Borax Characteristics

Borax is a glass making flux used to reduce the melting temperature of glass. 

It is almost colourless - grey, white, or yellowish; seldom bluish or greenish; and colourless in transmitted light.

The chemical composition of Borax is:  Na₂(B₄O₅)(OH)₄ · 8H₂O

It has a hardness rating of 2 – 2.5, about half that of glass at approximately 5.5.

The melting point is 878°C. At this temperature borax dissolves numerous metal oxides. In spite of this high melting temperature, it acts as a flux reducing the softening point at the surface of the glass at kiln forming temperatures.

The specific gravity of borax is approximately 1.7, considerably ligther than glass at ca.2.5.

Borax is sparingly soluble in cold water, although readily soluble in boiling water. It is insoluble in ethanol.

Description of devitrification
Temperature range

Devitrification spray

Does Wider Foil Give Greater Strength

The strongest part of a stained glass panel, whether leaded or copper foiled, is the glass.  The weaker points are the matrix that holds the panel together.

Of the matrix, the solder is the strong part.  The copper foil is weaker (and much thinner) and the adhesive is the weakest part of all.

A wider bead gives more apparent strength, but on the surface. It provides a broad line to grasp the glass.  But wide beads are often not what is visually desirable, nor practical.  And the wider the bead, the more solder will be used.

The most important part of a panel is the thin fin of solder between the top and bottom of the solder beads.  This is the connector between front and back. The strength of the whole panel depends on that fin.  So, it could be argued that very closely fitting foiled pieces lead to a weaker panel than loosely fitting ones.  I would not argue that, but it is important to have that connector of solder between the surface beads for a panel to be strong.

The solder connects the two solder beads together and forms the matrix which holds the panel together.

Here the fin of solder is a little thinner, so the matrix is marginally weaker

For larger panels, reinforcement will be required, either between the glass pieces, or on the surface.  The fact that reinforcement is so often used in the gap between the pieces, is confirmation that the fin of solder between the front and back is very important.

Cutting Fused Glass

The same principles of glass cutting are applied to fused glass as to the glass used to make the fused piece.  The differences relate to thickness and variations in thickness.

You still score on the smooth side.  More pressure is not required.  For glass thicker than 6mm, you may wish to use a wheel with a more blunt angle. as shown in this illustration.

www.oaklanddiamondtools.com

The main reason that people may feel it is difficult to cut fused glass relates to the additional thickness.  Just as breaking 4mm glass requires more force than 2mm, breaking a 6mm piece requires more force than a 3mm piece.  

floridastainedglass.net

Properly adjusted metal cut running pliers can do the job, but a cut runner designed for thicker glass can be a boon.  They are designed to provide greater leverage and so more force to the glass breaking.  With these the glass breaks along the score line cleanly.

www.glass-tool.com

The breaking of glass that is uneven in surface levels, as in tack fused pieces, can be more difficult.  One is that running the cutter over glass with distinctly different thicknesses can be difficult.  Maintaining consistent pressure and speed over the bumps of the tack fused pieces is difficult.  The second is that the running of the score will not always follow the score line.  For example, if the score line runs close to the edge of a thick piece, the break is likely to skirt around the thick piece, and possibly off to the edge of the piece, rather than continuing to follow the score line. Planning the score line on tack fused glass is important to avoid trying to break near the edges of thick pieces.

One possibility is to score the glass on the shelf side.  This is certainly possible, even though the surface is rougher.  It does avoid scoring across different levels and makes the break along the score line more probable.

Baffles in Side-Fired Kilns

The object of using baffles in side fired kilns is to keep the direct radiant heat from the edges of the piece(s) being fired.  If the edges receive direct radiant heat, they increase in temperature more rapidly than the interior of the piece.  This means the edges become sticky and seal before upper layer of the interior begins to conform to the lower layer.  This seals air into the piece.

fusedglass.org

The materials and placing of the baffles is important.

Baffles can be made from almost anything that can withstand the heat of the firing.  There is an argument that light-weight materials such as fibre board, vermiculite board, or fibre paper should be used to reduce electricity costs.  Heavier pieces such as brick and kiln shelf pieces require more energy to heat them up.  They then of course, store heat that needs to be released on cooling, so slowing the cool down and increasing the risk of devitrification.

