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.

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.