the firing. This is because of the high temperatures used in the first firing.  It means you may wish to be more cautious about a re-firing to eliminate bubbles, or for a fire polish for the surface.

Frit layers

If you are adding confetti or thin layers of frit or powder you have not significantly changed the piece.  You can re-fire the piece as though you are fire polishing any other piece of the same dimensions.

Additional layers

If you are adding more full layers in subsequent firings, you need to reduce the rate of advance to top temperature.  You also need to extend the soak and reduce the cooling rate according to the new thickness of the piece.  This is because the piece is thicker, so the rate of advance needs to be slower, the time required to adequately anneal is longer, and the cooling rate needs to be slower.  All of these changes in scheduling are to accommodate the additional thickness.

Tack fusing additional pieces

If you are tack fusing pieces to the top of an already fired piece, you need to go slower than you would by just adding a full layer.  Tack fusing pieces to an existing piece adds a significant complication to the firing.  Tack fusing requires a firing for thickness between 1.5 and 2.5 times the actual total height of the piece.  The complexity added is the shading of the base glass from the heat radiating from the elements. 

For example, if your piece from the melt is 9mm/0.375", it would have been annealed with a 90 minutes soak. The first cool would be at 69C/127F per hour, and the second at 125C/225F per hour with the cool to room temperature at 415C/750F. If it shows no significant stress, you can fire polish and anneal in the same way as your initial firing.

But

If you tack fuse pieces on top, then you need to treat the piece as though it were between 15mm/0.625" (a little over 1.5 the thickness) and 25mm/1.0" (a little over 2.5 times) thick.  This would require a soak of 3 or 4 hours.  A cooling rate of between 40C/72F and 15C/27F per hour for the first cooling stage is needed. The second stage between will need a rate between 72C/130F and 27C/49F per hour. The final cooling to room temperature will be between 90C/162F and 240C/432F to room temperature.

Conclusion

If you have made no significant changes in thickness or complexity, the second firing can be the same annealing as the first firing. If you have altered the thickness or complexity of the piece, the second firing will need to be slower.

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

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.

Pot Melt Contamination

Pot melting occurs at temperatures above that for which kiln washes are designed. This means the kiln wash most often sticks to the back of the melt.

If you put only fiber paper – Thinfire, Papyros, or standard 1mm or 2mm fibre paper – at the bottom, the dripping glass will tear and move it about.  It also tends to incorporate fibers from the refractory papers into the melt.  It is best to avoid fibre papers of any kind on the base.  Using fibre paper around the edges of dams, if you use them, is better than simple kiln washing of the dams.

From wikihow

If you have a sandblaster, it is easy to take the kiln wash off leaving a matt surface. You can live with this for many purposes, but if you want a more polished surface you can take the melt up to fire polishing temperature to shine up the surface. You will need to flip this over and fire again, if the original top surface is what you want to present.  Or if you like the new shiny surface, use it as is.

If you are going to cut the pot melt up for other uses, there is no need to fire polish as the surface does not matter, only the cleanliness, and removal of contaminants.


There is another thing you can do to avoid kiln wash contamination.

The best solution appears to be to put a disk or rectangle of glass on top of fibre paper. It can be clear or any colour you wish, but needs to fill the area enclosed by the dams. This seems to keep the fiber paper from tearing and being incorporated into the glass, even though the base will have the fibre paper marks.

It also works very well when you are confining the melt to get a thicker disk. Make sure you have kiln washed the sides of the container or dam very well, in addition to 3mm fibre paper arranged so that it is 3mm narrower than the expected final thickness, or any excess glass may stick to the dams. The means of arranging the fibre paper around the dams is given here. You may need to grind the marks off the edge of the disk, but this is much easier than grinding it off the bottom.

Analysis of Breaks during Fire Polishing

The analysis of breaks in fire polishing can be difficult.  The temperature and heat work are minimal, so the edges can look sharp, which would indicate that the break occurred on the cool down.

