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. 

Slumping and Annealing bottles

"Can a tack fuse schedule for fusing glass can be used to slump bottles?"

It may be that this person does not have the confidence to write a new schedule.  They may wish to use an existing schedule for another purpose. The short answer is “Although a Bullseye or Oceanside tack fuse temperature will be high enough to slump bottles, they are not suitable for annealing”.  There are reasons for this. 

The softening point of float glass, which is similar to bottle glass, is 720ºC/1330ºF.  Slumping would normally be done at about 20ºC/36ºF above this. You also need a slumping hold at this temperature much longer than a tack fuse schedule would use.

if you use a tack fuse schedule for a fusing glass, your annealing will be inadequate. Bottle and float glass tend to have an annealing point of around 540ºC/1005ºF. An annealing for fusing glass will be between 515ºC/960ºF and 482ºC/900ºF.  This is likely to be too low an annealing point for bottles.  Also, the annealing soak is likely to be too short. Slumped bottles are very thick at the base where it folds over the cylinder of the bottle.  This requires a longer anneal soak and slower cool than a schedule for a tack fuse of fusing glass.

Checking for stress in the completed work is normal.  It is essential for your finished bottle if you use a tack fuse to fire it.

 

Schedules should be devised for the glass and layup of each piece. Transferring a schedule for fusing to bottle glass is unlikely to be successful.

Bubbles in Bottle Slumps

 Any suggestions on how to avoid getting the oblong bubble under the neck of the bottle? This was my first try and I’m really happy with clarity, no devitrification in these.

I used this schedule:

Fahrenheit                    Celsius

300/1150/30                167/620/30

200/1370/20                111/740/20

400/1450/20                222/787/20

AFAP/950/60                AFAP/510/60

150/800/0                    63/427/0

300/100                       167/55/off

The bubble is kind of cool but not sure what it will do when I put it in a bottle mould.

 

To minimise the bubble, you need a bubble squeeze.  There isn't one of sufficient length or at the right temperature in the schedule. The softening point of bottle glass is approximately 720C. Starting the bubble squeeze at ca. 670C/1240F and progressing slowly (ca.50/90F or less) to 720C/1340F may give a better bubble squeeze. 

Also, the anneal soak is a bit low. Bottle glass and float glass both have annealing points of about 550C. You might make use of a lower annealing soak temperature to reduce the cooling time.  It is usually possible to anneal 30C below the published annealing temperature.  In this case that would be 520C.  

There is pretty thick glass in some places due to the way the bottom and neck of the bottle form. You may want to extend your anneal soak to one for 12mm/0.5”.  The soak time for this is 2 hours.  The first cooling segment would be 55C/100F per hour to 475C/888F if you use 520C/970F as the annealing soak.  The second cool segment should be at 99C/180F per hour to 420C/790F.  And the final rate at 330C/600F to room temperature.  It is important to include all three stages of cooling.  The research for my book Low Temperature Kilnforming (Or directly from stephen.richard43@gmail.com) has shown that to get the best stress-free results  use all three stages of cooling.

Bubbles at the shoulder of the bottle are common.  The change in circumference of the bottle at the shoulder means there is a greater amount of glass to “compress”.  Bottles with tapered circumference at the top of the bottle have fewer problems with creating bubbles.  The abrupt change in size at the shoulder causes bubbles to be more common.  A long slow bubble squeeze will allow the shoulder to form more closely in line with the neck. 

There are other things you can do to help avoid the bubbles. One thing is to insert a thin kiln washed wire into the neck of the bottle. This gives a path for the air to escape and allows you to pull it out, although a mark will be left.  You could also think of drilling a hole in what will be the underside at the shoulder to allow air out to the shelf. It does not need to be a big hole.

Bubbles at the shoulder of a slumped bottle are a common problem. It results from the greater amount of glass that has to slump into the space.  This leaves a cavity.  Slower bubble squeezes can help, as well as various venting methods.

Keeping Bottles from Rolling

A common problem in firing bottles is that they may roll into one another and stick, making both bottles useless.

One way to overcome this is to let the bottle find its heavy point by setting on smooth and level surface. It will gently roll to one direction before slowly coming back in the other. When it stops this oscillation, the heaviest part of the bottle will be on the bottom.  Mark the bottle in some way so you can move to the kiln in that position. If after this, it rolls in the kiln, then your shelf is not level. 

Additional assurance against rolling is putting a small piece of thin fibre paper (1 or 2 mm)at each side of the point the bottle touches the shelf.  Thinfire and Papyros are not enough to ensure there will be no movement. But the small bumps of fibre paper are enough to stop the bottle from rolling.

