Cool stuff about glass

Glass is essentially sand (silica) whose normal crystalline structure is broken down by heating and then cooled rapidly to ‘freeze’ it so that crystals don't re-form.

Silica sand is many-faced or faceted, somewhat like a diamond. Because of its structure it reflects light and is opaque. This process of melting and losing a crystalline structure is called ‘vitrification’; the material is said to have become ‘amorphous’, which means ‘without order’. The atoms are said to be in a disordered state. It’s called a supercooled liquid. 

In this amorphous state the particles are so small that light rays can “thread their way between them” and the glass is transparent. Other materials are added to lower the melting point (silica on its own melts at about 1710°C,  (3110°F), to make it easier to work, to make it clearer and more transparent, to prevent it dissolving in water. 

The most commonly encountered glass objects, such as bottles and jars, window panes, are made of a mixture of sand, soda and lime; plus a few extra ‘herbs and spices’.

To get more technical, that’s:

• Sand,(silica, silicon oxide, SiO₂) about 70 – 75%.
• Soda, (soda ash, sodium carbonate, Na₂CO₃)  about 12 – 16%
• Lime, (limestone, calcium carbonate, CaCO₃) about 10 – 13%.

For convenience, this type of glass is known as ‘soda lime’ glass. The silica isn’t mentioned, as it’s the basis of most types of glass. Another type of glass well known to most is ‘Pyrex’. It’s able to withstand rapid changes of temperature, is made of boron oxide (boric oxide, borax, B₂O₃) and silica, so it’s called a ‘borosilicate’ glass. 

Still other glass has substantial amounts of lead added. This gives added brilliance and clarity to the glass, making it ideal for cutting and polishing into sculptural and tableware products such as ‘Waterford’ crystal. As lead presents a significant health hazard it is being superceded by products from firms such as ‘Orrefors’ who have developed superior soda lime glasses free of lead.   

However,for most kilnforming applications the glass used will be ‘soda lime glass’ and includes clear glass and all of the vast range of coloured glass offered through art and craft outlets for kilnforming and leadlighting. 

Modern clear glass is made by floating the melt on a bed of tin and is referred to as ‘float’ glass. This gives it very true and even surfaces and uniform thickness.

Most coloured and all patterned glass is made by passing it through rollers, so the surfaces are not smooth nor true and the thickness can vary across a sheet. 

Colour in glass

Whilst generally regarded as ‘clear’, the normal commercial soda-lime float glass will usually have a greenish tint which is caused by iron impurities in the raw materials. Additives can remove this tint whilst others can impart any colour of the rainbow. The colourants are usually mineral oxides introduced into the melt during manufacture, so that the colour extends right through the sheet.

Multi-coloured sheet is actually additional melts poured on top of a base colour and rolled out thin like pastry, so that a particular colour may not extend all the way through; but will show only where the particular added colour has flowed during the rolling out process.

Glass and heat

Scientists often refer to glass as the fourth state of matter as, although it seems solid, technically it has the molecular structure of a liquid. Glass  is commonly made by combining any of the materials mentioned above at a high temperature in order to allow them to melt and fuse together. When cooled rapidly, the substance becomes rigid.

Whilst not strictly true, we can think of glass as having two states: solid and fluid. However, it doesn’t go from one state to the other at a precise temperature, as does ice to water at 0°C; instead, it occurs slowly, over a range of temperatures. All types of glass go through the same changes but not always at precisely the same temperatures.

There is a middle region where the glass is neither solid or fluid. It’s where it is passing from one to the other and for our purposes we will call it the ‘transition zone’.

Viscosity

To understand the nature of glass and to successfully kilnform with it, it helps if one understands the concept of viscosity.

Most liquids,such as oil or honey, become more runny the more they are heated and the warmer they get. A term to describe this is ‘a change in viscosity’. Viscosity is the tendency of a fluid to resist flow, so less viscous means more runny. Honey will get less viscous the more it is heated.

Let’s consider butter for a moment. Taken from the fridge it is hard, but becomes more spreadable when left out in a warm room for a while. It will still look hard. Put it in a pan to fry eggs and boost one’s cholesterol and it will become very runny. It will have been getting less viscous all the time it is warming but the change is not readily apparent in the early stages.

Glass can be heated to quite a high temperature before a change in viscosity becomes evident. In the early heating stage the glass will appear solid: as if the heat was having no effect on it. When it does become evident it can be seen to go through continuous change that has been described as  ‘bending like rubber to stretching like elastic to flowing like thick porridge to running like thin treacle’. (Cummins,Techniques of Kilnforming Glass, 1997.)

