Foiling and Soldering Small Pieces

There are several approaches to dealing with small pieces in copperfoiling:

No-foil approach
One approach is to have some of the pieces held in place by over-beaded solder without foil on the tiny piece, but it is patchy at best and likely to lose pieces in the long term.

Bevel approach
A very good and strong approach is to partially 'bevel' the edges of each piece on both faces. Grind at 45 degrees until the very edge is only 1 mm thick. Then use foil that is 4 mm wide for 3mm thick glass. For 4 mm glass, you will use 5.4 mm foil. Make sure that the foil covers only the bevelled edges and does not extend outside them.

Solder into the 'V' formed by the bevelled edges. Don't over-fill the joints as you don't want solder outside the 'V'. It also is best if the panel is supported underneath the area being soldered by a wet sponge to more quickly cool the solder.

With the solder contained by the 'V', the solder lines will be of constant width throughout the piece. Best to practice this technique on some scraps before you start the main job.

This approach will minimise the amount of light blocked by the foil - important with tiny pieces - while still providing the strength of fully foiled pieces.

Triming approach
If you have to have really small pieces, just foil them as you would any other piece, and burnish it as normal. Then take a very sharp craft knife (Exacto or similar) and trim the foil so that just a little tiny bit of foil is on the front and back of the piece.

No glass approach
Tiny pieces are really tedious to work with. So if the piece is going to be black or really dark, for example a small hummingbird's beak, or a bird’s eye, don't bother with glass but just fill the space with foil and solder.

Lead free Solder

There are some problems to overcome when using lead free solders. 

One is that all, except for expensive compositions, lead-free solders have a higher melting temperature than tin/lead compositions.  The table in this link shows the melting temperatures.

Most lead-free solders have a wide pasty range, so careful attention needs to be paid when selecting the composition, if you want a eutectic, or nearly so, solder.

Some eutectic solders are:

65% tin, 25% silver with a eutectic temperature of 233C.  It is known as “Alloy J” and patented by Motorolla.

99.3% tin, 0.7% copper has a eutectic temperature of 227C. It is expensive.

96.5% tin, 3.5% silver has a eutectic temperature of 221C.  This is slightly lower than the tin/copper composition but more expensive.  It is also likely to rob copper from the soldering bit, although it is easier to solder with as it has excellent wetting properties.

Lower eutectic temperature solders are available:

91% tin, 9% zinc has a eutectic temperature of 199C.  It corrodes easily and has a high level of dross.  This makes it a poor choice for copper foil work.

42% tin, 58% bismuth has a low eutectic temperature of 138C.  It is a well-established solder, but it is expensive.

48% tin, 52% indium has the lowest eutectic temperature of 118C, but it is very expensive.

Copper bearing solders

Another problem is that a solder without lead, robs copper from the soldering bit/tip, and even more so at the higher temperatures lead-free solders normally require.  One means of avoiding the rapid deterioration of the soldering bit is to use solder with a small amount of copper included in the composition. As little as 0.5% can be useful.  Normally, nothing greater than 1% is required to extend the life of the soldering bit.

Eutectic copper bearing solder

However, only one of the commonly available solders is eutectic. This is 99.3% tin and 0.7% copper with a melting temperature of 227C.

Copper bearing solders and pasty ranges

Other copper bearing solders are available. Most of them have high temperatures and wide pasty ranges making them less useful for copper foil work.

  

Near eutectic solders

97.25% tin, 2% Silver, 0.75% copper has a small pasty range of 217C – 219C, making it a nearly eutectic solder and suitable for copper foil, except for its high melting temperature.

91.8% tin, 3.2% Silver, 0.5% copper has a pasty range of 217 – 218C, also making it a near eutectic solder suitable for copper foil; again, except for its high melting temperature.  With its high silver content, the solder is expensive.

95.5% tin, 3.8% silver, 0.7% copper has a pasty range of 217-220C.  This also has a small pasty range, but may be similar in cost to the 91.8% tin composition.

95.5% tin, 4% silver, 0.5% copper has a pasty range of 217 – 225C.