Sometimes you want a particular part of the bottle up or down, but it won’t stay in place.  Then you need to put a slightly thicker piece of fiber paper against the bottle on each side.  It is better if it is not Thinfire or Papyros as they tend to disintegrate above 400C, long before the bottle begins to distort enough to keep it in place.

Other materials you can use to prevent the bottle from rolling are crumbled chalk, whiting, kiln wash, or even a few grains of sand.

Preventing bottles from rolling in the kiln is about finding the natural heavy spot, or propping the bottle in place with a variety of heat resistant materials.

Needling in Bottle Moulds

Sometimes people experience sharp, needle-like points on the bottle after it is slumped.

Causes

As the bottle expands and softens, it conforms to the surface of the mould.  When the cooling begins, some parts of the glass are trapped in the tiny pits of unevenness that always exist in the mould or in the separator.  As the glass retreats, the glass is stretched until it breaks off, leaving the sharp needles.

Prevention

Remedies relate to separators and temperatures.  This of course, assumes you already have a good coating of kiln wash or similar separator on the mould.

Separators

These additional separators can be fibre paper or powders.  Thinfire laid on the bottom of a bottle mould can provide additional separation between the bottle and the glass.  This works, because with a slow rate of advance, the Thinfire will have turned to powder before the bottle begins to slump. This powder will not interfere with any designs on the mould.  Papyros will work on smooth moulds, but not so well with textured bottle moulds, because of its more fibrous nature.

This use of powered paper indicates that you could use a cheaper solution.  Just dust a fine film of kiln wash on the mould.  I do this by placing the powdered kiln wash in a sock and shake the sock above the mould.  This will allow an almost invisible layer of fine powder to fall onto the mould.  This is enough to provide an additional layer of separation between the glass and the mould.

Temperature

It is quite common for people to slump bottles at tack fusing temperatures to do both the flattening and the slumping at one firing. This is quite hard on the mould and softens the glass enough to promote the needling. 

It may be better to use two firings – one to flatten using tack fusing temperatures, and one to form the bottle at slumping temperature.  This lower temperature will avoid the needling, as the bottle will not soften enough to form the needles during the slumping. The reason many people avoid this is because the bottles tend to devitrify on second firings.  If you do this two-stage slumping, you will need to apply a devitrification solution to the upper surface of the flat bottle to try to prevent it.

You can take a different solution to the two-stage firing.  As lower temperatures reduce the possibility of needling, you can simply soak for a longer time at the slumping temperature than a normal one stage tack and slump.  You will need to peek in at intervals to determine when the slump is finished, of course.  After a few firings though, you will get a good idea of the amount of time required to complete the slump. An additional advantage is that at the lower temperatures, devitrification is less likely.

Slumping Glass that is not Tested Compatible

Is it Possible?

It is possible to slump unknown glass. This glass might be art glass, window glass, bottles, or any other glass whose characteristics are unknown by you.  There are some suggestions about the characteristics of some glasses in this post that can be used as a starting point.

Preparation of the Glass

Prepare the edges to their final finish before slumping.  This because the slumping temperature will not be enough to alter the finish of the edge significantly.  This preparation can be done with diamond hand pads, or wet and dry sandpapers.  Start with a relatively coarse grit. You may wish to do the initial shaping on your grinder. This will be between 80 and 100 grit.  Continuing with a 200 grit and working your way through 400 and then 600 grit will give you an edge that will become shiny during the slumping.

Cleaning

Clean thoroughly.  This is especially important when using glass that is not formulated for fusing.  Devitrification is more likely on these glasses.  Water with a drop of dishwashing liquid can be enough unless your water has high mineral content.  Then distilled water or a purpose made glass cleaner such as Bohle or Spartan should be substituted.  Finish with a polish to dry with clean paper towels. More here. 

Firing the Slump

Fire up slowly.  You should advance at about 100°C to 150°C per hour.  Set your top temperature around 630°C for a simple slump, for soda lime stained glass.  For bottle or window glass you will need a temperature closer to 720°C although the also are soda lime glasses.

It is best to start with simple curves, as there are fewer difficulties in determining what the glass is doing.  It will help you to learn the characteristics of the glass before you tackle the difficult stuff, such as compound curves or texture moulds.

Observation

It is necessary to observe the progress of the slump as you do not yet know the slumping temperature.  You want to know when the glass begins to deform so that you do not over fire.  Start watching the glass at about 10 minute intervals from about 580°C for stained glass and 680°C for window and bottle glass.  There is not much light in the kiln at these temperatures, so an external light is useful.  You can also observe the reflections of the elements on the glass.  When the image of the elements begins to curve, you know the glass is beginning to bend.

