High Fast Slumps

What are the possible effects of fast rises to a high temperature for a slump?

Some of the possible effects of fast rises to a relatively high temperature slump are these:

Uneven slumps can occur. 

·         This largely due to differential heating of thicker/thinner parts. 

·         It can also emphasise anything off level.

·         Any unevenness in the heat across the kiln can also be emphasised by the rapid rise in temperature.

Uneven slumps can be promoted by contrasting colours. Dark and light colours heat at different rates, leading to one area of the glass slumping before another.

A dark/light contrast can lead to stress fractures in fast firings.

In a fast firing the top heats faster than bottom leading to the possibility of splits on the bottom of the piece

The edges of the piece heat faster than centre, increasing the possibility of spikes at the edge.

Fast slumps require higher temperatures to achieve the slump.  This means there will be more marking of the bottom surface.  It often includes stretch marks especially at the rim.

The Alternative to Fast High Temperature Slumps

Slow and Low

Slow rises in temperature means the slumps can be done at lower temperatures. Lower temperatures usually mean more control and fewer marks from the mould.  It does mean that you will need to observe at intervals to get the soak time you need, but this is required for all variations in rates and layups, as well as new moulds.

Melts, Apertures and Height Effects

The effects on the pattern of melts are a combination of several factors. The normal pattern is of spirals as a thread of glass moves down to the shelf and begins to spiral just as any other viscous fluid around the high spot of the drip.  The specific effects centre around three main elements.

Aperture size

The size of the holes determines the diameter of the thread of flowing glass.  Also, the larger the diameter, the quicker the flow.

relatively small apertures

large, long apertures

Height from shelf

The height from the shelf has the effect of determining both the thickness of the thread at touch down and the degree of spiralling.

Relatively low screen

Relatively high screen

Heat

The temperature and time determine the heat work.  The amount of heat (as well as top temperature) influence the flow of the glass.

These three elements interact

Aperture

Aperture size determines the maximum diameter of the thread.  You can thin the threads by having smaller grids or holes. 

The height affects two things. 

Height affects the relative thickness of the flowing thread. Higher makes for thinner strands. The reduction in size can be lessened by placing the apertures closer to the shelf.

Height also affects how the thread behaves on touching the shelf.  More spiralling occurs with height.  A low height will reduce the spiralling to just moving outwards.

Note that when talking about height, it is relative to the aperture sizes.

Heat affects how the glass flows

The higher the heat or the greater the heat work, the faster the glass flows.  Lower heat gives slow moving threads.  Faster flowing glass promotes thicker threads.  Slower moving threads can take up patterns other than spirals.

These factors give you three interacting elements

You could have, for example, a high screen with large openings and low heat to give thin threads with eccentric spiralling.

You could have low height with small apertures and high heat to give thick threads with minimum spiralling.

In theory, you could have at least twelve main combinations by using the extremes of each element, with multiple variations of dimensions in each case.

Experimentation Required

This is to illustrate the interactions are complex and require significant experimentation to be able to predict the probable outcome.  The outcomes will always be only probable, even though you can come to control more aspects of the process and you develop experience.

Uprisings at the Bottom of a Slumped Bowl

“I just finished slumping a dish and I got a big lump in the center of the bottom. This is not an air bubble, just a lump. What should I do to avoid this again?”

Several suggestions are possible.

Ensure there are holes at the bottom of the mould that allow air to get out into the kiln. Prop the mould up on stilts if the hole does not go directly out of the mould. Alternatively, drill a hole in the side to allow the air to escape from under the mould.

Firing for too long or at too high a temperature will cause the glass to continue sliding down. Having nowhere else to go, the weight causes the bottom to begin rising. This is a consistent experience across several kilns and with multiple users.

So keep the temperature down to the minimum required. To find that out, watch the slumping in stages (do not stare!). Look at the piece for a second or two every five minutes after you reach your desired temp.

If it already has slumped adequately, you are firing too high. Reduce your temperature in subsequent firings and watch to find what the required temp and time is. There is absolutely no substitute in slumping but to watch and learn what your mould and glass require.

If you are slumping at such a temperature to seal the glass to the mould, you are firing too hot anyway. Or put more positively, use a low temperature slump, that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice.

A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. Therefore, there are spaces for the air to escape from under the glass all the way to the top as well as through the air holes at the bottom. It also gives the most mark-free slump possible for your shape.

Glass on Drop Rings

When glass drops through a ring, you need to check on some things relating to the placement and firing.

