Devitrification Temperature Range

Devitrification is the beginning of crystallisation of the surface of the glass. It can look like a dirty film over the whole piece or dirty patches. At its worst, the corners begin to turn up and become “wrinkly”.

This piece shows both mild devitrification and more severe wrinkling on the right side.

This occurs in the range 730° – 760°C. This means that you need to cool the project quickly as possible from the working (or top) temperature to the annealing point. There is evidence to show that dwelling for a long time in this range on the way up to top temperature can promote devitrification too.

The lower graph line shows the  temperature relationships between annealing (glass transition), devitrification and blowing temperatures.

For more information see this post.

Specific Gravity, CoLE, and Colourants of Glass

I’ve been asked the question “is there is differential in specific gravity as related to COE or colorant used in the glass (white opal v clear)”? 

Using the typical compositions of soda lime glass (the stuff we use in fusing), both transparent and opalescent and combining the specific gravity of the elements that go to make up the glass, I have attempted to answer question - the last part of the question first.

Difference in specific gravity between transparent and opalescent glass

Transparent glass

Typical transparent soda glass composition % by weight (with specific gravity) Material                         Weight        S.G. Silicon dioxide (SiO2)           73%         2.648 Sodium oxide (Na2O)            14%         2.27 Calcium oxide (CaO)               9%         3.34 Magnesium oxide (MgO)          4%         2.32 Aluminium oxide (Al2O3)          0.15%    3.987 Ferrous oxide (Fe2O3)               0.1        5.43 Potassium oxide (K2O)             0.03       2.32 Titanium dioxide (TiO2)            0.02        4.23

There are, of course minor amounts of flux and metals for colour in addition to these basic materials.

The specific gravity of typical soda lime glass is 2.45.

Opalescent glass

Initially opalescent glass was made using bone ash, but these tended to develop a rough surface due to crystal formation on the surface.  The incorporation of calcium phosphate (bone ash) and Flouride compounds and/or arsenic became the major method of producing opalescent glass for a time.

The current typical composition by weight (with specific gravities) is:

Silicon Dioxide (SiO2) –             66.2%,     2.648 SG

Sodium Oxide (Na2O) –            12%,        2.270

Boric Oxide (B2O3) –                10%,        2.550

Phosphorus pentoxide (P2O5) –  5%,         2.390

Aluminum Oxide (Al2O3) –         4.5%,      3.987

Calcium oxide (CaO) –              1.5%,      3.340

Magnesium oxide (MgO) -         0.8%,      2.320

The combined specific gravities are within 0.03% of each other -  a negligible amount.  So, the specific gravity of both opalescent and transparent glass can be considered to the same. For practical purposes, we take this to be 2.5 rather than the more accurate 2.45.

Other glasses exhibit different specific gravities due to the materials used, for example:

Lead Crystal Glass

Lead Crystal glass contains similar proportions of the above materials with the addition of between 2% and 38% lead by weight.  Due to this variation the specific gravity of lead crystal is generally between 2.9 and 3.1, but can be as high as 5.9.

Borosilicate glass

Non-alkaline-earth borosilicate glass (borosilicate glass 3.3)

The boric oxide (B₂O₃) content for borosilicate glass is typically 12–13% and the Silicon dioxide (SiO₂) content over 80%. CoLE 33

 

Alkaline-earth-containing borosilicate glasses

In addition to about 75% SiO₂ and 8–12% B₂O₃, these glasses contain up to 5% alkaline earths and alumina (Al₂O₃).  CoLE 40 – 50

 

High-borate borosilicate glasses

Glasses containing 15–25% B₂O₃, 65–70% SiO₂, and smaller amounts of alkalis and Al₂O₃

All these borosilicate glasses have a specific gravity of ca. 2.23

Correlation between CoLE and and specific gravity?

This comparison of different glasses shows that the materials used in making the glass have a strong influence on the specific gravity.  However, there does not appear to be a correlation between CoLE and specific gravity in the case of borosilicate glass.  If this can be applied to other glasses, there is no correlation between specific gravity and CoLE.

Correlation between specific gravity and colourisation minerals and CoLE?

