Wednesday, 16 August 2017

Broken Base Layers

Sometimes in fusing, the base layer can exhibit a crack or break without the upper layers being affected.  This kind of break almost always occurs on the heat up.  In a tack fuse, the top layers may still be horizontal and unaffected by the break beneath them.  If a full fuse, the upper layers will slump into the gap, or apparently seal a crack that is apparent on either side.

An example of tack fused elements on top of a previously fused base


This is more likely to be seen where there is a large difference between thicknesses.  If the base is a single or double layer and there are several layers of glass – especially opalescent – on top, there is a greater chance for this kind of break to occur.

The reason for this kind of break is that the upper layers insulate the part of the lower layers they are resting upon.  Glass is an insulator, and so a poor conductor of heat.  This means that the glass under the stack is cooler than the part(s) not covered.  A break occurs when the stress of this temperature differential is too great to be contained.

An example of  stacked glass in a tack fusing

This kind of break can also occur when there are strongly contrasting colours or glasses that absorb the heat of fusing at different rates.  One case would be where the dark lower layer(s) were insulated by a stack of white or pale opalescent glass.  The opalescent glass will absorb the heat more slowly than the dark base.  This increases the risk of too great a temperature differential in the base.

Reducing the risk of these breaks.

Even thicknesses
One way to reduce the risk of base layer breaks is to keep the glass nearly the same thickness over the whole of the piece.  Sometimes this will not give you the effect you wish to obtain.

Slow the firing rate
Another way is to slow down the temperature rise.  Some would add in soaks at intervals to allow the glass under the stack to catch up in temperature.  As we know from annealing, glass performs best when the temperature changes are gradual and steady.  Rapid or even moderate rates of advance with soaks, do not provide the steady input of heat.

This prompts the question of how fast the rate of advance should be, and to what temperature. 

Rate of advance
The rate of advance needs to take account of the thickness differential and the total thickness together.  A safe, but conservative, approach is to add the difference in thickness between the thinner and the thickest parts of the piece to the thickest.  Then program a rate of advance to accommodate that thickness.  E.g., a 6mm base with a 9mm stack has a total height of 15mm.  The difference is 9mm which added to 15mm means you want a rate of advance that will accommodate a 24mm piece.

The rate of advance can be estimated from the final annealing cool rate required for that thickness.  In the example above, the rate would be about 100°C per hour.  The more layers there are, the more you need to slow the heat up of the glass. The Bullseye table for Annealing Thick Slabs is the most useful guide here.

Firing already fused elements
If you were adding an already full fused piece of 9mm thick to a 6mm base, you could have a slightly more rapid heat up, bu not by a lot. This is because the heat will be transmitted more quickly through a single solid piece to the base glass.  It is safer to maintain the initial calculation. If your stack is tack fused, this will not apply, as the heat will move more slowly through the layers of the tack fusing much the same way as independent layers on the initial firing.

The general point is that you need to dramatically slow the speed of firing when you have stacked elements on a relatively thin base.  Even a two layer base can exhibit this kind of break when there is a lot of glass stacked on it.

Wednesday, 9 August 2017

Stretching Lead Came

Stretching lead came is so ingrained into the literature and general thought that it is difficult to regain the purpose of the practice.  But I will try.

The purpose is to straighten the came

The purpose of putting the lead into a clamp and pulling on the other end is to straighten the lead came.  It is much easier to work with a straight came than one that is curved or kinked.  It gives visually straight lines, it provides smooth and sinuous curves without interruption in the line of the curve.

It is said that some came is “pre-stretched”.  This is really the result of alloys contained in some lead to make it stiffer.  It still needs to be straightened before use.  If the lead came is already straight, you do not need to do anything else before using it.  If you drop or otherwise accidentally bend the came, you need to straighten it before continuing.

Stretching can weaken the came 

Pulling on the lead came is not to stretch it, it is to straighten it. Stretching the lead can make it weaker. Lead drawn beyond its structural limits will break.  But you can weaken it before the break. You can test for this weakening of the came by observation. If you see "alligator" marks on the surface, you have weakened the came by putting too much effort into the pull. Straightening the lead must avoid so much force as to weaken the structure of the material.