When thinking about the relationship between the size of the flat glass and the size of the aperture, you need to remember how the glass behaves as it heats up toward the drop temperature.

Glass behaviour

The glass begins to sag at the middle of the aperture, however the glass is still relatively stiff.  The weight of the rim is not enough to keep it from rising from the ring. The rim of the disc maintains the angle from the centre of the drop to the edge, until it gets hot enough for the weight of the rim to allow the edge of the disc to settle back down onto the ring.  This is the source of a lot of the stretch marks at the shoulder of drops.

Rim width

To avoid the glass dropping through, you need to have an adequately sized rim.  The width of the rim sitting on the ring, needs to be related to the size of the hole.  

The consequence of an inadequate rim

I have found that for apertures up to 300mm diameter there needs to be at least 35mm on the rim.  The consequence of this is that your blank diameter needs to be 70mm more than the hole diameter.  For larger apertures – up to 500mm – you need 50mm, or 100mm added to the diameter of the hole.  I do not have the experience to say how much more is required for larger diameter drop rings.  There is more discussion on blank sizes here. 

Heat

The rate at which you heat the glass and the top temperature both have effects on the possible drop through.  

High temperatures. The higher temperature you perform the drop out, the more likely you will need larger rims or other devices to reduce the drop through possibilities.  It also promotes excessive thinning below the shoulder. 

Fast rates. The surface will become hotter than the bottom, but at different rates.  The glass over the hole is heating from both top and (to a lesser extent) bottom.  The rim is sitting on the ring and so heats only from the top.  The differential in heat may cause a break.

Weight. The thickness of the glass effects when the drop will begin.  The heavier the glass and larger the hole, the effective weight will be greater.  In these cases, you can use a lower temperature for the drop.

Additional methods.  You can use other methods to reduce the chance of a drop through.  Two of them are:

Weights. You can put kiln furniture on the glass rim to keep it from rising during the initial stages of the drop.  These must be placed symmetrically. Four or six pieces of kiln washed props or small dams would be sufficient up to 300mm diameter.  More would be required for larger apertures.  Of course, these will mark the rim, meaning that it must be cut off.

Inclined rings. Another possibility is to use an inclined ring, with the glass resting on the upward incline, so the glass is held above the aperture and is heating evenly until the drop begins.

Volume control

Glass has a surface tension (viscosity) that draws the glass toward 6-7 mm thick at kiln forming temperatures. 

To test this out, prepare three stacks of glass squares.  They all should be the same size.  Record the measurements. Place them in a stack of one, a stack of two and the last of three squares.  Fire them to a full fuse.  Compare the sizes of the original to the fired. Note the expanded size of the three-layer stack, the same size of the two-layer stack and the reduced footprint, and dog-boning of the single layer.

Credit: Paul Tarlow

Glass in a single layer behaves differently from the thicker set-ups. When the glass is hot it begins thickening at the edges. The viscosity of the glass is drawing from both from the edge and from the centre.  This means the footprint of the glass is getting smaller. The result is needling. The glass retreats leaving small threads where the glass was held in the small imperfections in the separator’s surface. 

If you do not need a full fuse, you can reduce this needling effect. Reduce the temperature and extend the soak.  This means that the glass does not expand on the heat up so much, and the greater viscosity reduces the needling effect.

If you need a thick piece of a certain size, you need to dam the glass to overcome the tendency to expand.  With experience, you can get to know how much a three-layer (or more) set up will expand and cut the glass accordingly.  In this way, you can often do without dams. There will be some thinning at the edges and a rounding of the corners.

An excellent document on volume control is the Bullseye Tech Note 5.  


Note that this 6mm rule applies at normal kilnforming temperatures.

At higher temperatures, the viscosity is less so the glass will become thinner than 6-7mm.  My experience has shown that at around 1200°C the glass will spread to about 0.5mm thickness.  This is just to point out there is a relationship between temperature and viscosity, and therefore thickness. As the temperature rises, so the viscosity reduces. This relationship allows the glass to become thinner.  At normal kilnforming temperatures, the 6mm rule applies, at higher temperatures it does not.

Further information is available in the e-book: Low Temperature Kilnforming.

Slumping a Form Flat

There are a variety of reasons that you might want to make a formed piece flat again for another kind of slump or drape.

There are lots things you think about when preparing to make a shaped piece flat.

I am going to assume there are no large bubbles in the piece.  You can see the posts  Large bubbles and Bubble at bottom  on the causes.

The following comments are things in five groups to consider when contemplating flattening an already formed shape.