Thermal conductivity

Is the ability of a material to conduct heat. Almost all materials do so, but some do it more rapidly than others. Copper and steel conduct heat rapidly, whilst wood or plastic do so much more slowly. We have all appreciated the wood or plastic handle on a frypan which slows down the flow of heat from the metal. Compared to copper, glass is a relatively slow conductor of heat. It takes time for the heat to flow through a piece from the outside to the inside; or from one side to the other.

Kilnformers often refer to glass as a ‘poor’ conductor of heat: they would prefer the heat to move instantaneously through it. On the other hand, building designers consider it a  ‘good’ heat conductor: maybe too good, as in a window it lets heat in or out too rapidly.

Coefficient Of Expansion

Most materials expand when heated and contract when cooled. The ‘rate’ at which they do so is called their Coefficient of Expansion or COE, sometimes referred to as Coefficient of Thermal Expansion or CTE. The COE of a material is defined as the percentage change in length per degree change in temperature.

Typically, for Bullseye glass this is .0000090 %/deg or 90 x 10^-7 %/deg. For convenience, only the significant numbers are used and Bullseye is referred to as COE 90. Another widely used art glass range is Spectrum System 96, which has a COE of .0000096 %/deg. There's a minute difference between the COE of the two, but enough to cause disaster if they are mixed together.

 This is because they join together at a high temperature when they have expanded and then shrink by different amounts as they cool. Stress developed because of this will cause the piece to crack.They are said to be incompatible.

Thermal shock

Because of the finite time it takes heat to flow through the glass, the side to which the heat is applied will expand more rapidly than the opposite side. Most people are familiar with the glass object that cracked when placed in hot water. The expansion of the side first receiving the heat, relative to the other side, created stresses that the glass couldn’t withstand.

Similarly when glass is in a kiln and heat is being applied from one side: too rapid heating can create such temperature and expansion differences across the glass that the resultant stress cannot be withstood.

Of course, this applies only whilst the glass is in its solid state. Glass in its fluid state is able to withstand rapid changes in temperature; it’s impervious to thermal shock until it cools back into the solid state.

Tensile strength, toughening

Unlike steel and other metals, glass in its solid state has almost no elasticity; it will bend only slightly under load, and then snap or crack. The load it will bear before cracking can be greatly increased by ‘toughening’.

This is a process in which the glass is heated and then quenched (usually by jets of air) to induce localized stresses. Toughening not only significantly increases the strength but it also causes the glass to break into small fragments if it does shatter, as seen when a ‘toughened’ car windscreen breaks into small pieces. This is referred to as ‘safety’ glass.

A more recent introduction is ‘heat strengthened glass’ which involves a process similar to toughening but in which a lesser level of strength is achieved.

Toughening processes involve the use of specialised commercial kilns and is beyond the scope of kilnforming. However, it is important that kilnformers are aware of these processes which render the processed glass unsuitable for salvage or rework. Other than for some small identifying stamp, toughened or heat strengthened glass looks no different to other glass 

Toughened or heat strengthened glass cannot be cut after processing without shattering.For your own safety, do not try to rework or salvage. See Float Glass for more on this.

There are also chemical toughening treatments but these have limited application.

Annealing

As glass cools from the fluid to the solid state it can develop internal stresses which can render it more prone to fracturing at room temperature. To minimise these stresses the glass undergoes a process called annealing.

The glass is held at a constant temperature for some time to allow the temperature to equalize throughout (anneal soak) and then slowly cooled (anneal cool) through a controlled temperature range to minimise the risk of stress being developed. Both the anneal soak temperature and the range and rate of anneal cooling varies with the type of glass and also with its thickness and mass.

Proper annealing is crucial for the durability of glass objects. If incorrectly done the piece may break at any time.

Devitrification

Holding the glass at elevated temperatures for extended periods allows the crystal structure to re-develop, referred to as devitrification. The surface of the glass can progressively become less shiny, then dull and looking like there is scum on the surface: when well advanced the surface becomes wrinkly and the glass becomes cloudy throughout.

The rate of change is dependent on temperature time and the type of glass. The higher the temperature and the longer the time at temperature, the greater will be the change in the glass. Some low temperature fusing glasses may undergo change at temperatures that don’t affect some types of float glass.