But this is where you really need to feel the edges.  If they feel very sharp, then you can be more confident that the break occurred on the cool.  But if there is even the slightest smoothness to the edge as you feel it, the break probably occurred on the heat up to fire polish.

In this picture, there appears to be an annealing break, because of the hooked ends of the break.  That is typical of a break due to inadequate annealing.  It is important to know when the break occurred, so that appropriate remedial action can be taken for future firings of similar pieces.

To determine if the annealing break occurred because the initial anneal was inadequate, it is important to do a touch test. Just looking at it will not be enough.

If the edges were even slightly smoothed, the anneal break occurred on the way up.  This would mean that the anneal of the original blank was not adequate, assuming a reasonable rate of advance was used for the thickness of the piece.

If the edges are razor sharp, the break occurred on the way down, indicating that the anneal after the fire polish was not adequate.  This would mean that in future the annealing needs to be done more carefully on fire polished pieces.

Being too quick to apply a diagnosis of a break during a fire polish can lead to the wrong conclusion, and so the incorrect alteration of future schedules.

Freeze and Fuse

"Freeze and fuse" is a term devised to describe a technique to obtain complex edge shapes and some bas relief.

The basic method is as follows, although there are a number of variations that can be successfully adopted.

Mix enough water with fine frit to make a damp slurry.

Then place about 3mm into your mould and tap on a hard surface. Tap quite vigorously to bring any air bubbles to the top and compact your powder.

Use a paper towel at this point and blot off any water that has risen to the surface.

Continue to layer, tap and blot until you're level with the top of the mould.

The more you tap and blot out any water (and every time you tap, more will rise to the top) the better your results will be.

When your paper towel won't absorb any more water, you're ready to put your mould into the freezer. One to three hours should be enough, but it must be frozen throughout.

Take the frozen glass from the mould. Letting it sit while you programme the kiln will allow it to come from the mould more easily. Place the glass form on the kiln shelf. Raise the temperature as fast as you like to 90C. Soak there for at least half an hour to remove any water in the piece. Then raise the temperature at about 100C/hour (depending on the thickness and size of the glass form) to a low fire polishing or sintering temperature (about 720C to 740C). Higher temperatures will flatten the form and change its shape. Soak at this sintering temperature for an hour or so.  Check on the progress of the firing by peeking at 10 minute intervals and advance to the next segment of the schedule when  the surface begins to shine.  

If you are planning another firing, you should not fire beyond the first hint of a shine appearing.

Some experimentation is required to get the best combination of rate, time and temperature.

Experience will show you variations on this basic method.

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

Terminology for degrees of fusing

Can anyone describe what a contour fuse is?

No one can satisfactorily describe, to a high level of acceptance, what a contour fuse is. For me it is just before a full fuse. That will not be acceptable for many, just as describing something as a rounded tack fuse is not a contour fuse for me.  A sharp-edged tack fuse is sintered glass. This will be important to observe as you move to other glass processes such as pate de verre.

There is not yet an accepted terminology and will not be as long as people choose to invent new descriptions for what are essentially the same things.

The closest you can get to a sensible range of descriptors is in the Bullseye document "heat and glass" where the temperature ranges are the important constants.

  

The fourth column of this document gives names for the process. It would be a good idea to adopt these terms, as Bullseye is the company doing the research in the area of kilnforming.

Bullseye terminology gives the following:

A slump or bend occurs in the 540C – 670C range

Fire polishing and sintering occur in the 670C – 730C range

Tack fusing (a rounding of edges) occurs in the 730C – 760C range

A rounded tack fusing that begins to sink into the base glass occurs in the lower end of the 760C – 816C range.

Contour fusing occurs in the middle of the 760C – 816C range.

Full fusing (flat) occurs at the upper portion of the 760C – 816C range.

Slumping unknown glasses

I had a recent request for help from an old friend who has taken up kiln formed glass. The problem is common enough, that (with her permission) I am adding it to the tips section.