95.5% tin, 4% silver, 1% copper has a smaller pasty range of 217 – 220C, but may be more expensive.

Other copper bearing solders 

94.6% tin, 4.7% silver, 1.7% copper has a wide pasty range of 217 – 244C.

96.2% tin, 2.5% silver, 0.8% copper, 0.5% antimony has a

smaller pasty range of 217 – 225C and may be slightly cheaper because of the reduced silver content.

  

95.5% tin, 4% Copper, 0.5% Silver has a pasty range of 217 – 350C and is the usual lead-free plumbing solder.  The high melting temperature of 350C makes it unsuitable for most copper foil applications.

97% tin, 0.2% silver, 2% copper, 0.8% antimony has a high melting temperature and wide pasty range of 287 – 318C., which makes it unsuitable for copper foil.  It is known as “Aquabond”. 

95.5% tin, 4% silver, 0.5% copper has a pasty range of 217 – 225C.

95.5% tin, 4% silver, 1% copper has a smaller pasty range of 217 – 220C, but may be more expensive.

94.6% tin, 4.7% silver, 1.7% copper has a wide pasty range of 217 – 244C.

96.2% tin, 2.5% silver, 0.8% copper, 0.5% antimony has a

smaller pasty range of 217 – 225C and may be slightly cheaper because of the reduced silver content.

Lower temperature copper bearing solders

94.25% tin, 2% silver, 3% bismuth, 0.75% copper has a pasty range of 205 – 217 which is smaller than many of the other copper bearing solders.

90.7% tin, 3.5% silver, 5% bismuth, 0.7% copper, with a pasty range of 198 – 213C, has a lower melting point than many other copper bearing solders.

93.4% tin, 2% silver, 4% bismuth, 0.5% copper, 0.1% germanium has a relatively small pasty range of 202 – 217C, but because of the incorporation of rare earth metals may be expensive.

Cleaning the Kiln of Dust

Dust is promoter of devitrification. You should do the most you can to keep your kiln free of dust.

Dust can come from the kiln lining materials.  Regular gentle vacuuming of the kiln surfaces will help prevent particles from falling on to you work or other surfaces in the kiln.

It can come from the separators you put in the kiln.  I often see pictures of used fibre paper at the side, or under, the kiln shelf.  This should be cleaned out after each use to provide clean firing conditions.

The main reason for this obsessive cleaning is that dust particles within the kiln will be disturbed by the air movement involved in closing or opening the kiln lid or door. There also is air circulation within the kiln during the heating and cooling phases, although it is not as much as when opening the door/lid.  These disturbed dust particles will settle on the glass and defeat your cleaning of the glass.  

Eutectic Solder

This a term for solder which becomes liquid and solid at the same temperature.  How is this possible?

An explanation is given by Wikipedia:

" … each pure component [of a homogeneous mix of materials] has its own distinct bulk lattice arrangement. It is only in this atomic/molecular ratio that the eutectic system melts as a whole, at a specific temperature (the eutectic temperature) the super-lattice releasing at once all its components into a liquid mixture. The eutectic temperature is the lowest possible melting temperature over all the [possible] mixing ratios for the involved component species.

Upon heating any other mixture ratio, and reaching the eutectic temperature, … one component's lattice will melt first, while the temperature of the mixture has to further increase for (all) the other component lattice(s) to melt. Conversely, as a non-eutectic mixture cools down, each mixture's component will solidify (form its lattice) at a distinct temperature, until all material is solid."

[https://en.wikipedia.org/wiki/Eutectic_system]

When soldering with 63/37 solder, the solder is heated above its melting (liquidus) point and so remains liquid for a short time until is reaches its solidification temperature.  The important element is that this is the lowest temperature that a mixture of materials can melt.  In the case of lead/tin solder, it 183C.  Other solders have different eutectic temperatures, e.g., a 96.3% tin and 3.7% silver solder has an eutectic point of 221C.

Slumping an Unknown Shaped Glass

A request for suggestions on how to slump found glass that had been shaped by some method was received. The request included a schedule for flattening - open side down – in a mould.