The minerals that colour glass are a very small proportion of the glass composition (except copper where up to 3% may be used for turquoise).  The metals are held in suspension by the silica and glass formers.  That means the glass is moving largely independently of the colourants which are held in suspension rather than bring part of the glass structure. There is unlikely to be any significant effect of the metals on the Coefficient of Linear Expansion.  The small amounts of minerals are unlikely to have an effect on the specific gravity.  So, the conclusion is that there is no correlation between CoLE, specific gravity, and colouring minerals.

The short answer

This has been the long answer to the question.  The short answers are:

·         The specific gravity of soda lime transparent glass and opalescent glass is the same – no significant difference is in evidence.

·         There appears to be no correlation between specific gravity and CoLE.

·         There is unlikely to be any correlation between colourant minerals and CoLE or specific gravity.

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

Devitrification on Repeated Firing

 Devitrification is defined as the crystallisation of the glass, making it a non-vitreous substance.

Molecular level difference between vitreous and devitrified silica
from Digitalfire.com

You can see that there is not much difference between the the two states of the glass in structure, but mainly the arrangement of molecules.

The appearance of devitrification has a range of appearances from a mild smeary look through a dull surface to a crazed, crumbly aspect in severe cases. 

Mild devitrification

Medium level devitrification requiring abrasive cleaning

Causes of devitrification are related to slow changes of temperature (up or down) and most importantly nucleation points such as dust, oils, or cleaning residues. So, thorough cleaning is most important. 

Causes in repeated firings of the same piece relate to:

        Cleaning. 

It is important to thoroughly clean the piece before each subsequent firing.  Many times abrasive cleaning such as sandblasting is important to clean out impurities from the previous firing.  The resulting surface from any abrasive cleaning requires further cleaning with lots of clean water and a thorough drying with clean cloths or paper.

        Slow cooling or heating. 

Devitrification normally occurs in the range of 670⁰C to 750⁰C. This is the reason for the rapid rates of advance in this temperature range rather than other factors.  It can form both on the rise and on the fall in temperature. Slower rates in the devitrification range allow enough time for the crystallisation to begin.

        High temperatures.

Both high temperatures and long soaks can promote devitrification.  It is not just the slow rise or fall in temperature, but long periods at high temperature can lead to devitrification even though other precautions have been taken.

Changes in the composition

High temperatures and many repeated firings of the piece can lead to changes in the glass.  Some metals and fluxes are more likely than others to change composition or oxidise at extended soaks at high temperatures.  This can reduce the ability of the glass to resist devitrification.

Prevention/Correction

Prevention relates to thorough a) cleaning and b) firing rates.

All correction of devitrification relates to the modification of the surface.  If the problem is only at the surface, you can use either abrasive cleaning or the addition of fluxes to the surface, or a combination of the two. 

Where you have a mild dulling of the surface due to devitrification you can apply a flux.  This softens the surface by reducing the melting temperature of the glass and so reverses the crystallisation at the surface. The devitrification solution can be a proprietary spray such as Super Spray. Be aware that some sprays use lead particles as the flux, so are inappropriate for pieces intended to be food bearing. You can make your own devitrification solution by dissolving borax in distilled water.  When the devitrification is wide spread or deep, abrasive cleaning is required.

Abrasive cleaning can be by hand with sandpapers or diamond pads.  Be sure to keep them damp.  This keeps dust from rising, and the sanding surfaces clean for better working.  Sandblasting can be quicker, especially on uneven surfaces or where there are deep imperfections.  The surfaces resulting from abrasive cleaning need to be scrubbed clean with sufficient water, and then polished dry as for a finished piece.

It is possible to combine both these methods to be more certain of a shiny finish.  When combining, you need to do the abrasive cleaning first, then the wet cleaning and finally add the devitrification solution.

A fourth possibility is to sprinkle a fine but consistently thick layer of clear fine frit or powder over the piece.  This, when fused, provides the new surface concealing the devitrification below.  Again, this must be done at a full fuse, so it is not applicable to items you wish to remain tack fused.