Shape/form

• ·         Shallow forms with no angles have the fewest difficulties.  Take it out of the mould, put it on the prepared shelf and fire to the slump temperature.  Observe when it is flat and proceed to the annealing.
• ·         Forms with angles or multiple curves are a little more difficult.  If the piece has stretched in some areas to conform to the mould, you will have some distortion in the pattern and possibly some thinner areas.  It should be easy to flatten pieces on a prepared shelf with the same schedule, but a slightly higher top temperature as used to slump it.
• ·         Forms where the sides have pulled in will become flat, but continue to have curved sides.
• ·         Deep forms are possibly the most difficult.  The glass may have stretched, giving thin areas.  It may be that the process of flattening the glass will cause a rippled effect as the perimeter of the piece is a smaller size than the original footprint.  These deep forms are the least likely to flatten successfully.

Orientation

• ·         Which way up? Upside down or right side up?  Shallow forms are easiest to flatten by placing them right side up on a prepared shelf.  For deep or highly formed pieces, it may be best to put it upside down to allow the now higher parts to push the perimeter out if it is necessary.

Thickness

• ·         Thick glass will flatten more quickly than thin glass, so you need to keep a watch on the progress of the work to avoid excess marking of the surface of the glass.
• ·         Very thin pieces are likely to develop wrinkles as they flatten.  Even if they do not, there will be thick and thin areas which might cause difficulty in subsequent slumping.
• ·         Tack fused pieces are likely to tend to flatten at different places and times due to the differences in thickness and therefore weight. This makes shallow forms easier to flatten.

Temperatures

• ·         In all these processes, you should use the lowest practical temperature to flatten.  This means that you will need to peek at intervals to see when it is flat.
• ·         Your starting point for the top temperature to use will be about 10°C lower than that at which the original was slumped, normally.  The amount of time may need to be extended significantly. The reason for this is to avoid as much marking on the finished side as possible.
• ·         Shallow forms and thick pieces will flatten more quickly than others, so a lower temperature can be used.  You will still need to observe the progress of the flattening.
• ·         Angled shapes and deep forms will need more heat and time than the shallower ones. 
• ·         Thin pieces may require more time than thick pieces.
• ·         Tack fused pieces need more attention and slow rates of advance to compensate for the differences in thicknesses.

Separators

• ·         Kiln washed shelves are usually adequate for flattening.
• ·         Thinfire or Papyros are needed when flattening upside down to ease any sliding necessary.
• ·         Powdered kiln wash or aluminium hydrate can be dusted over the kiln washed shelf when it is felt the form will need to slide on the shelf while flattening.

It may be that after all this, you feel it is not worth it to flatten.  It certainly is worth the effort, if only to learn about the characteristics of the form and its behaviour in reversing the slump or drape.

CoE Varies with Temperature

Information from Bullseye shows that the Coeficient of Linear Expansion changes rapidly around the annealing range.

The following is from results of a laboratory test of Bullseye clear (1101F)
Temperature range.......................COE
20C-300C (68F -­ 572F).................90.6
300C-400C (572F - ­752F).............102.9
400C-450C (752F - 842F).............107.5
570C-580C (1058F-1076F)............502.0

Bullseye glass is probably typical of soda lime glasses designed for fusing.

The change of CoE by temperature is further illustrated by Kugler (a blowing glass) who state their CoE by temperature range. Remember CoE is an average expansion over a stated range of temperatures)
CoE 93 for the range 0C-300C
CoE 96 for the range 20C - 300C
CoE 100 for the range 20C - 400C

The extension of the range by 100C has a distinct effect on the average expansion over the (larger) range. 

This shows why it is not helpful to refer to CoE without also mentioning the range of temperature.

In addition, here is an illustration of the effect. 

(If the owner of this illustration comes across this, please let me know, as I have lost the source)

Devitrification on Ground Edges

The first element in preventing devitrification is cleaning.  Making sure all the edges of the glass are clean will help.  OK, you have cleaned the edges well after grinding. You still get detrification, so you want to know

Why do ground edges get devitrification? 

To answer this question, you need to think about how glass behaves in the kiln. As it heats up the glass expands, pushing the cut edges into the separator on the shelf. The pits caused by the grinding have not yet become fire polished.

When the glass retreats on cooling the pits in the edges of the glass, although very small, pick up some of the separator. These small particles act as the nucleation points for the crystallisation of the glass which is generally called devitrification.

The glass of a single 3mm layer retreats further on a single piece than on a 6mm piece. This rolls the devitrified glass upward onto the upper edge of the piece.