My response:

I would not attempt to do both the actions in one step. Flatten first, slump second. 

Before you start the flattening, clean it well, as any dirt trapped will be permanently imbedded.

During the slumping onto a flat surface, watch to see when it slumps during the flattening. When the form definitely begins deforming, note that temperature. The rate of advance should be moderate – no more than 150C per hour.

Observe the progress of the slumping.  When it begins to deform and change shape this will give you the slumping temperature. Record this temperature as this will be the temperature at which to conduct the slumping of the flattened form.

The temperature at which the deformation begins, minus 40C, can be taken as the middle of the annealing range. This will give you an idea of the annealing temperature as this method is not exact, but good enough to get an adequate anneal.  You can begin your annealing at this temperature without worry of it being too high.

Annealing Point and Range

A question has been asked about whether the statement that “annealing longer never hurts” is true.

To understand why this statement is not always true, you need to be aware that annealing is not just the soak at the stated annealing point.

The annealing point has a mathematical description, but in lay terms it is the temperature at which the stresses in the glass are most quickly relieved.  Annealing at this point is only possible in large industrial processes.  It is reported that float glass manufacturers can anneal glass in 15 minutes because of excellent temperature control in their lehrs.  For those of us who do batch annealing such speed and accuracy is not achievable.

As we cannot achieve such accuracy with our kilns, annealing for kiln formers consists of a temperature equalisation soak at the annealing point and then slow cooling through the lower strain point.  That is the point where the glass becomes so stiff that no further annealing is possible. 

Most kilns have relatively cool areas.  They mainly are in the corners and at the front of top hat or front-loading kilns.  You should know where these cool spots are.  They can be checked for by a simple test as described in Bullseye Technote 1.   This will enable you to know if and where any cool spots may be.  In smaller pieces, you can just avoid those areas in the placing of your pieces.

Annealing of large pieces, parts of which must be in the cool areas, is possible.  But not with excessively long anneal soaks.  If the kiln has temperature differentials, a long soak will impose those variations in temperature upon the glass. This means that the glass will begin its annealing cool with variations in temperature across the piece.

During the anneal cooling, research at Bullseye Glass Company has shown that to achieve as stress free a piece of glass as possible, the temperature variation across and through the piece should not vary more than 5°C. This is relatively difficult to achieve if you have cool areas in your kiln.  But it is possible.

To alleviate the possible difficulties of temperature variations in the kiln, the anneal soak should not be extended beyond that recommended by its thickness.  What should be extended is the anneal cool. The rate of cooling should be slowed to the rate for a piece at least twice the thickness of the current piece.

If it is a tack fused piece, this reduction should be for a piece four times the thickness of the thickest piece you are annealing.

The conclusion is that it is possible to anneal too long, if the piece is large and the heat in the kiln is not uniform. If you are concerned, remember that the soak at annealing point is to equalise the temperature throughout the substance of the piece. The annealing cool - the first 110 degrees Celsius - is very important. If you are concerned, it is best slow that rate of decrease dramatically. This provides a safer option for an adequate annealing of large pieces.

Flat Kiln Shelves

A question has been asked about using tiles in addition to standard kiln shelves to fire glass upon.  Yes, you can use the unglazed backs to fire on, assuming they are not ridged or in other ways not a regular surface.

It is important to have flat shelves, as ones with even small shallow depressions can promote bubbles at higher temperatures. Tiles for walls and floors do not need to be flat to do their intended job and so are not checked for be flatness.

A magnified view of a shelf surface that is not perfectly even

You can do a quick check for flatness, by placing a ruler on edge across the tile or shelf to see if any light comes through underneath the ruler.  The light areas are the places where the surface is lower than the rest.  If these are few and small you can make corrections in the surface of the tile by grinding.

You can make sure they are flat by putting two tiles back to back and grinding them together. The initial grind will show you the high spots as they will have the grinding marks there. 