However, if the devitrification has progressed to a crazed appearance, it is so deep as to be almost impossible to reverse.  The piece will also probably have developed incompatibilities. So the only real option in crazed pieces is to dispose of them.  They will not be useable in combination with any other glass. They will make any glass with which they are combined subject to devitrification and possible breakage.  These are pieces which truly cannot be cut up and re-used.

Use of Untested Glass - Kiln Forming Myths 22

You must use art glass rather than recycled glass.

This seems to refer to the use of untested glass in kiln forming.  If you are going to use untested glass for kiln forming, it does not much matter which you use.  Because, in every case you will need to test for forming and annealing temperatures to be able to make use of the glass with unknown properties. 

Of course, people use glass that is not tested fusing compatible in many circumstances.  Float glass is frequently used in many kiln forming applications.  And bottle glass is of very little different in composition.  So-called art glass can be used in a variety of ways also.  There are many other variations of glass including handmade, casting, lamp working, and borosilicate, among others.  Each has their own set of characteristics, which overlap with each other.  The forming and annealing temperatures must be determined to enable you to use them. Some of this information is often available from the manufacturer’s web site or other sources.  Many times you have to do the testing for yourself.  One guide to help determine the critical temperatures is here. 

One characteristic that all untested glasses share is a tendency to devitrify by the second or third firing, so attempting to get the most work done in the fewest firings is a good idea.  This tendency to devitrify is frequently shown when manipulating bottle glass.

Defining the Glass Transition Phase

We often treat glass as a simple material. However it is a very complex and as yet not fully understood material. One of the most curious aspects is the transition between plastic and solid states. This is the temperature range of glass annealing – called the glass transition by scientists. This note comes largely from "Glass Properties" produced by Schott. The text in brackets [ ] is my additional explanation.

The glass transition comprises a smooth but very large increase in the viscosity of the material. Despite the massive change in the physical properties of a material through its glass transition, the transition is not itself a phase transition  of any kind [in this case from a liquid to a solid] and involves discontinuities in thermodynamic and dynamic properties such as volume, energy, and viscosity.

Below the transition temperature range, the glassy structure does not relax in accordance with the cooling rate used. The expansion coefficient for the glassy state is roughly equivalent to that of the crystalline solid. [Thus the CoE, which is taken as an average of expansion per degree Celsius over the range of 0C to 300C, is an inadequate guide to how the glass will behave at the glass transition and higher temperatures.]

Glass is believed to exist in a kinetically locked state, and its entropy, density, and so on, depend on the thermal history. Therefore, the glass transition is primarily a dynamic phenomenon. Time and temperature are interchangeable quantities (to some extent) when dealing with glasses.

[Viscosity shows a relatively regular change with temperature changes.] In contrast to viscosity, the thermal expansion, heat capacity, shear modulus, and many other properties of inorganic glasses show a relatively sudden change at the glass transition temperature. Any such step or kink can be used to define T_g [the transition phase of glass].  To make this definition reproducible, the cooling or heating rate must be specified.

Annealing - Effects of Chemistry

Affects of Chemistry on Annealing Point

The change in the transition temperature is affected by the rate of cooling; it is also affected by the chemistry - or composition - of the glass. The transition temperature in silicates (glass of various compositions) is related to the energy required to break and re-form covalent bonds in an amorphous (or random network) lattice of the tetrahedra form of the glass molecules.

A covalent bond is one that involves the sharing of electron pairs between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is what covalent bonding refers to.

The transition temperature is influenced by the chemistry of the glass. For example, addition of elements such as Boron, Sodium, Potassium or Calcium to a silica glass helps in breaking up the network structure, thus reducing the transition temperature and the melting temperature. Alternatively, Phosphorus helps to reinforce an ordered lattice, and thus increases the transition temperature.

The modifiers commonly used in glass-making are: sodium oxide, potassium oxide, lithium oxide, calcium oxide, magnesium oxide, and Lead oxide. Although there are over 2,000 known additives to glass. The minerals used to colour the glass seem to have minor affects upon the glass composition as they generally are in a colloidal suspension without forming bonds to the silica atoms.