Altering the Schedule

Soak for at least 30 mins at the temperature when the glass begins to visibly drop. This may or may not be long enough.  Continue checking at 5-10 minute intervals to know when the slump is complete.  If the glass is completely slumped before the soak time is finished, advance to the next segment.  If not fully slumped, you need to extend the soak time. This means that you need to know how to alter your schedule in your controller while firing.  Consult your controller manual to learn how to do these things.

Stop the soak when complete and advance to the anneal. Continue the slumping soak if not complete after the 30 mins.  In some cases, you may need to also increase the temperature by 5-10°C.

Annealing

The annealing point will be about 40°C below the point that the glass visibly starts the slump. If you want a more accurate determination of the annealing point, this post gives information on how to conduct a test to give you both the slump temperature and the annealing point.  It also helps to determine the lower part of the tack fusing range (the lamination state), since it is not far above the slumping point that you will observe.

The annealing soak for a single layer, 3mm glass need not be long – 15 to 30 minutes.  The annealing cool can be as fast as 120°C down to 370°C.  For thicker glass and slumped bottle glass you will need a longer soak – 30 to 60 minutes – and a slower cool.  The annealing cool in this case could be about 60°C/hour to 370°C.  You can turn the kiln off at 370°C, if you wish, or keep the temperature controlled to about 50°C.  The rate for the final cooling can be approximately double the first cooling rate.  For a single layer of stained glass this could be 240°C, and for thicker glass about 120°C

Moulds for Bottles

One of the many styles of commercially prepared moulds

An alternative to buying moulds for slumping bottles is to use a sand bed. You can place the bottle into the sand and roll it a little from side to side to create a depression in the sand that then becomes the mould.

I use a fine sand (not builder's or garden) and coat it with alumina hydrate (slaked alumina). I use about 1 part alumina to 5 sand, but the mix is not critical, just enough alumina to coat the sand particles. It can go directly on your kiln floor if you have an easy way to pick it back up, as it is re-usable. Or you can put it in a stainless steel tray or any open topped box that will withstand the temperature.



Make your depressions and then sprinkle or sift a fine layer of alumina over the area - I use an old sock to hold some and dust it over the sand. Then lay the bottle in the depression.

The amount of sand impression you get is dependent on the temperature you use - the higher, the more sand texture you get. 





This way of slumping bottles eliminates the need for a mould and it is variable for different sized bottles.

Bottle slumping

Results

Devitrification is a major problem on bottle slumping. Some means of reducing the problem are

• clean labels off thoroughly
• remove all adhesives
• scrub the whole bottle and polish dry
• apply a devitrification spray before firing.
• Spend as little time in the devitrification range (700C – 750C) as possible
• Vent the kiln up to 540C if you are using fibre papers
• sandblast off the devitrification and fire again if the devit is slight
• in severe cases of devitrification, throw the bottle away.

Champagne and large bottles of sparkling wine split easily due to the differences in thicknesses. Firing of these needs to be very slow until you get past 715C.

Bottles with printed labels need particular care. The enamels used in the printing may contain lead and any other elements in the paint may contaminate your kiln.

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Bottle Slumping

Firing

The amount of slump you want will determine the top temperature combined with the rate of advance.

You have the choice of firing slow and low or fast and high. If you choose the former, you can also choose a lower final temperature. As a starting point for considering your firing schedule you can assume that bottle glass is very similar to float glass. This means that you can start with an annealing temperature of about 548C and a softening point of 720C. The strain point is around 510C, so all the annealing needs to be finished by that temperature.

Bottle glass is fairly robust, so an initial rise of 150C/hr to 600C may be slow enough, considering the differences in thickness that most bottles exhibit and then faster to your top temperature. A little experimentation is required. The minimum temperature required for a slump will be around 720C. The slumping temperature you choose depends on how fast you want to achieve your slump and how flat you want the result to be. Slow slumps can be done at around 720C, but if you want faster or flatter you need to consider temperatures around 770 -790C. You will need to observe to determine what temperatures are best for your desired results.

Annealing needs to be done carefully for two reasons. You do not know how consistently the glass has been made and you have a range of thicknesses involved in the now slumped piece. So it is safest to assume the piece is now about 20 mm thick. This would require a 3 hours soak at 550C; an initial annealing cool rate of 25 C/hr to 495C; a secondary cool rate of 45C/hr to 440; and a final cooling rate of 150C/hr to around 50C. If the bottles are clear or light enough, you should do a stress test on them by placing the bottle between polarised filters and over a light source. This will tell you if you have annealed properly.

You also need to consider whether you can fire bottles from different brands of drink and different parts of the world together. Some say don't try it. Others say there is so little variation in glass the world over that careful annealing will compensate for the differences. Experimentation is the only way that you will be able to tell.

Allow a day after firing before any cleaning or washing of the bottles is done.