You can eliminate these higher areas by rubbing the tiles together with a coarse grit (ca. 80) between the tiles to speed the grinding. If you are concerned about the dust or don’t have good ventilation, you can make a slurry of the grit by adding water. When the whole surface has the same marks, both will be flat. To double check, scribble with a paint marker over one and let it dry.  Then add grit between to grind again. When all the paint marks have come off they are both flat on the back.

This sounds time consuming and lots of effort, but you will be surprised at how quickly you can achieve flat smooth surfaces even on larger tiles.  This also works for larger kiln shelves.

Preventing Devitrification on Cut Edges

“Question-when cutting up a Screen Melt, using a tile saw. How do you NOT get devitrification when laying the slices cut sides up?”

Devitrification occurs where there are differences in the surface.  This means that the surfaces exposed to the heat must be both clean and smooth.  It is not enough for only one of these to be the case, both are required.

First, the sawn edges need to be clean.  A good scrub with a stiff bristle brush is essential.

Second, devitrification sprays of whatever kind do not seem good enough to prevent the devitrification. This is probably due to the thin covering of the differences (scratches, pits, etc.) on the surface.

Beyond that, I know of two ways to prevent or reduce devitrification. That is, providing a smooth surface to resist devitrification.

1 – Grind

This can be done with hand pads, grit slurry or machines such as a Dremel with damp sanding pads or belts, wet belt sanders, or a flat lap.  The grinding should go down to at least 400 grit before cleaning and arranging to fire.

2 – Clear glass

This method relies on putting a layer of clear glass that is less likely to devitrify than the cut edges over the whole surface.  You could use a sheet of glass, although that would promote a multitude of bubbles due to the spaces between the strips and the naturally uneven heights of the strips.

Placing a layer of fine frit on top of the arranged pieces before firing is a way of allowing air out and forming a smooth upper layer by filling the gaps. It is best to avoid powder, as this promotes a multitude of fine bubbles, giving a grey appearance. The layer you apply needs to be an even layer and at least 1mm thick. If you are concerned at getting lots of bubbles, you could use medium frit instead.  In this case, the layer will need to be thicker than 1m to get an even coverage. The whole of the surface of the piece needs to disappear under the layer of frit, and that may be a good guide to the thickness of frit to apply.

Composition of Glass

Glass can do most anything. From bottles to spacecraft windows, glass products include three types of materials:

• Formers are the basic ingredients. Any chemical compound that can be melted and cooled into a glass is a former. (With enough heat, 100% of the earth's crust could be made into glass.)
• Fluxes help formers to melt at lower temperatures.
• Stabilisers combine with formers and fluxes to keep the finished glass from dissolving, crumbling, or falling apart.

Chemical composition determines what a glass can do. There are many thousands of glass compositions and new ones are being developed every day.

Formers
Most commercial glass is made with sand that contains the most common former, Silica. Other formers include:

• Anhydrous Boric Acid
• Anhydrous Phosphoric Acid

Fluxes
But melting sand by itself is too expensive because of the high temperatures required (about 1850°C, or 3360°F). So fluxes are required. Fluxes let the former melt more readily and at lower temperatures (1300°C, or 2370°F). These include:

• Soda Ash
• Potash
• Lithium Carbonate

Stabilisers
Fluxes also make the glass chemically unstable, liable to dissolve in water or form unwanted crystals. So stabilizers need to be added. Stabilisers are added to make the glass uniform and keep its special structure intact. These include:

• Limestone
• Litharge
• Alumina
• Magnesia
• Barium Carbonate
• Strontium Carbonate
• Zinc Oxide
• Zirconia

Based on an article from the Corning Museum of Glass

Float Glass

A reported 90% of the world's flat glass is produced by the float glass process invented in the 1950's by Sir Alastair Pilkington of Pilkington Glass. Molten glass is “floated” onto one end of a molten tin bath. The glass is supported by the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in a lehr. The finished product has near-perfect parallel surfaces.

An important characteristic of the glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch.

Float glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.

More information on float glass in the kiln is here.