If an oxide, such as sodium oxide, is added to silica glass, a bond in the network is broken and the relatively mobile sodium ion becomes a part of the structure. With increase in the amount of modifier, the average number of oxygen-silicon bonds forming bridges between silicon atoms decreases. The principal effect of a modifier is to lower the melting and working temperature by decreasing the viscosity. An excess of modifier can make the structural units in the melt sufficiently simple and mobile that devitrification (crystallization) occurs in preference to the formation of a glass. The skills of the glass makers lie in the balance of factors relating to the transition and working temperatures, and the maintaining the resistance to devitrification.

Reference: http://glassproperties.com

What is Viscosity

What is Viscosity?

An example of differing viscosities

There are a variety of definitions, but these two capture the main elements.

Informally, viscosity is the quantity that describes a fluid's resistance to flow. Fluids resist the relative motion of immersed objects through them as well as to the motion of layers with differing velocities within them.  Source

Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion.  Source

A demonstration of the resistance of different viscosities of oil to a weight moving through the liquid.

Almost all liquids are viscous fluids having viscidity. For example, when rotating a drum container filled with water on its vertical central axis, the water that was at rest in the beginning starts moving as it is dragged by the container’s inside wall and then whirls completely together with the container as if it were a single rigid body. This is caused by the force (resistance) generated in the direction of the flow (movement) on the surfaces of the water and the container’s inside wall. A fluid that generates this kind of force is regarded as having viscosity.

Temperature is a very important factor for measuring viscosity. In fluids, as temperature goes up, viscosity goes down and vice versa. In the case of distilled water, if the temperature changes 1 centigrade, it produces a difference of 2 % to 3 % in viscosity.  Source

Viscosity is the measurement of a fluid's internal resistance to flow. This is typically designated in units of centipoise or poise but can be expressed in other acceptable measurements as well. Source

Why is viscosity important?

“Near the strain point the expansion increases rapidly and sometimes erratically.” The links between the molecules has reduced in strength and so have a lesser role in the forces acting at higher temperatures. “In those upper ranges – the temperatures where glasses are formed and re-formed with heat – viscosity is a much more useful indicator of how glasses will behave.

“The combination of viscosity and COE are what make glasses more or less compatible, i.e., containing stress in amounts low enough to allow them to hold together without breaking at room temperature for extended periods of time under normal circumstances.

Bullseye found in the early 1980s in their efforts to mix coloured glasses in streaky colour combinations that the COE could not be used to predict compatibility. In trying to correct the compatibility of certain mixed glasses, the closer they brought together the COEs, the more incompatible became the mixes.

“The reason that we could not use COE to successfully predict whether a coloured glass would fit the base clear glass was/is because, as the base glass composition is altered with the addition of the necessary oxides to colour it, the viscosity is inevitably changed. This viscosity change causes the coloured glass and the clear base glass to strain themselves in the cooling cycle of the fusing process (a viscosity mismatch). Therefore once the two glasses reach room temperature they have undue residual strain that may lead to failure.

“In order to prevent this undue residual strain an equal but opposite strain must be introduced into the coloured glass to cancel out the strain induced by the viscosity mismatch. This is accomplished by introducing an expansion mismatch of equal but opposite strain. The two mismatches cancel each other out, leaving the two glasses nearly strain free.

“It is this phenomenon (viscosity mismatch cancelled out by an equal but opposite expansion mismatch) that enables glasses of very different compositions to be formulated to fit each other. The very fact that the expansion of a coloured glass has to be altered to make it fit a base clear glass implies that COE cannot be used as an indicator of compatibility. It is also why it only makes sense to describe these glasses as tested compatible to a specific manufacturer's base glass for a specific glass forming process.“ [L. MacGreggor]

Even different formulations of glass have different viscosities and different rates of softening with temperature increases.

How does viscosity apply to us?

Although viscosity is of major importance to the manufacturer, it does have some relevance to kiln formers too.

Understanding that glasses have different viscosities – most often referred to as hard and soft – can help in the choice of colours and styles of glass to combine. Some glass will spread more, and also allow other glass to sink deeper into the layer than others. It might help avoid combining extremely hard and soft glasses next to each other.

It should also help explain some results that were not planned. It may help in when thinking about uneven slumps.