The placing of the baffles is important too.  Baffles protect the glass edges from radiant heat until the general heat of the kiln can come into effect over the whole of the piece.  This means that if the baffles are placed against the elements at shelf level, the element above can still give radiant heat to the edges.  Therefore, baffles placed near the glass are better.  They protect the edges from radiant heat at whatever level the side elements are placed.  This is more important for pieces that are further from the edge of the shelf, than those nearer the edge, as the centrally placed glass can “see” the radiant heat from the upper elements. 

Anneal according to the Number of Firings - Kiln Forming Myths 17

You should anneal longer each time a piece is fired.

It seems the idea is that each time you do something to a piece the risk of poor annealing increases. So, an increase in annealing soak will reduce that risk.

In fact, you should anneal for the thickness of the piece plus any additional complicating factors introduced.  No further annealing based purely on the number of times fired is required.

If you have merely added a layer of powder to the glass, you only need to anneal for the thickness of the base.  It is when you begin to add significant amounts of frit or other glass pieces that you need to increase the annealing soak and also slow down the annealing cool.  If you go on to complicate matters by only tacking those elements to the base glass, you will need to slow down even more.  This blog post gives some indicators on how much you need increase the soak and slow the cool.

Score coated glass on the back - Kiln Forming Myths 16

Cut iridised glass on the back

The idea seems to be to get a more even score and avoid chipping of the iridised surface.

First, the iridised surface is almost microscopic in thickness.  It is put onto the surface as a mist of metallic oxides as it begins its run through the annealing lehr.  This thickness will not affect your scoring.

The back is usually rougher side of the glass and so will be more difficult to get a smooth, even score than the front iridised surface.

Chipping of the iridised surface is caused by too much pressure during the scoring.  Reduce your pressure and review your scoring practice.

Scoring the iridised surface with appropriate pressure will produce a clean break without chipping the surface.  These comments apply to dichroic and flashed glass too.

Elevation of Moulds - Kiln Forming Myths – 15

All moulds must be elevated to allow air out from between the glass and mould.

This is not a big problem, as it will not create any problems, but it does show a lack of thinking about the mould itself. 

There are some things you need to check.

Are the holes in the mould at last touchdown point(s)?  Sometimes the vent holes in moulds are made at convenient points rather than at the places where the glass will last touch down in the mould.  On a simple ball mould, a hole at the centre will be appropriate, as this is the last place the glass will touch.  On a bowl with a square base, the last places the glass will touch are the corners, so that is where the holes need to be.

Are there holes in the side of mould to allow air out from under the mould?  If there is one or more, there is no need to elevate the mould.  The air will move out from under the mould through the hole in the side. In general, moulds are not so uniform on their base that they fit the shelf enough to seal the displaced and expanding air underneath the mould. 

Are the holes clear?  This is more important.  If the vent holes are not open due to kiln wash or other things blocking the space, there will be no escape for the air.  The vents need to be checked on each firing to ensure they are open.

Does the mould need holes at all? There are a number of shallow slumpers and other simple moulds, such as a wave mould or any cylindrical mould form, that to not need vent holes, either because they are so shallow, or because the air can escape along the length of the mould.

How Much Annealing is Needed - Kiln Forming Myths 14

You don’t need to anneal as much on initial firings as on the last firing

This seems to be based on the theory that only the last firing matters to the soundness of the piece.  However, re-firing a poorly annealed piece may be at too great a risk from this kind of ill thought-out practice.

Even for properly annealed pieces, there is risk at each re-firing of a piece from thermal shock.  The rate of advance on the second firing of a fused piece needs to be reduced to accommodate the now thicker piece than the two or more original pieces.  Re-firing of tack fused pieces needs even slower rates of advance, because of the uneven thicknesses.   

The risk on each heat up to poorly annealed objects is even greater than indicated above for well annealed pieces.  Poorly annealed pieces can’t absorb temperature changes as easily, so are at greater risk of heat shock during temperature increases.  