Bottle Slumping

Placing

Production made bottles are not evenly thick all the way around. So they roll until the heaviest part is at the bottom.

One way to avoid any rolling in the kiln is to put the bottle on a level surface and let it roll to the heavy side. Mark the up side so that you can place it in the kiln with the heavy side down.

Some times though the seam lines – which will show in the final piece – are in the wrong place. To keep the seam lines to the side, you can place a small piece of 0.5 mm fibre paper against each side of the bottle at the base if you are working on a hard surface.  On a softer surface, the weight of the bottle or a gentle push will sink it into the surface enough to avoid rolling.

The distance to place bottles apart is important to know so they do not become attached when several are slumped together.

You can slump test bottles of various diameters to determine their final width.

However, if you wish to calculate the distances, The width of slumped bottles is approximately 1.6 times the diameter of the original bottle. If you want the base of the bottle to be flat too, the final size will be wider, dependent on the amount of glass in the base.

You also need to leave some additional space than this calculated distance between bottles, as the final size is narrower than the size when hot. You should leave at least an additional 10 mm to each side of the bottle.

So a 50 mm diameter bottle will become 80mm wide. You need an additional 10 mm each side, so the spacing from the centre of one bottle to the next should be at least 100 mm.

The surface that you are slumping onto is important too.

You can place the bottles on a sand base that has been dusted with kiln wash powder. This has the advantage of allowing a gentle push into the sand to prevent rolling. But it imparts a grainy texture to the back.

You can get a smoother surface by using whiting or sifted plaster that has been screeded smooth. This also allows the bottle to be gently pressed into the surface.

You can use fibre papers as separators from the shelf, but they are relatively expensive.

Kiln wash works very well. It can provide a very smooth surface, but if you want more texture, you can sprinkle some kiln wash through a sieve over the shelf.

Bottle Slumping

There are four major considerations in bottle slumping: cleaning, placing, firing, results.

Cleaning

A major consideration in slumping bottles is the cleaning required. This requires a lot of time, as everything has to be clean inside and out.

Labels and the glue attaching them must be cleaned off. The interior needs to be clean. And the bottle needs to be dry before being placed in the kiln.

Soaking first helps the cleaning process. If you are doing large numbers you will need to find large containers that you can leave the bottles in to soak for a couple of days. For small numbers, a few bottles soaking in a bucket of soapy water will do. The bottles should be upright to allow the internal residues to float to the top. After a few days the labels should have fallen off and the internal deposits floated to the top or easily washed out. Screen or plug the sink to be able to remove the residues from the sink before it gets into the drain. Then you can proceed to rinse and clean the bottles.

The bottles will need to be wiped free of the glues used to stick the labels to the bottles.  Any glue residue left on the bottle will show up on the finished piece, usually as devitrification. If the glue or label has not come off with the cold water and soap soak, you can soak them in very hot water. You can then use a variety of solvents to remove the most persistent adhesives.

For the difficult internal deposits, you can use one or two short lengths of small chain and slosh that around with water. The chain used for hanging small stained glass panels is ideal.

As you can see the cleaning process is lengthy and can be time consuming. So you might want to see if you can get new bottles at a reasonable price. Home brew shops may have bottles they are willing to dispose of. Bottling plants may also have supplies of bottles. New bottles will greatly reduce the labour of bottle slumping, although it does not fit the re-cycling ethos that brings people to bottle slumping in the first place.

Cutting Bottles

Cutting bottles seems to have a fascination for many people. There seem to be three methods – heat and cold, scoring, sawing.

There are various ways to apply heat and cold to assist with breaking the bottles.

- A string tied around the bottle and soaked in a flammable liquid is a common way to apply heat. As soon as the flame has gone out, you immerse the bottle in cold water; the temperature differential should crack the glass where the string was.

- Filling the bottle with water to the level where the break is wanted and then applying gentle heat with a torch flame at that level should promote a crack.

- Alternatively, the bottle can be scored and put into the freezer for a while and then into hot water.

Scoring is the common method to start a crack.

- This is followed by tapping from inside the bottle with tools from a purchased kit or home-made tappers – a metal ball on the end of a curved piece of metal.

- The score line can also be the preliminary step in the application of heat or cold.

These provide the cleanest edges to the cuts. However there is quite a high failure rate using these methods.

Sawing is method that provides a higher success rate, but is wet, and leaves rough edges to the cut, requiring further cold work.

- Band saws designed for glass can be used, but usually do not have a high enough throat to allow the thickness of the bottle to pass through.

- Most tile saws cut from underneath, so rotating the bottle can lead to a cut completely around. This requires a lot of skill to do free hand, so you need a jig to keep the bottle at right angles to the blade and the bottom the same distance from the blade while rotating the bottle all the way around.