Figure Rolled Glass

The elaborate patterns found on figure rolled glass are produced by in a similar fashion to the rolled plate glass process except that the plate is cast between two moving rollers. The pattern is impressed upon the sheet by a printing roller which is brought down upon the glass as it leaves the main rolls while still soft. This glass shows a pattern in high relief. The glass is then annealed in a lehr.

Rolled Plate Glass

The glass is taken from the furnace in large iron ladles and poured on the cast-iron bed of a rolling-table. It is rolled into sheet by an iron roller. The rolled sheet is roughly trimmed while hot and soft and is pushed into the open mouth of a lehr, down which it is carried by a system of rollers.  The method is similar to table glass, except in size and thickness.

Table Glass

This glass was produced by pouring the molten glass onto a metal table and sometimes rolling it. The glass thus produced was heavily textured by the reaction of the glass with the cold metal. Glass of this appearance is commercially produced and widely used today, under the name of cathedral glass, although it was not the type of glass favoured for stained glass in ancient cathedrals. It has been much used for lead lighting in churches in the 20th century.

Modern example of rolling glass. The operator is waiting to take the rolled sheet off the table

Broad Sheet Glass

Broad sheet is a type of hand-blown glass. It is made by blowing molten glass into an elongated balloon shape with a blowpipe. Then, while the glass is still hot, the ends are cut off and the resulting cylinder is split with shears and flattened on an iron plate. (This is the forerunner of the cylinder process). The quality of broad sheet glass is not good, with many imperfections. Due to the relatively small sizes blown, broad sheet was typically made into leadlights.

According to the website of the London Crown Glass Company, broad sheet glass was first made in the UK in Sussex in 1226 C.E. This glass was of poor quality and fairly opaque. Manufacture slowly decreased and ceased by the early 16th Century. French glass makers and others were making broad sheet glass earlier than this.

Drawn Sheet Glass

Drawn sheet glass -sometimes called window glass or drawn glass – is made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardens just out of the vat. This film or ribbon is pulled up continuously and held by tractors on both edges while it cools. After 12 meters (40 feet) or so it is cut off the vertical ribbon and tipped down to be further cut.

This glass has thickness variations due to small temperature variations as it hardens. These variations cause slight distortions. You may still see this glass in older houses.

In more recent times, float glass replaced this process.

Flashed Glass

Red pot metal glass is often undesirably dark in colour and very expensive. The method developed to produce red glass was called flashing. In this procedure, a semi-molten gather of coloured glass was dipped into a pot of clear glass. As the bubble became enlarged, the red glass formed a thin coating on the inside. The formed glass was cut, flattened and annealed as any other blown sheet.

There were a number of advantages to this technique. It allowed a variety in the depth of red – and other deep colours - ranging from very dark and almost opaque, and sometimes merely tinted. The other advantage was that the colour of double-layered glass could be engraved, abraded, or etched to show colourless glass underneath. 

Other base colours are also used in making flashed glass, for example red flashed onto a pale green base.  Also see this post on finding the flashed side of glass.

There still exist a number of glass factories, notably in Germany, USA, England, France, Poland and Russia which continue to produce high quality glass by traditional methods primarily for the restoration of ancient windows.

Cylinder Glass

Cylinder blown sheet is a type of hand-blown window glass. Large cylinders are produced by swinging the cylinder in a trench or blown into a cylindrical iron mould. The glass is then allowed to cool before the cylinder is cut. The glass is then re-heated and flattened. The result is much larger panes and improved surface quality over broad sheet.

Trench method

Cylinder blown sheet glass has been manufactured in France, Germany and Poland since the 18th Century, and continues today. It began to be manufactured in the UK in the mid 19th Century, although the only small remaining company has ceased manufacturing in the late 2010's.

Mould method

Machine drawn cylinder sheet was the first mechanical method for "drawing" window glass. Cylinders of glass 12 m (40 feet) high are drawn vertically from a circular tank. The glass is then annealed and cut into 2 to 3 m (7 to 10 foot) cylinders. These are cut lengthways, reheated, and flattened. This process was invented in the USA in 1903. This type of glass was manufactured in the early 20th century (it was manufactured in the UK by Pilkington from 1910 to 1933).