Prevention of devitrification of the ground edge is to have the pits in the glass edge finer than the particles of the separator. This is more than just washing the glass immediately after grinding to remove the glass powder from the grinding scratches.  Yes, this will reduce the chance for devitrification, but not totally prevent it.  As noted above, the pits in the glass will pick up particles of separator on expansion, giving nucleation points for the devitrification.

Further coldworking beyond the initial grinding is required to reduce the devitrification possibilities.  This involves using finer grinding bits or smoothing by hand with finer grits.  This does not have to take long, as the shape has been achieved by the grinder.

The logic of prevention is to have the glass edge smoother than the particle size of the separator, so the finer and smoother the separator, the smoother the surface of the glass edge must be.  

But my devitrified edge was on top of other glass

The follow-on question is about why devitrification occurs on ground edges that are not near the kiln shelf.  There are two elements to consider.

It is claimed that the fumes of the binder burning off can settle in the pits of the ground glass, providing those nucleation points for the glass crystalisation. The suggested solution is to vent the kiln to about 400C to allow the combustion fumes out of the kiln rather than keeping them inside the kiln.

The second and more certain element is that the grinding creates microscopic pits and fractures in the glass where the powder from grinding settles.  Almost no amount of cleaning will completely remove this residue from the tiny pits and fractures resulting from grinding. 

There are at least two solutions to this cleaning problem. Don't grind unless absolutely necessary - groze instead.  The second is to lightly cover any ground edges with clear powder frit.  You could of course consider ultrasonic cleaning or power washing, either with a dishwasher, or outdoor power washer.  Both these seem to be so completely out of proportion to the problem, that I have never used them.

Firing Rates

Top temperature is, to a small extent, variable between kilns, even from the same manufacturer.  But it is a small part of variations in top temperature required to get the same results in differing kilns.

An example of a firing schedule

It is, more importantly, a function of how the heat is put into the glass. Firing as fast as possible to the top temperature does not allow all the glass to be at the same temperature. This is because glass is a good insulator and the transfer of heat from the top or the sides is relatively slow.  For small things, you can fire very fast, as there is a small mass of glass to absorb the heat.  But a speed of 250°C is fast enough for anything more than 100mm square and at least two 3mm layers thick.  (Thicker glass requires slower rates of advance as surprisingly do single layer projects).  The slower rate of advance allows the glass to be all of a similar temperature from top to bottom, allowing the desired effect to be achieved at lower temperatures or shorter soak times. 

For example, a slower rate of advance will give rounded edges at shorter soak times than a rapid rate of advance will require.  Alternatively, it might require a lower temperature with the same soak time.  Keep in mind that, in general, lower temperatures with slower rates of advance, give better results.

The faster your rate of advance, the more the glass lags behind the air temperature (which is what pyrometers are measuring). Therefore, a reasonable pace will give better results than the as fast as possible rate of advance. 

In short, the variations in top temperature required and length of soak is not about the kiln firing cooler or hotter as much as it is about the firing rate.

The 6mm Rule - Kiln Forming Myths 11

Glass always wants to be 6mm thick

This is true only at some temperatures.  

The surface tension or viscosity of the glass, together with gravity determines the extent to which the glass will thicken or thin.  The viscosity of glass is such that at high temperature tack and full fusing heats, the glass does tend to become 6mm - 7mm thick. This is taken advantage of in kiln forming to obtain rounded edges, and in making frit balls.  A single layer of frit up to about 10mm will become a round dome due the action of the viscosity and weakness of gravitational forces acting on a small mass. 

Larger pieces of single layer glass begin to shrink as the viscosity is great enough to overcome gravitational forces to allow thickening at the edges.  This causes dog-boning.   At the same time the glass is thickening at the edges, it is thinning in the interior allowing large bubble formation on thin pieces. It also is the cause of the needle points on thinner pieces at higher temperatures.  The glass is soft enough to conform to any imperfections in the surface and so be stretched thin as the main mass of the glass contracts. 

This contraction also applies to low mass items such as frit in casting moulds.  The glass particles contract to form a single mass of material, leaving some stuck to the mould. These pieces may be completely separate as tiny frit balls, or if attached to the main mass, a series of needle points on the edge of the finished piece.

However, the viscosity at full fuse temperatures is not great enough to keep thicker glass in its original shape.  So the effect of gravity on glass of 9mm or thicker overcomes the weakening viscosity force and the stack begins to expand. The extent of the expansion is the result of both viscosity (heat dependent) and gravity (mass dependent).