Thorough cleaning is essential before firing as devitrification can be promoted by dust or surface contamination. In addition, dirt perspiration fingerprints and smears can be fired into the glass.

Forces working on glass in the fluid state

There are two main forces at work on glass which influence the way it will move and the shape it will assume when in the fluid state. They are:

• Gravity.
• Surface tension.

Gravity at work

What’s gravity? Simply stated, it’s the force pulling all matter towards the centre of the earth. It acts on matter all the time, but for our purposes the focus will be confined to glass in its fluid state.

The force of gravity causes things to droop, slump or sag. Slumping of glass occurs in a vertical or near vertical direction. To do this most effectively the glass and mould needs to be placed horizontally.

Glass doesn’t change form whilst it’s rigid. It’s only when it is fluid, able to flow, that it will change shape.

Temperature affects the way the glass behaves. The hotter and more fluid the glass, the easier it will flow. The hotter the glass the more readily it will conform to the fine detail on what is supporting it.

The change in the glass is the result of a combination of temperature and time at temperature. Potters have long referred to it as ‘heatwork’. Higher temperature for shorter time and lower temperature for longer time can both achieve similar results on the glass and similar degrees of slump. That’s similar, but not necessarily exactly the same. More on this elsewhere.

Surface Tension

The second force acting on glass in the fluid state is surface tension. This is a property of liquids that causes the surface layer to behave like an elastic sheet. It wants to achieve the minimum possible surface area.

For a given volume, a sphere or ball has the smallest surface area of any shape, so surface tension tries to pull the fluid into a spherical shape. It’s the reason why falling water forms into droplets and mercury forms into balls. The strength of this surface tension varies between materials: with some it’s strong, with others weak, and glass is somewhere in between.

How surface tension and gravity interact

Whilst surface tension is trying to pull a fluid into a sphere, gravity is trying to flatten it out. The effect on different materials can vary widely.

The surface tension of water is insufficient to prevent a drop from breaking up into myriad droplets and splashing over a large area when it strikes a solid surface.

Mercury has no such problem, its strong surface tension enabling it to retain its shape remarkably well.

If glass is allowed to remain fluid enough for long enough it will reach a stage where the force of gravity pulling it down and surface tension pulling it up will equalize.

The glass will try and achieve this ‘equilibrium thickness’ which, for most soda lime glass, is about 7mm, (that’s a vertical height of 7mm). The more fluid the glass the faster will be the response to the effect of these competing forces.

Fig 3A-01

These identical stacks of float glass, subject to different temperatures and soak times,  show the progressive changes due to increased ‘heatwork’. Each stack comprised six pieces of 4mm float, 24mm total height.

Note how they slumped, flowed outwards, moved more toward the shape of a flattened sphere.
Of course, the height of the stacks were more than equilibrium thickness, so the glass wanted to become thinner; flow sideways.

Fig 3A-2,.

When the height is less than equilibrium thickness the opposite occurs. The edges will roll in, the overall size is reduced. On a typical 3mm piece the corners will become rounded and the sides scalloped, as in Fig 2.

Most newcomers to kilnforming will be fusing or slumping 3mm glass, so most work will be under 7mm thick. A single 3mm layer will behave as in fig. 2, whilst a fuse of two layers will show only a minor tendency to pull in as it is close to equilibrium thickness.

The skin effect

Molten glass develops a skin when it comes into contact with air. A stream of molten glass can be cut with shears and the cut faces will immediately develop a skin so that each portion becomes a separate ‘package’.

If packages are placed together so that parts of the surface are in contact, when heated to a sufficiently high temperature the skin will ‘dissolve’ and the two packages will combine into one.

This is what happens when two pieces of flat glass are placed one-on-another in a kiln: as heating proceeds the pieces will soften, the contact faces will conform in shape to one another and the skin will eventually dissolve.

Result!. A fused piece.

This is unlike the ‘skin’ on custard, which retains its form when stirred back into the mix: it won’t dissolve again.

• When a glass blower ‘gathers’ a blob of molten glass from the furnace it comes out nicely rounded, under the influence of surface tension. It will slump out of shape if it isn’t constantly turned to counteract the effect of gravity.
• A lava flow from a volcano is a spectacular sight. Consisting substantially of molten silica, it’s also a glass. Seen often on video, observe how rounded is the ‘nose’ of the flow, how it stays in a single mass: unlike a flood of water which splashes and leaps all over the place.