Crown Glass

Crown glass: The earliest style of glass window

The earliest method of glass window manufacture was the crown glass method. Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool. Centrifugal force forced the hot globe of glass into a round, flat sheet. The sheet would then be broken off the pipe and cut into small sheets.  

This glass could be made coloured and used for stained glass windows, but is typically associated with small paned windows of 16th and 17th century houses. The concentric, curving ripples are characteristic of this process.

At the center of a piece of crown glass, a thick remnant of the original blown bottle neck would remain. They are known as bull's eyes and are feature of late 19th century domestic lead lighting and are sometimes used with cathedral glass or quarry glass in church windows of that date. Optical distortions produced by the bullseye could be reduced by grinding the glass. The development of diamond pane windows was in part due to the fact that three regular diaper shaped panes could be conveniently cut from a piece of crown glass, with minimum waste and with minimum distortion.

This method for manufacturing flat glass panels was very expensive and could not be used to make large panes. It was replaced in the 19th century by the cylinder, sheet and rolled plate processes, but it is still used in traditional construction and restoration.

Types of Glass

Glass Types by manufacturing method

There are several ways of categorising glass and this overview of glass types looks at the way the glass is manufactured.

Crown Glass

Crown glass is the oldest method of producing sheet glass and continued to be used until the 19^th century.  This method consisted of blowing a very large bubble of glass.  It was then spun rapidly over a pit until the bubble collapsed into a disc that ranged from 1500mm to 1800mm diameter.  

This gave the thinnest and least marked glass at the outer portion of the disc.  The centre was the thickest and became known as the bullseye.  The glass was cut to provide the best use of the disc.  This limited the size of panes to what could be cut from the disc.  Diamond shapes were often cut from the remainder and the central bullseye was used in less expensive glazing.

Corning Museum of Glass

Cylinder Glass

Cylinder Glass is a handmade process that includes broad sheet glass. It was widely used from the 17^th to the 19^th century, and now is limited to a few manufacturers.  

"Among the Glass Workers" Harry Fenn, 1871

An elongated bubble was blown.  The top and bottom of the bubble are broken off and annealed.  Later the cylinder is placed in the lehr for reheating.  It is scored and when it breaks open along the score, the glass is flattened. Characteristically, it has a gradation of thickness with thicker edges where the top and bottom of the cylinder were cut off.

From IdoStuff

Flashed Glass

A development in cylinder glass was to make the bubble of two colours, with the dark colour gathered first and then encased in clear (or sometimes other pale colours) and blown into a cylinder.  This made dense colours more transparent and enabled more detail through abrading and etching.

Drawn Glass

Industrialisation of glass production began with the development of drawn glass.  This method of mass production of window glass was invented and developed by Emile Fourcault in Belgium. Full scale production began in the early 1900’s.  

The glass is drawn upwards from a vat of molten glass until it cools enough to be cut into sheets at the top of the tower.  The process is subject to slight variations in thickness due to uneven cooling and gravity. It enabled much larger panes of glass without the astragals that are common in Georgian and later houses.  It was the most common method of producing window glass until the 1950’s.

Table Glass
Table glass is the process of putting molten glass onto a flat surface (the table) and rolling the glass flat.  This has been used from the latter part of the 19^th century to the present.  It enables textures to be pressed into the glass from the rolling cylinder.  It is easier to produce streaky and wispy glass by combining different colours on the table. 

Kokomo Glass Co.

This can be done as single sheets or further mechanised to roll out long ribbons of glass.  This is now mostly referred to as machine or hand rolled glass depending on the amount of mechanisation.

Float Glass

The glass that we now rely on for large clear windows began with the development of experiments by Alastair Pilkington and the company named after him.  This consisted of floating near molten glass on molten tin, hence the name, float glass.  This has been the standard method of glass for windows since the 1950’s.

Annealing Large Pieces

A question was asked about how long to anneal a large piece in relation to smaller pieces.

“Large” is in relation to the size of your kiln.  A large piece for a 300mm square kiln would be something 250mm square.  For a kiln of 600mm square, 250mm would be a small piece.  It would contain a large piece of 500mm square as a large piece. 