When considering whether to reduce the annealing at the early stages of your project, you need to consider at least: 

• time saved versus risk, 
• the size of the piece,
• The degree of fusing.

The time saved will depend on how you fire.  If you fire overnight, there usually is no saving in time, so why increase the risk of breakages?  Time savings normally relate to how many firings you can get out of your kiln in a day.  So the number you need is an element in thinking about the risk too.

If firing during the day when there might be time savings, consider the size.  Smaller sizes usually can survive being under-annealed more easily than larger ones.

The degree of fusing is an important element in the risk.  The closer you are to a lamination fusing, the greater the risk of under-annealing causing a break on the next firing.  A full fused piece is more likely to survive an inadequate annealing on the subsequent firing.  These factors indicate that it is less advisable to under-anneal tack fused pieces that are going to have several firings.

You also need to be careful about applying the practice of blown glass workers to flat glass.  Blown glass, because of its curved shapes can withstand more stress than flat glass.  Although blown glass pieces can be under annealed before going through another process of heating, it is more difficult to do this with flat glass.

If you decide to reduce the annealing process at the early stages of a project that must be fired many times you need to be careful to avoid breaks. To avoid thermal shock of inadequately annealed pieces, the rate of advance must be reduced.  This reduction should be at 75% or less of the normal rate of advance for a piece of the size and nature of your project.

To have the best chance of survival throughout the kiln forming process, each piece needs to be properly annealed every time the temperature rises to or beyond the annealing temperature.

Peeking into the Kiln at Low Temperatures - Kiln Forming Myths 13

Do not peek between 100C and 540C – it will break the glass

Not necessarily.

To think about this systematically, you need to remember that the temperature readout is of the heat in the air, not of the glass.  On the way up, the temperature of the glass will be less than that of the air. On the way down the air temperature readout will be lower than the glass.

You can see from the readout how quickly the air temperature recovers to the original temperature.  This is generally, more quickly than the glass can lose its temperature.

The glass will be increasingly brittle as the temperature falls below the annealing point. 

The risk of thermal shock increases as the difference in air and glass temperature increases.  So shock is likely to be less at a readout of 100°C than at 400°C. 

The risk of thermal shock also increases with the thickness of the piece.  A piece of 25mm is more likely to be shocked at any given temperature than one of 6mm.

Whether you can peek depends on several things:

·        Temperature – e.g., just above the annealing soak a quick peek is less likely to cause problems than one at a lower temperature.  The shape of the glass will not change significantly below 600°C, so a peek while the kiln is cooling to the annealing point will not affect any but very thick pieces.

·        Length of peek – The key element is peeking is to affect the temperature as little as possible. So the opening should be as brief as possible.  The essential element in peeking is to take a mental snapshot of the glass, close the peep hole or lid, and think about what you saw.  Do not look or stare while the kiln is open. 

·        Size of opening – The smaller the opening you can manage during the peeking, the less risk of shock.  This is because less relatively cold air can enter the kiln.

·        Use of peep holes – set up your piece in the kiln in such a way that you can see your work through them.  At lower temperatures you will need the assistance of small intense light to illuminate the work.

·        Thickness – remember that thicker work is more likely to thermal shock because of the slow transfer of heat from the internal parts of the piece.  Peeking needs to be more cautious as the thickness increases.  Again, peeking above the annealing point should tell you everything you need to know about the final shape of the piece, making peeking in the brittle range unnecessary.

It is a good idea to minimise the viewing of your piece below annealing, but it is not impossible, if you follow the principle of avoiding drastic temperature falls during your peeking.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Slumping and Draping materials - Kiln Forming Myths 12

Always slump into ceramic, drape over steel

This myth is based on the fact that steel expands and contracts more than glass and ceramic expands and contracts less.

So, the myth goes, slumping into steel means the glass will be trapped or crushed by the contracting steel.  But draping over means the steel will contract more than the glass making the removal of the glass easy.

The reverse is the expectation for ceramic.  Slumping into the ceramic allows the greater contraction of the glass to be removed from the mould without sticking.  But draping over means the glass traps itself against the ceramic as a result of its greater contraction.