At lower temperatures, the viscosity is much greater.  This can be used for low temperature tack or laminating temperatures. The glass can be adhered with heat without distortion of the single layer, as the viscosity is so high the glass does not change shape, even retaining sharp edges, although stuck together.

At temperatures above full fuse the viscosity decreases further allowing the glass to flow.  This is used in casting, blowing, and various higher temperature processes, such as aperture melts and stringer formation.  Here the viscosity is low enough to allow gravity to make thin and elongated shapes.

There is a range of temperature above which glass will thin more than the 6mm – 7mm “rule”.  I do not know the exact correlation between temperature and thickness, but at around 1150°C  the glass will become only a little under one mm thick.  This can be seen from the results of kiln runaways. The glass that is melted onto the surface of the shelf is extremely thin, showing that the viscosity was so low that gravity was able to thin it to a fraction of what we think of as normal thicknesses.

The 6mm myth arises from the behaviour of glass at a specific heat range and is the result of the combined forces of viscosity and gravity.  Knowledge of how these interact can enable you to understand the outcome of various projects.  This knowledge of the forces can be used to help create the effect you want.  It also enables you to employ various means to counteract the natural forces of gravity and viscosity. 

More information is in the e-book: Low Temperature Kilnforming.

Writing a Schedule

Making your own Schedule

I've been asked about making a schedule rather than using a pre-programmed one. My response is this, but please join in with amplifications and questions.

In principle, a firing schedule for glass follows these stages:

1 – a gradual, steady heat up to a temperature above the annealing point to avoid thermal shock

2 – a soak or slow rise around the slump temperature to allow any air to escape

3 – a more rapid rise to top temperature to avoid devitrification

4 – a rapid fall in temperature to an annealing soak, saving time and avoiding devitrification. The soak at annealing temperature is to equalise the temperature throughout the glass

5 – a steady slow fall in temperature to well below the lower strain point to complete the annealing

6 – a controlled cool to near room temperature to avoid thermal shock.

The details of schedules can appear complex, but the purposes of these six stages are reasonably simple.

Segment 1 is to heat the glass evenly without causing it to break from too fast an increase in temperature. At minimum this steady increase in temperature must continue to about 40ºC above the annealing point. (This will be about 540ºC)

Segment 2. This segment can include a “bubble squeeze” to enable air to get from between sheets of glass before the edges seal, or it can be a separate segment in your schedule. The slow rise in temperature will occur from about 600ºC to 680ºC. The bubble squeeze soak occurs at around 660ºC to 680ºC. In both cases there is normally a soak of half an hour at least at the end of the range.

Segment 3 is to go through the devitrification range (say 700ºC to 760ºC) as quickly as reasonable, but usually no faster than 330C per hour.

Segment 4 is to get back through the devitrification range to the annealing soak, which will be as long as required to equalise the temperature within the glass. This soak time increases exponentially with the thickness.

Segment 5 is the annealing cool, which should be a slow steady fall in temperature to ensure the glass all cools at the same rate (to around 370C).

Segment 6 continues the cool, although faster than previously, and often is achieved by turning the kiln off and leaving it closed until room temperature.

A schedule for a 6mm piece up to 2/3 the size of your kiln could be even simpler:

Segment 1 - 220 dph to 670C for 30 minutes

Segment 2 - 330 dph to 800 (flat fuse) for 10 minutes

Segment 3 - afap to 516 for 30 minutes

Segment 4 - 80 dph to 370, no soak

Segment 5 - off

You may find a schedule that will work, but you still need to know why it works, or at least what each segment is doing. So, for example, you need to think about what a 15 minute soak at 225C will do. What is the glass doing at that temperature? What do you want to achieve in that temperature range? Is there another way to achieve your objective? These are the kinds of questions you need to think about so you can construct your independent schedule when you move outside the parameters of the pre-programmed schedules.

To make a schedule for yourself can be worrying. But you can see from this example that it does not need to be complex. The principles are simple, although the details can be confusing. It is essential to know something about how heat affects the glass and this Bullseye Tech Note is one of the best descriptions. 

Knowing what the heat up events are is useful too. 

Effects of Multiple Layers

Stacking layers of glass fully or partially over the base layer has significant effects on the firing of the whole piece.