Straightening not Stretching 

The amount of effort to be put into straightening the lead came is just enough to make it straight. This will vary depending on how straight the came is at the start.  The reason for drawing the lead toward yourself is that you can see as you look down the length when the lead came is straight. If you are pulling vertically, it is more difficult to see when the lead becomes straight. 

If the lead is badly kinked or twisted, it may be best to cut that section out. If you continue to pull to straighten a difficult section, you can weaken the whole length of came.  First, ease the kinks and twists out as much as you can by hand. Then do an initial straightening pull.  This initial straightening pull will show where the problem(s) lie.  You can cut that section out and straighten the remaining pieces without stretching the lead to the point of weakness.

Of course, you must employ some basic safety rules.  Make sure the lead is securely clamped.  In the cleat style lead vices, you can give the lever a thump with the pliers to ensure the teeth are set into the lead before pulling on the other end.  Other vices need to have other ways to ensure that the end is held securely.

The other basic safety rule is that you should brace yourself against any break of the lead, or slip from the vice.  One foot should be placed behind you so that in case of breaks or slips you will not overbalance and fall.  This has the added advantage of ensuring you cannot put your body weight into the straightening effort.

There are other common sense rules, such as gloves, removing obstructions behind you, etc.


Remember that the purpose is to straighten, not stretch the lead came. 

If you are putting your foot on the bench to add force to the puilling of the lead in a vice on the bench, you are putting too much effort into the job and risk falling when the came breaks or slips out of the vice.  If your whole body weight is being used to draw the lead toward you, you are using too much force. If you can see signs of a pattern developing on the surface of the lead, you are using too much force.

Straightening the came is not an exercise in a workout programme.  It is a steady firm drawing force until the came is straight.  If you have to use more than usual force, stop and figure out why.  Cut out the difficult section so you do not weaken the came.  Then straighten the remainder and continue leading.

Wednesday, 2 August 2017

Smooth Surfaces on the Bottom of Bowls

A frequently asked question is how to get a smooth shiny surface to the outside of slumped bowls. There are two certain ways – have the shape blown, or do a free drop. 

Avoid Moulds

In blown glass work the hot glass can be shaped in a cold mould, which means that the glass does not take up all the mould imperfections.  However, the glass must be put back into the glory hole to remove the chill marks from the cold mould.

A free drop is the process where the glass blank is placed over an opening which allows the glass to fall without touching any mould.  You need to observe periodically during the firing to arrest the drop when it is at the stage and shape you want.  You then need to remove and polish the rim that rested on the elevated ring that supported the glass during the drop.

Failing these techniques, you need to use a mould 

The surface of the glass that is in contact with the mould will take up the texture of the mould surface. When the glass is hot enough to take up the shape of the mould, it will be soft enough to take up some texture from the mould. The hotter you fire, the more texture will be imparted to the glass. 

You can minimise the texture of a mould 

Prepare the mould with the smoothest surface you can.  If the shape is simple enough, you can use very fine sandpaper - 6000 grit is useful.  This will give the smoothest possible mould surface.

Use the finest kiln wash you can find to coat the mould. The finer the powder is ground, the less texture is present.  You can also smooth the kiln washed surface with a balled-up piece of soft cloth or tights.  Do this very lightly, so that you do not rub off the kiln wash. Remove the excess powder before firing.

Minimise the temperature

A major way to reduce the texture is to fire at a slow rate to the lowest temperature you can, using a 30 to 90 minute soak. This will give you less texture than a fast rate to a higher temperature with a shorter soak. To determine how long is required at a low temperature , peek periodically to see if the slump is finished.

The principle is to fire as slowly and to as low a temperature as is practical.  This will reduce the chances of marking as long as the glass does not slip down steep sides.  

Wednesday, 26 July 2017

Cutting Hour Glass Shapes

Hour glass shapes, wasp waists, or those that are thinner along the length than the ends, should be avoided as much as possible.  They are difficult to break out from the score.  More importantly, they are an inherently weak shape. The longer the piece is with the narrow part along its length, the more likely it is to break; in cutting or in the long term, in the panel.  However, these shapes are sometimes unavoidable.