It is important to recognise that glass chemistry is extremely complicated, and to see that the expansion characteristics have to be balanced with the viscosity characteristics as the two main elements in compatibility. There are others, of course, but these appear to the two main ones.

Thin Glass Uses

• Thin glass is often used in jewelery as it allows more layers of differing colours to be built up. It also is very useful when building a channel in the piece.
• Thin glass allows more layers to be built up before going over the 6-7mm when the glass begins to expand due to the height overcoming the surface tension of the glass.
• This also allows colours that are not present in the manufacturer's palette through combinations made from two or more colours.
• Thin glass versions of dense colours provide a lighter tone which can fit in well with other lighter colours.
• Thin glasses are also useful in tack fusing, as the height is not so great and so can be used over 6mm thicknesses without adding greatly to the volume.

Moretti Rods

Information on Moretti/Lauscha specifications:

Coefficient of Expansion (C.O.E): 104

Strain Point: 448C

Working Temperature: 926C

Annealing Range: 493 – 498C


Softening Point: 565C

Rods per pound: 15-17

Rod 
Length: Approx 1metre

Rod Diameter:5-6mm

from yglass.com

Keeping Flashed Glass the Right Side Up

Once you have determined the flashed side on a sheet of glass, mark it with a felt tip or wax marker of some kind so that you will not have to perform this action each time. This should be carried over to each piece as you cut it away from the main piece.

When you have cut a piece from the main sheet, it is easy to turn it over and work on the clear rather than the flashed side. It is essential to know which the flashed side is if you are going to do any etching of any kind. So, as soon as you have cut the piece, mark the flashed side. This will keep you certain that you are working on the flashed side.

Another method to keep track of the flashed side is to mark across the intended score line. After scoring and breaking you will have both pieces of glass marked. All you need to do is make sure you always mark the same side - flashed or clear. Some like to cut on the clear side and some the flashed side. All you have to do is to determine which your practice is.

Distinguishing the Coloured Side of Flashed Glass

On smaller pieces of flashed glass you can determine which the flashed or coloured side is by putting it to the light and viewing it through the edge. If the flash is very thin or you cannot determine which the flashed side is, you can alter the angle a little. If you tip the glass down slightly and the light is coming through the clear side, there will be very little variation in what you see.

If you tip it down and you see the colour very distinctly, then the flash is on the upper side.

Also note that on the left side of the glass you can see the effect of the cutter pressure on the glass.  These little hook like marks are evidence of the stress caused by scoring the glass.  This is the kind of mark you will see on glass that has adequate, but not excessive pressure applied during the scoring.

Now back to the subject of the flash.

On larger pieces this is more difficult, and dangerous to you and the glass, as you risk breakage by holding large sheets horizontally. So you can use your grozers to nip a little glass off the edge. If there is no change in colour of the chipped edge, you have taken glass off the clear side. When you chip off the flash, there will be a little bit of clear showing which the coloured side is. Here are two examples.

Once you have determined which the flashed side is, mark it and all off-cuts with a felt tip or wax marker of some kind so that you will not have to perform this action each time.

Glass Colours

Glass normally has little or no colour because the electrons in the material are tightly bonded so no electronic movement in the energy range of visible light is possible. Glass is given colour by addition of various materials to selectively absorb light in the visible spectrum.

There are three processes: addition of ions of transitional metals; addition of colloidal particles; and addition of coloured crystals.

Ions of transition metals provide electronic excitations in the visible light range. Some of the common ions are:

• Chromium with two positive ions gives a blue, but
• Chromium with three positive ions gives a green.
• Cobalt with two positive ions gives pink.
• Manganese with two positive ions gives an orange.
• Iron with two positive ions gives a blue-green, as can be seen by looking at the edge of much of modern window glass.

Addition of colloidal particles of various sizes causes absorption of some parts of the visible spectrum and reflects the complimentary colours. These are very small particles ranging from 4 to 170 nanometers. For example,

• Gold of 4-10 nanometers will give a pink.
• Changing the size to the range of 10-75 nanometers will produce a ruby.
• As the size of the gold increases to the range 75-110 nanometers a green is produced.
• Between 110 and 170 nanometers browns are produced.

The addition of very small coloured crystals that are dispersed throughout the glass will produce coloured glass.

