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.