The principle to use in scoring and breaking out the glass is to remove less glass than that you are retaining at each stage of the process.

This has consequences: 
  • ·         breaking the first score is the easiest
  • ·         only a rough outline of the final piece should be scored and broken from the sheet
  • ·         Relieving scores and breaks will be necessary.  The number will depend on the relative thickness of the thin and thick parts.

You can make the first score and break of one side of the shape from the main piece of glass – usually with little difficulty or need for relieving scores. (1)

You then should score and break off the piece to be retained from the larger sheet.  Be sure to give a margin for the final piece. (2)

Now score the other part of the hour glass shape.  Do not tap the score. Begin gently to run of the score from each end.  Don’t worry if the runs do not meet up.  Do not tap to make them meet up. (3)

If running the score from both ends is not enough to make the run complete, you will need to use relieving scores.  These scores can be like onion rings – generally concentric curves running in the same sort of shape as the curve to be broken out.  

Or you can use the fish scale approach – overlapping crescents.  These are most useful for deeper inside curves.

Either way, each score needs to be planned.  Each relieving score should be smaller than the width of the piece to be retained.  In general, this means the outer relieving scores can be wider apart.  As you approach the final shape, the distance between the scores will need to be less and less. (4,5,6)

More information on scoring and breaking out concave curves can be found here:

Wednesday, 19 July 2017

Lead Free Solders

Lead free solders have been created in response to concerns about lead, especially in the electronics industry. The following tables present a selection of available solder compositions.  The characteristics of these lead free solders can be compared to the common lead bearing solders in the last table.

Abbreviations for the metals of the compositions:
Ag=Silver; Bi=Bismuth; Cu=Copper; Ge=Germanium; In=Indium;
Sb=Antimony; Sn=Tin; Zn=Zinc

Melting Temperatures of Lead-Free Solders

Alloy  %                     Melting Temperature    Comments
Range (ÂșC)
Sn 65, Ag 25                         233           High strength; patented by Motorola (“Alloy J”)
Sn 99.3, Cu 0.7                     227           Eutectic
Sn 96.5, Ag 3.5                     221           Eutectic. Excellent strength and wetting
Sn 98, Ag 2                          221 – 226
Sn 77.2, Ag 2.8, In 20           175 – 186
Sn 95, Sb5                           232 – 240 Good high-temperature shear strength
Sn 42, Bi 58                         138           Well established; expensive
Sn 91, Zn 9                          199   Eutectic. Corrodes easily; high dross
Sn 95.5, Ag 0.5, Cu 4            217 – 350 Lead-free plumbing solder
Sn 97.25, Ag 2, Cu 0.75        217 – 219
Sn 91.8 Ag 3.2, Cu 0.5          217 – 218
Sn 95.5, Ag 3.8, Cu .07         217 – 220
Sn 95.5, Ag 4, Cu 0.5            217 – 225
Sn 95, Ag 4, Cu 1                 217 – 220
Sn 94.6, Ag 4.7, Cu 1.7         217 – 244
Sn 89, Zn 8, Bi 3                   192 – 197
Sn 97, Ag 0.2, Cu 2, Sb 0.8    287 – 218  High melting range; “Aquabond”
Sn 96.2, Ag 2.5, Cu 0.8, Sb 0.5      217 – 225
Sn 90.5, Ag 2, Bi 7.5             190 – 216
Sn-91.8, Ag 3.4, Bi 4.8          201 – 205
Sn 93.5, Ag 3.5, Bi 3             208 – 217
Sn 94.25, Ag 2, Bi 3, Cu 0.75   205 – 217
Sn90.7, Ag3.5, Bi 5, Cu 0.7     198 – 213
Sn 93.4, Ag 2, Bi 4, Cu 0.5, Ge 0.1         202 – 217
Sn 42.9, Bi 57, Ag 0.1           138 – 140
Sn 48, In 52                         118           Eutectic. Lowest melting point. Expensive


Liquidus Temperatures (°C) of Candidate Lead-Free Solder Alloys for Replacing Eutectic Tin-Lead Solder

Alloy Composition%     Liquidus             Melting Range
98Sn-2Ag                                             221-226
96.5Sn-3.5Ag              221                    221
99.3Sn-0.7Cu              227                    227
96.3Sn-3.2Ag-0.5Cu     218                   217-218
95.5Sn-3.8Ag-0.7Cu     210                   217-210
95.5Sn-4.0Ag-0.5Cu                             217-219
95Sn-5Sb                                            232-240
42Sn-58Bi                   138                   138
89Sn-3Bi-8Zn                                      189-199

Where there is a single temperature in the melting range column, the solder is eutectic.