These things are true.  But….

The most important thing in considering a mould is the draft.  This not about cold air, but the angles of the mould. A mould with vertical sides will not release the casting or kiln formed object even if the expansion characteristics of the two materials are identical. To release, the mould must have a slight angle from the vertical away from the glass.  This applies whether a slump or a drape.  This is called a positive draft, as illustrated.

www afsinc org

 And here

If the draft is sufficient, it does not matter whether you are slumping or draping into steel.  In using a stainless steel mixing bowl for draping, you can only use the lower portion where the angle is shallow.  If you rest the glass on the rim, the draft will be too steep to allow the glass to slide upwards as the steel contracts on cooling.

www evetsourcesolutions com

Even when draping over steel, you need to have a draft to aid the easy removal of the glass, as in this example:

creativeglassguild co uk

When draping over ceramic, you need to be careful that you have sufficient draft over the whole of the mould. In the case of this ceramic draping mould you need to make sure the glass is not fully formed as the steep portion at the top will be where the glass grabs the mould.

glassartbymargo com

And if you were to use this casting mould as a slumping mould, the steep straight sides would make it difficult to get the glass out of the mould. 

sundanceglass.com

Although the facts behind the statement “slump into ceramic, drape over steel” are established, you need to understand that the draft of the mould is as important as the way in which you use the material.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

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.

Absolute Firing Temperatures - Kiln Forming Myths 9

There is a given temperature for each level of fusing – slump, tack, full, etc.

You will often see statements about the temperature for achieving a particular effect.  It is as if all glass under all circumstances does the same thing at a given temperature. These temperatures can only be understood in relation to several things.

• ·         Kiln characteristics
• ·         Speed of firing – i.e., heat work
• ·         Time at forming temperature

The relevant factors about the kiln are:

·         Insulation.  The two main types of insulation in kilns are fibre blanket and insulating brick.  Fibre blanket is often the main insulating element in kilns as it does not absorb a lot of heat. It of course loses heat more quickly than refractory brick.  Most often the floors of kilns are made of brick for rigidity and resistance to damage.  (They also can be replaced individually if one is damaged.)  Refractory brick comes in two densities.  The light weight one is not rated to such a high temperature and loses heat more quickly than the higher temperature rated dense brick.  Both lose heat much more slowly than fibre blanket.  This means the top temperature can be reached more quickly in a fibre insulted kiln than in brick insulated kilns. The brick insulated kilns radiate the heat back into the kiln upon cooling, making for long safe anneal cools without much effort in controlling the cooling rate. Thus the temperatures for an effect are different for kilns with bricks all around than with fibre blanket, and no comparison is easy between kilns with different insulations.

·         Size.  The size of the kiln has an effect on the temperature cited to achieve an effect.  A small kiln can heat up very rapidly, but the glass cannot heat evenly as quickly.  A large kiln takes more time to heat up, as there is more insulation absorbing the heat input.  So working temperatures for small and large kilns are different.  The size of the piece(s) of glass also have an effect.  Small pieces can be heated much more quickly than large or thick pieces, so the top temperature for an effect will be different for the two sizes.

·         Temperature variation across the kiln shelf affects the rate of firing possible and (as noted later) will affect the top temperature.  The more even the heat the faster it is possible to go and that affects the temperature chosen.

·         Element placement.  Some kilns have only side elements, some only top elements, and some have both.  All these variations affect the temperature required to obtain an effect.  In general, top fired kilns can be fired faster than side fired kilns.  Kilns with both, require an intermediate rate, unless the side and top elements can be fired independently.

Speed of firing, i.e., heat work

·         Heat work factors make the top temperature different in different circumstances.  This is mainly about the speed at which you fire the glass.  Generally, the slower you fire, the lower temperature you need.  Allowing the glass to absorb the heat gradually usually means that you can achieve a particular effect at a lower temperature.  A fast rise in temperature requires a higher temperature.

Soak times 

·         The amount of time you soak at the working temperature will also affect the temperature chosen.  A longer soak allows a lower temperature to be used (although that can get into the risk of devitrification from spending too long at the top temperature – it is a balancing act).  A higher temperature can be used to keep the soak time reduced. 