I tried an experiment today to use some of my nice (non-fusing) glass. I cut at 270 mm diameter circle from a 3mm thick sheet and wanted to slump it into my 270 mm bowl mold. I set the mold up carefully and checked it was dead level in all directions and that the glass was absolutely centered on it. I have no idea what the COE is so decided just to use the S96 recommended slumping temperature of 650C. When I checked the kiln no more than 2 minutes after it had reached 665C, the glass had slumped almost to the bottom of the mold but it had slumped very asymmetrically. There was also a small burp on one side which has never been an issue when slumping bowls in this mold before.

The schedule I used was as follows:
200C/hour to 540C, 0 hold
650C/hour to 665C, 10 hold
Then standard S96 anneal programme

Also, the edges were still a bit rough from the cutting, i.e., they hadn‘t fire polished at all. Can you help?

Finding out about the softening characteristics of the glass

Slumping a single layer of glass with unknown characteristics – the CoE is not really relevant – requires that you watch it and other similar ones until you have established a slump temperature for the glass.

There is a way to do it:
Cut a piece of glass 305mm long by 20mm wide. Support it 25mm above the kiln shelf with the posts being 290mm apart. Put kiln furniture on top of the glass where it is supported. Make sure you can see the shelf just under the middle of the suspended glass when you are setting up this test. You can put a piece of wire or other dark element there on the floor of the kiln to help you see when the glass touches down.

Set the kiln to fire at 100C/hour to about 680C. Peek at the suspended glass every 5-10 minutes after 560C to see when the glass begins to move. Then watch more frequently. If your kiln has an alert mode on it, you could set it to ring at each 5C increase in temperature, otherwise use an alarm that has a snooze function to make sure you keep looking. When the glass touches down to the witness sitting on the shelf, record the temperature. This will approximate the slumping temperature in a simple ball curve mould.


Getting smooth edges

You need to have smooth edges before slumping. You can fire polish the piece of glass to get rounded edges, or you can cold work the edges with diamond hand pads, working from the roughest to the finest you have available. If that does not give you the edge you want, you will need to fire polish before you try to slump.

You can do at least two things to find the fire polish temperature. You can do a little experiment by using the cut off pieces of the glass and roughing them up a little before putting in the kiln. Make sure you can see it through the peep hole(s). Set the kiln to fire at about 250C to say 750C. Look in from about 700C to determine when the edges begin to round.

The other is to put a strip of the same glass in with the slump test and set the kiln to go up to 750C rather than just 680C. You can check on progress just as for the separate firing to determine the fire polish temperature. I think about 40C above slump temperature should be enough, but your test will determine that.


Avoiding uneven slumps

Most uneven slumps occur because of too fast a rate of increase in temperature. The piece can hang up on the mould sometimes causing the glass elsewhere to slide down to compensate. The real difficulty in the schedule was the 650C/hour rate up to the top temperature. This was so fast that the glass at the edges would have the opportunity to soften and so hang before the centre was soft enough to begin to bend. 150C / hour would be fast enough from 540C to achieve the slump.  In fact, 150C/hour all the way to the slumping temperature would be fast enough.  The glass reacts well to a steady input of heat rather than rapid rises, even with soaks at intervals on the way up.

Other things can be done too. You mentioned the edges were rough from the cutting. This can cause difficulties of hanging. To avoid that, you should smooth the edges before placing the glass on the mould. A further precaution against uneven slumping is to give a slight bevel to the bottom edge so that it can slip more easily along the mould.

You had already done the leveling of the mould and the centralization of the glass on the mould. These are two other things that can cause uneven slumps.


Avoiding “burps”

The glass slipping a long way down the mould is often accompanied with burps or bubble like up-wellings. These are both indications of too high a temperature being used to slump. I would begin looking at the glass from about 600C in the slumping of any unknown piece of glass. That would apply to any new configuration of the glass or mould too. The fact that the glass slid to the bottom and had a burp means that the temperature was too high and too fast. Once you have established the lowest slumping temperature, by watching to see when it begins, you then can add about 30 minutes soak to that temperature. The length of this soak will have to be determined by observation and experience, though.