Based on:
V. Solberg, “No-Lead Solder for CSP: The Impact of Higher Temperature SMT Assembly Processing,” Proc. NEPCON West 2000 Conf. (Feb. 28 - Mar. 2, 2000) Anaheim, CA (Source: Indium Corp.) # N.-C. Lee, “Lead-Free Chip-Scale Soldering of Packages,” Chip Scale Review, March-April 2000.

Solidus and Liquidus Temperatures of Some Leadfree Alloys on Copper

Alloy  %                             Solidus (°C)        Liquidus (°C)
98Sn-1Ag-1Sb                      222                   232 
89Sn-4Ag-7Sb                      230                   230
91.2Sn-2Ag-0.8Cu-6Zn          217                   217
89.2Sn-2Ag-0.8Cu-8Zn          215                   215
89.2Sn-10Bi-0.8Cu               185                    217
85Sn-10Bi-5Sb                     193                   232
52Sn-45Bi-3Sb                     145                   178
42Sn-58Bi                            138                   138

Based on:
M.E. Loomans, S. Vaynman, G.Ghosh and M.E. Fine, “Investigation of Multi-component Lead-free Solders,” J. Elect. Matls. 23(8), 741 (1994)

Eutectic Composition of Solders

Most solders and especially tin-lead alloys have a melting (or pasty) range between which the metal has moved from a proper solid (solidus) to a completely liquid (liquidus) state.  Wide melting ranges are ideal for plumbers, they are not for electronics, or stained glass.  It is much easier to run a nice bead with a narrow range of melting (pasty) temperatures.

Some alloys of solder have what is known as an eutectic characteristic.  This is where the liquidus and solidus states occur at the same temperature.  A composition of 61.9% tin and 38.1% solder is both eutectic and the melting occurs at a minimum temperature.

For comparison with lead free solder characteristics the following % compositions of Tin (Sn), Lead (Pb) and Silver (Ag) solders are given.

Element % of solders  Melting point        Comment
Sn 62, Pb 36, Ag 2       179                    Eutectic; traces of antimony
Sn 63, Pb 37               183                    Eutectic; traces of antimony
Sn 60, Pb 40               183-191             Traces of antimony
Sn 96.3, Ag 3.7           221                    High melting point. Eutectic
Sn 10, Pb 90               275-302
Sn 3, Pb 97                275-320
Sn 5, Pb 93.5, Ag 1.5   296-301



Most of the alternative solders contain tin as it assists in the formation of bonds with a wide variety of metals.  These solders are also mechanically weaker than tin-lead solders.  Lastly, they are much more expensive than tin-lead solders.  Even within the tin-lead solders there is a variation in price, as tin is much more expensive than lead. If high temperatures were not a problem, you could use a high lead content solder.  However, that also raises the liquidus temperature and increases the pasty range.

The choice in lead free solders is between the high liquidus temperatures of the less expensive compositions and the high price of the eutectic, or nearly so, ones.  The lowest eutectic composition is the Tin-Bismuth solder, but it is also among the most expensive to buy.  You should also note that the inclusion of copper in the composition prolongs the life of the solder bit, as low lead content of the solder leads to the incorporation of small amounts of copper from the tip into the solder joint.

Saturday, 15 July 2017


Needling is a description of the fine points emerging from the edges of glass.

This occurs in two conditions mainly.