• The Egyptians made scarlet glass by the addition of red copper oxide. Other examples are
• Lead hexachrome (Pb2CrO6)which produces red, and
• Green is produced with chromium (III) oxide (Cr2O3) crystals, often called viridian.

Based on MIT Solid State Chemistry Notes, p.15-16

Glass Transition Point

This is the temperature range at which a super cooled liquid becomes a glass. At higher temperatures the molecules are able to reorganise quickly as in a liquid. At temperatures below the transition range, the movement among the molecules virtually ceases and the resulting material is known as a glass.

Two characteristics should be noted here. The temperature range for the transition phase is dependent on the speed of cooling. The slower the cooling, the more time there is for reorganisation and so there is a lower transition temperature. The quicker the cooling of the material through the transition phase, the greater the volume of the material, i.e. it is less dense, although the more slowly cooled glass is still much less dense than the crystalline material.



Based on MIT Solid State Chemistry Notes, 7, pp.7

Formation of Glass

There are a lot of glasses – natural and laboratory created – in addition to the silica based one that we work with. However understanding how glasses in general are created helps to understand “our own”. In general, when the liquid phase of a material is cooled below its freezing temperature it usually transforms into a crystalline solid. But some materials do not crystallise when cooled to their freezing temperatures. Instead they create a rigid network which is known as glass. It is very similar in structure to a liquid – hence super cooled liquid.

At temperatures just above their freezing points, most materials have viscosities that are similar to water at room temperature. They are so fluid that the molecules can rapidly form crystalline structures. But many inorganic silica materials form glasses on cooling because their viscosity at and above their freezing points is very high. There are also high energy bonds between the silicon and oxygen molecules. The viscosity increases very rapidly as the temperature is reduced. These prevent the flow required for crystallisation. In organic glasses, e.g. resin, crystallisation is difficult because of the long chain molecules that the material is composed of, preventing the molecules from sliding past one another, i.e., the difficult structural re-arrangement that would be required to form crystals.


Based on MIT Solid State Chemistry Notes, 7, pp.5-6

Temperature Characteristics of Various Glasses

Over the years I have collected temperature information for a number of glasses. They are of comparative interest and can assist with choosing a temperature or range of temperatures for the work you are doing. If the work is important, or critical, refer to the manufacturer for the latest information.

Bullseye
There has been a lot of information published about this glass. One interesting characteristic has been the different temperatures for the complete range of glass they produce. So there appears to be a difference between the transparent, opalescent and gold pink glasses.
Transparent:
Full Fusing 832C ; Tack Fusing 777C ; Softening 677C ; Annealing 532C ; Strain 493C
Opalescent:
Full Fusing 843C ; Tack Fusing 788C ; Softening 688C ; Annealing 502C ; Strain 463C
Gold Bearing:
Full Fusing 788C ; Tack Fusing 732C ; Softening 635C ; Annealing 472C ; Strain 438C

This also illustrates that not all the characteristics of a glass range are linear. The most apparent one is that the full fusing, tack fusing and softening points of the opalescent glass are higher than transparent, although the annealing point is lower.

Desag GNA
Full Fusing 857C ; Tack Fusing 802C ; Softening 718C ; Annealing 516C ; Strain 427C

Float Glass
Full Fusing 835C ; Tack Fusing ca. 760C ; Softening 720C ; Annealing ca. 530C ; Strain 454

Spectrum S96
Full Fusing 788C ; Tack Fusing 718C ; Softening 677C ; Annealing 510C ; Strain 371C

Uroboros
Full Fusing 788C ; Tack Fusing 732C ; Softening 663C ; Annealing 538C ; Strain 427C

Although the information above may be dated, the important element is that there is little correlation between glasses in the relationship of annealing point to other characteristics of the glass.

This listing also shows that the temperature characteristics are not linear between glasses. For example, Spectrum and Uroboros have the same full fuse temperatures, but different tack fusing, softening, annealing and strain temperatures. Sometimes one is higher than the other, and other times it is reversed.