Glass is a poor conductor of heat, so you need to be careful to allow the heat to penetrate to the base layer to avoid thermal shock. There also is the effect of the (very small) insulating space between each sheet. The effects of multiple, even layers can be seen from this table based on Graham Stone's* work:

3mm layers

1 sheet – Initial Rate of Advance =1000ºC to 475ºC (less than half an hour)

2 to 3 layers – IRA = 240ºC to 475ºC (ca. 2 hours)

4 layers – IRA = 100ºC to 475ºC (4.75 hours)

6 layers – IRA = 25ºC to 125ºC, then 30ºC to 250ºC, then 40ºC to 375, then 50ºC to 475 before 150C to top temperature (ca. 15.5 hours)

This shows the dramatic effect increasing the number of layers has on the firing schedule to make sure the heat gets to the bottom sheet evenly. If you compare the initial rates of advance (IRA) with the same thickness, but fewer sheets you can see the space between layers is important.

6mm layers

1 sheet – IRA = 320ºC to 475ºC (ca. 1.5 hrs)

2 layers – IRA = 240ºC to 475ºC (ca. 2 hrs compared to 4.75 hrs for 4 layers of 3mm)

3 layers – IRA = 200ºC to 475ºC (ca.2.5 hrs compared to 15.5 hrs for 6 layers of 3mm)

These are the fastest safe firing speeds for evenly covered sheets. 

This difference in firing times for stacks of thicker glass, shows how important it is to fire sections of the stack before the final firing of all the layers together.  It also reduces the risk of bubbles developing within the stack. 

If you are thinking of tack fusing with thicker and thinner areas, you need to take account of the differences in thickness in the various areas of the piece when preparing your schedule. You will need to decrease your IRA by quite a bit. So you might want to be thinking of firing some of your pieces to be added to the base layers before tacking them in an additional firing to reduce the risk of thermal shock to the base layer.

* Firing Schedules for Glass; the Kiln Companion, by Graham Stone, ISBN 0646 39733 8

Annealing - Physical Changes

Physical changes of Glass at the Annealing Point

What happens at the annealing point and what is its relevance to compatibility? There are two main changes that occur – physical and chemical. They both affect the temperature of the annealing point, but in different ways. These notes are an attempt to understand these changes and how they affect compatibility.

The first requirement is to understand what the annealing point is. First it is a range of temperature during which the glass transforms from a liquid to a solid. It has a definition:

The annealing point is the point at which the material reaches the glass transition temperature. It occurs in a temperature region at a point where stresses can be relieved in a very short time. It is defined mathematically by a specific viscosity. In simple terms, this is the temperature below which viscosity prevents any further configurational changes.

Any contraction beyond the transition temperature range is due only to the lower kinetic energy of the groupings of the tetrahedra molecules. Thus, the compatibility of the glasses is determined at the annealing range as a combination of expansion/contraction and viscosity at the annealing range of temperatures rather than at the lower CoE which is more suited to crystalline solids. The transition temperature of a given “glass composition” depends both on its constituents and upon the rate of cooling.

The physical changes of glass during the transition/transformation range of temperatures are various:

• Viscosity has a very large increase with temperature reduction, but without any discontinuity. Viscosity has an enormous effect on the activity of molecules in glass. As the glass cools below its transition temperature it causes the progressive immobility of the molecules.
• The expansion rate (CoE) shows a relatively sudden change around the annealing temperature. Below the annealing point, the glass expansion and contraction behaves much like the CoE at the lower, measured temperatures. This means viscosity may be the most important element in creating a stable fusing compatible glass.
• The amount of heat required to increase the glass temperature rises quickly rather than the previous regular heating rate needed to achieve unit changes.
• The shear modulus changes rapidly, making the glass much more brittle below the annealing point.
• The rate of heating or cooling can affect the exact temperature at which the glass transition point occurs.

The annealing phase (glass transition) is a dynamic process where time and temperature are to some extent exchangeable. This allows annealing to occur at the lower part of the range of the transition phase, but the glass then needs a slower cool from there. From the (higher) annealing point temperature - as defined by viscosity - the cool can be a little more rapid than at the lower temperature range of the transition phase. The anneal at the lower part of the transition saves annealing and cooling time for thick slabs, but for thinner pieces (less than 9mm), soaking at the annealing point and cooling from there is the simpler process.

Slow cooling results in a lower transition range because the tetrahedra forms of the molecules have more time to rearrange (to the degree that this is possible). This slower cooling results in tighter packing of tetrahedra as the mass reaches its transition range. When the glass reaches room temperature, its volume will be smaller when cooled slowly than glass melt which has been cooled rapidly. Hence, slower cooling from the melt results in a denser glass.

Reference: http://glassproperties.com