The one that is most commonly seen is in the fusing of single layers of glass. The surface tension of the glass pulls the glass in from its original size, trying to achieve the 6-7mm that is a thickness equilibrium at full fusing temperatures. If the surface the glass is resting on has any rough areas, and most surfaces do, some of the glass will stick and the rest retract. This leaves short, thin and extremely sharp “needles” extending from the edges. 

Two common surfaces allow these sharp edges. Fibre paper of 0.5mm and greater is rough enough to allow the hot glass to stick to tiny depressions in the paper.  Kiln wash is often not smooth enough to prevent this kind of sticking either.  You can smooth powdered kiln wash or aluminia hydrate over these surfaces to reduce the grabbing of the surface by the hot glass. However, the powder is often drawn back with the contracting glass. Thinfire or Papyros paper is fine enough to avoid the needling most of the time without any addition of powders.

The other main condition is in casting, mainly box casting or damming. In this case, the stack of glass sheets or cullet is higher before firing than its final thickness. This means the glass flows out to the dams and sinks down to its final thickness during the firing process. As the glass touches the fibre paper or other separator it behaves just as the single layer of glass does. Some sticks to the surface while the rest is dragged away by the surface tension and reducing thickness of the stack of glass.

Prevention of Needling
Lining dams
Separators for dams

Thursday, 13 July 2017

Quartz Inversions and Conversions

You need to know about this in both casting and when using ceramic pots in the kiln.

Crystalline solids are rather temperamental and quartz is no different. Quartz is a crystalline form of silica in that it has a three dimensional regular pattern of molecular units. These form naturally in nature because lengthy cooling times allow arrangement. Quartz is made of a network of triangular pyramid (tetrahedron) shaped molecules of silicon combined with four oxygens.

Unfortunately, the quartz delights in changing the orientation of the tetrahedron shaped molecules with respect to each other, thus loosening or tightening the whole mass (and thus changing its total size). It exhibits twenty or more “phases”. A change to another phase is called a “silica conversion”. The most significant phases are quartz, tridymite, crystobalite, and glass.

Changes which occur between these are reversible, that is, the change which occurs during heat-up is inverted during cool down. These changes are thus called “quartz inversions”. These inversions, unfortunately, often have associated, rather sudden, volume changes. That means that quartz conversions are something to consider when optimizing the fired properties; quartz inversions are something to consider when firing to prevent cracking losses. There are two important inversions you need to know about because of their sudden occurrence during temperature increase and decrease.

The first is simply called ‘quartz inversion’ and it occurs quite quickly in the 570°C range (1060°F). In this case, the crystal lattice straightens itself out slightly, thus expanding 1% or so. This is therefore an important temperature in casting as it is an expansion on the heat up and a contraction, “grabbing” the glass on the way down. This is the reason for various modifiers when silica or flint is used as the strengthener.

The second is crystobalite inversion at 226°C. This is a little nastier because it generates a sudden change of 2.5% in volume. This material has many more forms than quartz, so it is complex to say the least. However, while all bodies will have some quartz, you won’t have a problem with crystobalite inversion unless there is crystobalite in your body. Crystobalite forms naturally and slowly during cooling from above cone 3 (1104-1149°C). It forms much better if pure crystobalite is added to the body to seed the crystals or in the presence of catalysts (e.g. talc in earthenware bodies). Thus, this element exists in most ceramic moulds and moving slowly around 226°C should be observed when firing containers made of ceramic materials.

Wednesday, 5 July 2017

Simple Investment Mould Materials

There are a lot of differing recipe options for making plaster moulds. A simple general purpose investment mould making material and method follows:

Equal parts of powdered silica (sometimes called silica flour or flint), plaster of Paris and water by weight.  For example:

1 kilo silica
1 kilo Plaster Paris
1 kilo water
(Do not measure by volume)

Mix silica and plaster of Paris dry in separate bucket by hand.  If you can use a closed container that is best.  Otherwise use breathing protection and do the mixing outside.  Silica is very bad for your health.

Measure the water into a separate bucket with enough volume for three times the amount of water. Slowly sprinkle the entire contents of the dry mix into the bucket of water.  Do not dump it in!