All these variables mean that without being given the kiln characteristics and a schedule, you cannot evaluate the temperatures and rates of firing that are given out by others.  You need to know how closely their kiln fits with your kiln in its characteristics as outlined above.  When asking for a temperature or a schedule, you should indicate what kind of kiln you are using.  You need to know in any schedule what the ramp speeds are and the soak times.  They can then, of course, form the basis for your experimentation.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Exceptions to Slow and Low - Kiln Forming Myths 8

The principle of slow and low always applies.

Although the principle of attempting to get the effect you want at the lowest possible temperature with the slowest practical rate of advance should always be considered, there are times when it is not wholly applicable.

Among these are when working with small scale pieces, such as jewellery, and in general pieces below 100mm that are at least 50mm from the side of the kiln.  In these cases you can fire much faster, as the heat has less distance to travel through the glass to maintain an even heat.  You still should be using two stages – the first and slower to rise to the strain point and the second much faster one to reach the top temperature.  In these cases the target may have to be a little higher than in a larger, slower firing.

Another case is in fire polishing.  Fire polishing can often have a fast segment to avoid distorting the piece.  In this case you fire appropriately slowly for the thickness of the piece until you are past the upper strain point.  This can usually be taken as 540°C.  (For float and bottle glass the temperature is around 690°C).  As you have passed the brittle phase of glass by this time, you can advance the temperature quickly.  The objective is to achieve enough heat to change the surface, but avoid heating the interior to the softening point.  You may want to observe the finish of the surface, so that you can switch to the cool down phase of the firing as soon as the polish is achieved.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Soak ca 550C - Kiln Forming Myths 7

A soak at mid-500's °C can soften the glass to enable sealing against the mould.

The only glass that this might apply to is lead crystal.

The idea seems to be that it is possible to fire the glass in such a way that it conforms to the edge of the mould, so trapping air inside before the glass begins to slump. It goes on to indicate this is a cause of large bubbles from under the glass. This trapping of air is possible of course.  But not at a temperature of ca. 550°C.  Although this is above the strain point of fusing compatible glasses, it is not in the range where most glass begins to soften.

Bullseye, Spectrum, and Uroboros all only begin to bend near 600°C.  This means there is no possibility of the glass conforming to the edge of the mould at 550°C before it begins to slump.  The softening point of Float glass is around 720°C, and although with a large span, a 4mm piece will begin to bend at about 610°C (thicker glass will begin to bend earlier), it takes up to an hour for the glass to take up a gentle bend at that temperature.

It is possible that you can seal the edge of the glass against the mould in a side-fired kiln that is quickly fired. This is where the edge receives greater radiant heat than the top surface.  However, I doubt that this will happen even in a side fired kiln until nearer 600°C.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Bubbles are Art - Kiln Forming Myths 6

Big thin bubbles are art

Unless you have designed the bubbles, they are mistakes, not art.  Even when designed, they are delicate and when broken are very sharp.  So, they cannot be sold or used as they are even by yourself. 

Bubbles within the glass in a plate.

fusing101.files.wordpress.com

People frequently make the suggestion that the bubble should be broken and the cavity filled with frit.  Of course this can be done, but almost always appears a fix rather than a design choice.

The more important thing is to learn the cause so it can be prevented in the future.  Bubbles can be between layers or from underneath the whole piece.

Bubbles between the layers of glass are usually the result of inclusions or layup and firing rates.  Anything which holds the upper layer above the lower one has the potential to induce bubbles.  Most often, with a bubble squeeze, these are relatively small and are 2mm or more thick.  These may be acceptable or seen to be unsightly, but are not dangerous.  The bubbles can become large and/or thin with high temperatures or fast rises in temperature.  Be sure to have a good bubble squeeze, and a moderate (ca. 300°C) rise in temperature from there.

Bubbles can also rise between the shelf and the glass.  This happens most often when firing single layers above a low temperature tack fuse.