Another example is shown by the Desag GNA and Float glasses. Desag GNA has higher full fuse and tack fuse temperatures than float, but lower softening, annealing and strain temperatures. This helps to make the point that you need to know the glass you are using as it will not have a proportional relationship at every point in the kiln working temperature range.

I emphasise that these temperatures have been collected over a period and may not be the current or absolutely correct information. They are used here to illustrate the differences within and between the glasses of various manufacturers.

Properties of Some Basic Glass Types

Various types of glass have differing properties which make them suitable for a variety of applications. Some of the characteristics of three glasses are given here. The glasses are quartz, soda/lime, and lead crystal.

Quartz glass
Softening point (C) 1508
Annealing point (C) 1048
Strain point (C) 956
CoE at 10-7 metres/degree C: 3.1
Density (kg/m3) 1973
Refractive index 1.459

Soda/Lime glass
Softening point (C) 693 - 732
Annealing point (C) 516 - 549
Strain point (C) 471 - 493
CoE at 10-7 metres/degree C: 56 - 100
Density (kg/m3) 2203 - 2275
Refractive index 1.51 – 1.52

Lead glass
Softening point (C) 438 - 671
Annealing point (C) 366 - 527
Strain point (C) 343 - 449
CoE at 10-7 metres/degree C: 47 - 55
Density (kg/m3) 2505 - 4867
Refractive index 1.54 – 1.75

Solarisation

Old glass can show changes in colour as evidenced by the different colour of the glass under the lead came where the light cannot reach the glass.

Drew Anderson has provided the explanation.

This change in color of some glass is known as solarisation.

The main ingredient of most glasses is silica, which is usually introduced as a raw material in the form of sand. Silica itself is colorless in glass form but most sands contain iron as an impurity, and this gives a greenish tint to glass. By adding certain other ingredients to a molten glass, it is possible to change the greenish color and produce colorless glass.

These ingredients are known as decolorizers, and one of the most common is manganese dioxide (MnO2). In chemical terms, the manganese acts as an oxidizing agent and converts the iron from its reduced state - which is a strong greenish blue colorant - to an oxidized state which has a yellowish, but much less intense, color. In the course of the chemical reaction, the manganese goes into a chemically reduced state, which is virtually colorless.

When pieces of decolorized glass containing reduced manganese are exposed to ultraviolet radiation for long periods of time, the manganese may become photo-oxidized. This converts it back into an oxidized form. Even in low concentrations this imparts a pink or purplish color to glass. The ultraviolet rays of the sun can promote this process over a matter of a few years or decades.

Selenium and cerium have also occasionally been used as a decoloriser and can produce solarisation colors, just as manganese does. The colors developed by these two elements are said to range from yellow to amber.

Tempered or Toughened Glass

Toughened or tempered glass is a type of safety glass that has increased strength and will usually shatter in small, irregular pieces when broken. It is used when strength, thermal resistance and safety are important considerations.

Toughened glass is made from annealed glass by a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace which heats it to above its annealing point. The glass is then rapidly cooled with forced draughts of air to below its annealing point, causing it to harden and contract, while the inner portion of the glass remains free to flow for a short time. The final contraction of the inner layer induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass.

It is this compressive stress that gives the toughened glass an increased strength - typically four to six times the strength of annealed glass. The pattern of cooling during the process can be revealed by observing the glass with polarised light, which shows the strain pattern in the glass.

See also Prince Rupert's Drops

Properties of Glass

Mechanically Strong
Glass has great inherent strength. It is weakened only by surface imperfections, which give everyday glass its fragile reputation. Special tempering can minimize surface flaws.

Hard
The surface of glass resists scratches and abrasions.

Elastic
Glass gives under stress - up to its breaking point - but rebounds exactly to its original shape.

Chemical Corrosion-Resistant
Glass is affected by only a few chemicals. It resists most industrial and food acids.

Thermal Shock- Resistant
Glass with stands intense heat or cold as well as sudden temperature changes.

Heat-Absorbent
Glass retains heat, rather than conducts it. It absorbs heat better than metal.

Optical Properties

• Reflects
• Bends
• Transmits
• Absorbs light with great accuracy.
  

Electrically Insulating
Glass strongly resists electric current. It stores electricity very efficiently.