Large also relates to the distance from the edge of the kiln.  Although some kilns have much more even heat than others, all have areas that are relatively cooler than others.  It is important to know where those are, so that you can avoid those cool areas, by placing pieces to avoid those spots or by altering the rate of cooling.  Bullseye has a tip on determining the relatively hot and cool temperatures are in your kiln. 

In a rectangular kiln, there are usually cool spots in the corners.  Front opening kilns often have cooler areas at the front of the kiln.  Knowing where these are will give you the information to know the area of the kiln that has even heat.   This area tells you what the size of a large piece for your kiln is.

You can alleviate many of the differences in temperature in your kiln by remembering that annealing is not simply a given temperature.  It is a range. 

The popular perception is that the soak at the annealing temperature is all that needs to be done to anneal.  The soak at the annealing point equalises the temperature throughout the glass. But it does not complete the annealing. That continues through the gradual cooling of the glass down the next 110°C.

Simply soaking longer at the annealing point, in the circumstances where the temperature in not equal all over the glass, “locks” the stresses of uneven temperatures into the glass.  Instead, a gradual, slower than usual annealing cool is required.

Of course, the rate of cooling is relative to the thickness of the piece and the degree of temperature variation in your kiln.  If you must utilise the area of the kiln with slightly cooler temperatures, the minimum requirement would be to use a cooling rate for a piece at least two times thicker than the thickness of the one you are annealing at present.


But, to answer the original question - how long to anneal a large piece in relation to a small one of the same thickness?   Given the precautions above, the size of the piece is not the major determining factor.  The thickness of the piece is the important dimension when considering annealing.

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.

Glass Stuck to Element

First consideration you need to think about when you discover glass stuck to an element is the nature of the metal of the elements.  Once fired, kiln elements become brittle.  This means that they are likely to break if disturbed when cold.  So, you need to make sure you absolutely must do something to rescue the kiln.  It may be that you can just leave the stuck glass alone.  Where the glass is, and how much of it, is stuck to the elements is important when considering what to do.

Where

This brittleness of the elements means that the location of the glass in relation to your firings needs to be considered.  If the glass is on an element below your normal firing position, you can think about just leaving it.  This applies to glass stuck to the side elements too, unless you are in the habit of firing very close to the side elements. The heating elements of the kiln form an external layer of oxidisation that protects the inner metal.  This means that small amounts of glass will not affect the operation of the elements, nor your future pieces.

If the glass is stuck to top elements, you are likely to be more concerned about future drips of the glass onto your future work.  The glass is not likely to become hot enough to detach or drip onto your work except at extended full fuse or casting temperatures.  This means that you can observe the progress of any possible drip at each firing and only remove the glass when it begins to begin to hang down from the element.

How Bad

How much glass is stuck to the element?  Normally, if it is only a small amount, it can be left.  Ceramics kilns often have a bit of glaze (a glass carrier of the colour) stuck to the elements and continue to be fired for years without damage.

If there is a lot of glass stuck to the elements you will need to remove most of it to avoid dripping onto future work. 

Methods of Removing

In most cases where there are significant amounts of glass stuck to the element, it is on the brick or fiber lining of the kiln too. 

My recommendation is to heat the glass just a few centimetres from where it is attached to the element. Use a hand-held blow torch to do this. When the glass is red hot - enough to begin moving - you can pull it away between the lining and the element with long handled tweezers.  Do not attempt to pull it off the element right away.  You can later chip the glass off the lining without damaging the element as the connection is separated.

As the element has begun to be warmed by the heat used to separate the glass on the lining and the element, you can continue to warm the element, moving the torch in a slow waving motion at least 10cm each side of the stuck glass.  When the glass and element are red hot, you can begin to pull the glass off with long handled tweezers, bit by bit.  Keep re-heating the element and glass as much as necessary so the temperature does not drop below cherry red.  This ensures the elements continue to be flexible and will not break.

Of course, glass can be melted onto its kiln furniture and there are different considerations for those circumstances.