Let the mixture sit for 2 minutes (slaking).  Then mix by hand slowly to prevent bubbles. Using your hands allows you to feel any lumps that are present and break them down gently. Depending on temperature and amount of water, you have 15-20 minutes before the mix begins to become solid.

When mixed thoroughly, pour carefully and slowly into a corner of the mould box or container to reduce the occurrence of bubbles within the investment material or against the master.

When the pour is finished, tap the mould container to encourage any bubbles to the surface.

You can take the investment and master from the container once it is cold to the touch. Remove the master from the investment material carefully to avoid damaging the surface of the investment.

For pate de verre, you can use the mould almost immediately.  For casting, it is important to have a dry mould.

Let the whole air dry. Depending on the temp, humidity and density this can last from several days to several weeks. A way to tell how dry the investment is, is by weighing the mould when it has just hardened. When it has lost on third of its weight (the water component), it is ready for kiln drying. This removes the chemically bound water from the investment material. 

This is only an outline of what to do.  Investment moulds are extremely complicated in their chemistry, physics, and use.

Wednesday, 28 June 2017

Stencils vs. Saw


Frequently when people want to make a complicated shape they resort to a saw to create the shape.  This is used in both stained glass and fused glass work.  Although it may be necessary in stained glass applications, it is not as necessary in fusing.

One of a variety of saws


There is an alternative to an expensive saw – stencils and frits.  You can make a stencil from stiff card. Place the stencil in the appropriate place. Then sift powder or sprinkle frit over the stencil.  Lift carefully and the shape is there ready for fusing.

Example of sifting powder over a complicated stencil

To get the depth of colour obtained from sheet glass, you need to apply the powder or frit to at least the thickness of sheet glass. This also means that you need to go to a full fuse with the powder or frit on the top surface.  You can, of course, later cap and fire again.

Example of the cutting of a stencil

More guidance on stencils is available here

Wednesday, 21 June 2017


What it is

Mica is widely distributed throughout the world and occurs in igneous, metamorphic and sedimentary rocks. Mica is similar to granite in its crystalline composition.  The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.

Mica can be composed of a variety of minerals giving various colours and transparency. Purple, rosy, silver and grey colours come from the mineral called lepidolite.  

Dark green, brown and black come from biotite.  

Yellowish-brown, green and white come from phlogopite.  

Colourless and transparent micas are called muscovite.  

All these have a pearly vitreous lustre.

The melting point of mica depends on its exact composition, but ranges from 700⁰C to 1000⁰C.

Glass has a specific gravity of about 2.5, and mica ranges from 2.8-3.1, so it is slightly heavier than glass.

Tips on uses of mica powder and flakes

The naturally occurring colours are largely impervious to kiln forming temperatures.  Other added colours have various resistances to the heat of fusing. This is determined by the temperatures used to apply the colour to the mica.  Cosmetic mica is coloured at low temperatures and will not survive kiln forming with their colour in tact.

Mica does not combine with glass, but is encased by glass as it sinks into the glass surface.  You can use various fluxes to soften the surface of the glass.  Borax is one of those.  The cleaving of the mica results in only the layer in contact with the glass sticking.  The upper layers brush off.  This applies to both powder and flakes. One solution is to fire with mica on top in the initial firing and then cap for the final one.

When encasing mica exercise caution. Micas flakes must be applied thinly, as air is easily trapped between layers which leads to large bubbles from between layers of glass.  This is the result of the shearing of layers of the flakes allowing air between layers.  Although powdered mica is less likely to create large bubbles, air bubbles are often created for the same reason.  This is the reason it is most often recommended to fire the mica on top. 

Of course, one use of the mica to make complicated designs is to cover the whole area and fuse.  Then sandblast a design removing the mica from areas of the glass. You can then fire polish, or cap and re-fire to seal the mica.

Mica safety

MSDS for mica only mentions the inhalation of the dust as a risk. Mica is resistant to acid attack and is largely inert.  Inhalation of the dust is a (low level) risk.  Any significant health and safety problems relate to the coloured coatings.

Wednesday, 14 June 2017

Deep slumps

One of Karl Harron's deep slumped bowls

Deep slumps require multiple stages to get even drops without thinning the sides.  There are several makers of staged slumping moulds which allow progressively deeper slumps in a series of firings into deeper moulds.