A single layer piece with large, burst, healed and emerging bubbles.

www.warm-glass.co.uk

It can also occur when there is either debris between the glass and shelf, or when there is a depression in the shelf.  Both these cases allow air to remain trapped between the shelf and the glass.  Slower rates of advance and bubble squeezes can help reduce these, but the shelf needs to be checked for debris and high or low spots.

The piece below is disfigured by the random bubbles at the left and in the centre of an otherwise acceptable platter.

fusedglass.files.wordpress.com

Evaluate your pieces before you declare a single or series of large thin bubbles art.  Of course, you should play around with the piece to learn from the mishap.  You can use the pieces of it in other projects.  But unless it is truly exceptional, it is a mistake, not art.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Short Holds in Schedules - Kiln Forming Myths 5

Frequent short soaks on the way up will make a schedule safer

Safer in this context usually means less subject to thermal shock.  To determine the validity of this requires a bit of understanding on how the glass takes up heat, and as effected the lay-up.

Glass is a good insulator, both of heat and electricity, although we are only concerned about heat here.  This means that glass transmits heat poorly or, as it may be thought of, slowly.  A steady input of heat at an appropriate rate is less likely to shock the glass than quick rises with (catch up) soaks. 

In general, there is not much change in the rate required when you go over to a single rate without soaks.  For example, a ramp rate of 200°C from 20°C to 400°C with a 20 min soak, then 300°C to 540°C with another 20 minute soak could also be written as 193°C/hr to 540°C - both take 2.8 hours to achieve the same temperature. So the rate is not very different, but the way the heat is put into the glass is.

The glass is subject to heat shock below its softening point, and so rapid increases in temperature at the start of the schedule increase the risk of thermal shock below the 540C region.

When you have uneven coverage of the base glass, as most of us do, more care is required than when we have evenly thick glass.  This relates to the poor heat conductivity of glass.  The need is to have all the glass heat up at the same rate.  This is relatively simple when there are no partial layers on top as when doing a decorative tack fusing.  The pieces on top insulate the heat from the glass immediately below.  This gives a cool spot under the top glass, in relation the uncovered glass. To avoid this difference in temperature, which causes stress, becoming too great you need to slow the rate of advance as well as keeping it a steady increase.  This indicates you should be scheduling the rate of increase as though there were two more layers over the base glass.

The steady input of heat also becomes more important with thicker glass or more than two layers of glass.  The rate of heat input needs to decrease rapidly with increasing thickness – there is not a linear relationship.  For example, doubling the thickness from 6 to 12mm requires a reduction of 2.3 times the rate of advance.  Increasing the thickness by 4 times to 25mm requires a reduction of 10 times the 6mm rate of advance.

Other factors that require slower and steady increases in temperature are where you have dark and light glasses next to one another.  The same applies where you have a viscous and a less viscous glass together.  The classic is black, the least viscous of the glasses, and white, the most viscous.

However there is at least one circumstance where soaks are useful.  When draping over steel or ceramic, the free hanging glass heats up more than the centre where it is resting on the mould.  In this case, the mould forms a heat sink, drawing the heat away from the glass into itself.  You need to go very slowly or insert a few soaks to allow the supporting mould to heat up. More information here.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Relationships of bubbles to more holes in moulds - Kiln Forming Myths 4

More vent holes reduce the possibility of big bubbles

The position of the vent holes is more likely to prevent bubbles than simply the number.  A ball mould only requires one at the centre bottom.  A rectangular bowl with sharp curves needs the holes in the corners, not the centre.

The holes in a mould that are intended to allow air to escape should be at the places where the glass will last touch down on the mould.  When placing the holes, you need to think where the glass will last conform to the shape of the mould.

In a square or rectangle mould, the corners are the last places the glass will stretch into.  So the vent holes in the mould need to be there rather than in the centre, or along the straight edge of the bottom.  If it is a square slumper, it may be that there is no actual need for a vent hole, as the curve is gentle, but it is safest to have one at the centre. 

If the firing is too hot or too long in any but gently sloping moulds, large bubbles will be created even though there are adequate or multiple vent holes, as explained by the glass slipping down the mould and pushing the bottom up.


More information on big bubbles is available here.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.