A slow heating allows the glass to be at an even temperature throughout its thickness. A rapid rise with a thick piece will sometimes reveal a tear like opening on the underside of the glass that does not come through to the top. This is because the upper surface is sufficiently hot to begin slumping while the bottom is just a little too cool. If there is too great a difference, the glass just breaks all the way through.

Also slow heating allows the slump to be accomplished at a lower temperature, leading to fewer problems and to less texture being taken up from the mould.

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

Tack Fuse vs Fire polish

Are tack fuse and fire polish the same thing?

Maybe

They both occur in the same temperature same range, depending on the degree of tack fuse you want.

What you are doing in the fire polish process is heating the top surface enough to appear polished. Very little time is needed in a fire polish at top temperature as opposed to a tack fuse.

In a tack fuse, you want the bottom of the upper pieces to be hot enough to stick to the bottom layer. This requires a higher temperature or longer soak than a fire polish.

At around 730C, depending on your kiln, you will be softening the upper surface of the glass enough to give a polished appearance.  To determine whether the polished surface has been achieved, you can peek into your kiln at the chosen temperature to see if the polish is complete.

This is also the temperature at which sintering, or a lamination of the glass pieces occurs.  The edges will still be sharp, but cannot be pulled apart.  This kind of fusing needs careful annealing – long soaks and slow cools.

Tack fusing of various degrees occurs in the temperature range from 730C to 770C.  To determine which temperature and soak time will give you the result you desire will require experimentation and observation.  Generally, you can achieve the desired level of fuse with lower temperatures and longer soaks, as you can at higher temperatures and longer soaks. 

It is also possible to give a fire polish to your glass at a really low temperature, such as 550C, with a very long soak. This will avoid significantly flatening the surface of your piece.  This is the effect of heat work.

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

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.

Flattening a Bubble

Sometimes a large shallow bubble appears from under the glass.  If it has not thinned there are some things you can do. 

First – do not drill holes.

One flattening method is to place the piece on 1mm to 3mm fibre paper and fire to a slump temperature.  The fibre paper of these thicknesses will allow air out from under the glass.  With sufficient time, the bubble will flatten.  It will take some time as the weight of the bubble is slight.

Another method is to fire upside down.  It does not matter whether the bubble is central or not. This will likely take less time than the first method, but requires an additional firing.  To use this method, place the glass upside down on the shelf with an appropriate separator underneath.  Take slowly to around 620C maximum for as long as it takes to flatten. A low slumping temperature will reduce any marking that later needs to be fire polished away.

When flat and cool, clean and fire polish.

If the bubble has become large and thin, this proposed process will not work. My suggestion for these is to avoid the effort to do an unsatisfactory repair.  Instead use it for one of the many inventive process that use unsuccessful projects.

Removing Fibre Paper Marks

We all at times take short cuts or economies which lead to less than desirable outcomes.  One of these is to piece together fibre paper.  Often the marks of the join – which are always there – are just too obvious to leave.  The question becomes whether the fibre paper join marks can be eliminated.

Yes, there are at least two ways to remove these marks.  

One is to cold work the bottom with a flat lap or wet belt sander.

The other is to use the kiln to re-fire the piece.

One method would be to put fresh fibre paper or kiln wash on the shelf and fire.  This will require temperatures near the full fuse to achieve enough heat at the bottom of the glass to effect a significant change in the markings.

My suggestion for removing fibre paper marks - while it is still flat - is to fire upside down to fire polish to get rid of the marks without much changing the desired final texture of what will be the top. This is because the underside of the glass will not have the same heat effects as the top side. This also has the advantage that you can observe when the marks are eliminated.

When fired, flip over, clean the piece well to remove any fibre or kiln wash, and take to a quick fire polish to remove any marks - if necessary - caused by the upside-down firing.  This quick fire will be a slow rise to ca. 600°C, and then quickly to the 740°C to 770°C range.  This will cause the minimum change in the surface of the piece.

You will need to observe when both the evidence of the line disappears, and in a subsequent firing, when the final top surface of the piece is fire polished.

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.