If you have a steep-sided mould, you will find slumping directly into the shape difficult.  There will be uneven slumps, thinning of sides, hang-ups, etc., among your attempts to achieve the slump in one firing.  It is possible to mimic this series of moulds without buying the whole set. 

To avoid these difficulties, you can build up the inside bottom of the mould by placing powdered kiln wash in the bottom and smoothing it to a gentle curve. You should aim for a gentle shape as in a ball mould. 

After the first firing, remove some of the powder, placing it in a clean container.  Shape the remaining powder into a deeper slump than the first one. 

It takes some time and practice to achieve a smooth even curve.  You can ease the shaping process by cutting the intermediate shapes from stiff card.  This can be rotated to achieve an even curve in the powder.  Remove any excess powder and do a final rotation to give the powder a final smoothing.  Place the glass back on the mould and fire.

It may be that you will need to repeat this several times to get the full slump.  Separate template curves need to be cut for each slump if you are doing more than one intermediate slump. It does depend on the steepness of the mould sides and the depth of the slump as to how many stages are required.  Sometimes the slump can be achieved in only two stages.

After firing the powder, pour it back into your kiln wash container, as it still is good for mixing to apply to shelves, moulds etc.

This method is useful for any mould that is too deep for achieving the slump in one firing, and without buying intermediate moulds.  Remember the final result will be smaller than the size of the deep mould, as the span of the glass becomes less with each deeper slump.

Wednesday, 7 June 2017

Effects of Annealing at the Top End of the Range

It is possible to begin your annealing at any point in the annealing range.

The annealing point is the temperature at which the glass most quickly relieves the stress within.  This occurs at the glass transition point

The  annealing range is between the softening point and the strain point of the glass.  No annealing can be achieved above the softening point, nor below the strain point.  This range, for practical purposes can be taken to be 55°C above and below the published annealing point.  For thick slabs, Bullseye has chosen to start the anneal 34°C below the published annealing point of 516°C.

High Annealing Point

They could have chosen to use a higher point, even up to 571°C, the approximate strain point of the glass.  The effect of this is an extended anneal cool.  The reasons are as follows.  

The anneal soak does not need to be extended, as the purpose is to get all the glass at the same temperature in preparation for the annealing cool. 

The cooling rate must be slower (approximately one third the rate) than an anneal soak at a lower temperature, as the glass must be maintained at the same temperature throughout the long cool.  

Also, the initial rate of cool needs to be maintained down to the strain point, which is 110°C below the softening point.  Of course, after that initial cool, the speed of cooling can be increased.

Low Annealing Point

Starting the anneal cool closer to the strain point requires a longer soak to ensure the glass is all at the same temperature (+/- 5°C) before the anneal cool begins.  Typically, this initial soak would be for an hour before the initial cool begins (for a 6mm to 9mm thick piece).

Effect of the Differences in Approach

The advantages and disadvantages centre around these needs to 

  • soak long enough to get all the glass to the same temperature and secondly, to 
  • cool slowly enough to maintain the even temperature distribution throughout the glass.


If you think of an example of a piece of Bullseye glass 12mm thick, it will show the differences in approach.

High temperature soak
A soak of 30 minutes at 571°C (the highest possible start for an annealing soak) is required to even the temperature.  To ensure the temperature differentials in the glass do not deviate from the +/-5°C, the cool needs to be at 18°C per hour down to 461°C.  It is possible then to increase the speed to 36°C down to 370°C.  This gives you a total annealing cool of just over 5 hours.

Low temperature soak
Starting the anneal at 482°C requires an hour soak followed by a decrease in temperature of 55°C per hour to 427°C, and an increased rate of 110°C to 370°C.  This gives an anneal cool time of 3 hours and 30 minutes.

The example shows how, although the annealing result may be the same, there is considerable time saved (for thicker pieces) in using the lower part of the annealing range to begin the annealing.  It also will save some electricity.

However, an anneal of 30 minutes at 516°C with a cool of 80°C per hour to 370°C will still give a perfectly adequate anneal for 6mm thick pieces.