Narrow Grozing Pliers

 

Top pliers are used but undamaged.  The bottom pliers have rounded tips and will not grasp thin pieces of glass anymore.

Narrow grozing pliers are very useful in many circumstances, but their use is different from the standard 10mm grozing pliers.

Use narrow pliers to grasp small pieces and pull away from score. A firm but not hard grasp of the glass is required to pull the small pieces off. The square tips are important to the function of the pliers. If the tips are rounded, it is not possible to grasp the small pieces of glass to pull them away from the scored part without slipping off.

Squeezing hard crushes the glass and wears away the jaws quickly, which rounds the ends of the pliers, making them unsuitable for their intended purpose.

They are not meant for grozing. And in any case grozing is done with the serrations further away from the tip of grozing pliers. Misuse of the narrow pliers causes rapid wear and greatly reduces the useful life of the tool.

Chipping Glass while Sawing

Frequently there are chips on the bottom surface or breakouts at the end of cuts while sawing glass. There are several methods to reduce these effects.

Saw blade depth

The blade on an adjustable depth overhead saw should be set to just below the saw table depth. This reduces the break outs on the bottom surface. It helps to make the angle at which the saw blade meets the glass more acute, helping to reduce the chipping of the surface.

Of course, on an adjustable overhead saw blade could be set to just mark the surface to reduce chipping on the top. Then the table drawn back to adjust the blade to the full cutting depth. However, that is a lot of adjustment to reduce minor chipping that will be remedied in further cold work or fire polishing.

For saws that do not have adjustable depth, bottom surface chipping can be reduced by placing sacrificial glass below the main piece. This raises the main glass and creates a more acute angle between the glass and the blade, also reducing chipping on the upper surface.

Ends of Cuts

Break outs often occur at the ends of the cuts. Placement of a sacrificial piece of glass vertically at the exit of the cut helps to give a clean cut at the end. This will apply whether using a fixed or adjustable saw blade.

Of course, the two can be combined:

White as a Difficult Glass

Description of the Project

A white 3mm base with 3mm and 6mm decorations made up of mosaic pieces from previously fused glass (all the same CoE). At the end of the firing three corners had broken and their edges rounded. The fourth corner had sharp edges. The tentative conclusion was that there was incompatibility between the white and the previously fired pieces. There were no other cracks visible on the white or between the mosaic pieces. The author did not indicate what the schedule was for either firing, nor what the profile of the last firing was, but asserts white is a particularly difficult glass which does not work well with a wide variety of colours.

My observations are: 

• Compatibility is not an issue on the heat up. It is only a problem at annealing and cooling.
• Breaks on the ramp up (showing rounded edges at the conclusion of the firing) are normally the results of too fast rates.
• Breaks during cooling (showing sharp edges) are due to annealing, compatibility, cooling rates, or some combination of these.
• Previously fired glass can show some shift in compatibility and so needs slower up ramp rates than normal for the profile and thickness.
• Incompatibility between the base and the mosaic pieces would show up as breaks in the white glass under each top mosaic piece.
• Not all glass of the same CoE from different manufacturers is compatible.

Could this have been from incompatibility?

On the way to top temperature the pieces have not yet combined. The incompatibility will only show up during the cooling, as it is the imbalance of  viscosity and contraction between the fused pieces that cause the breaks.

Only one of the broken corners has those sharp edges, making incompatibility an improbable cause of the breaks. Further, incompatibility between the base and upper layers present either a crazed appearance at the connections, or simple breaks around the base of each decorative piece. Incompatibility would have multiple breaks all over the base, if not the top too. Finally, if the fired mosaic pieces were incompatible with the white glass, there would have been breaks throughout the whole piece, not just at the corners.

A further possibility is that the corners were very close to the sides of the kiln, because only the corners broke away from the piece,. If it was side fired, much slower rates are required. And all kilns tend to be cooler near the sides on the heat up than toward the centre, even if top fired.

My guess, based on the description, is that the up ramps were too fast, and the anneal was too short and the cool too fast. Unless the previously fused pieces were tested for stress it is not possible to know whether those were stressed before the final firing, which could have caused the break off of the three of the corners. The fourth corner break was on the cool down and is most likely to be too short an anneal and/or too quick a cool.

Is white glass especially difficult?

There is nothing in this piece to identify white glass as an extraordinarily difficult glass, or that a multiplicity of colours added to white would provoke breaks. The problems exhibited are most likely related to fast heat up ramp rates, and inadequate annealing and cooling.

Sintered Snowflake

Credit: Janet Wager

"My snowflakes are fragile. Will more glass make them stronger?”

My experiments for the e-book Low Temperature Kilnforming showed more frit does not make these sintered pieces stronger. Sintered pieces made from frit are made stronger with a combination of packing, time and temperature.

Packing

The amount of packing makes the whole more dense by linking more particles together. The most dense packing is done by putting down thin layers and packing each layer into the previous one. This allows more connections to be made between the pieces, which is important to the resulting strength.

Temperature

The temperature at which the frit is fired is important too. Firing at 650°C/1202°F will produce a weaker piece than one fired at 690°C/1274°F, if both are soaked for the same amount of time.

Time

Generally, the soak at the top temperature will be about two hours. The interplay of time and temperature are particularly important when sintering frit. The connections at the atomic level take more time to form at low temperatures than at high ones. The piece fired at 650°C/1202°F can be strengthened by extending the time to 4 hours instead of 2 hours, which will give it about two thirds of the strength of firing at 690°C/1274°F, for 2 hours.

Rates

The ramp rates need to be slow to achieve maximum strength. For example. A rate of 150°C/270°F to 690°C/1274°F needs a 2 hours soak, but a rate of 600°C/1080°F needs 6 hours to achieve the same strength. An alternative is to use a fast ramp to the strain point and then 50°C/90°F to the top temperature for a 3 hour soak.

Texture

The texture of the sintered piece will vary according to time and temperature. A firing at 690°C/1274°F for two hours will give a shiny top surface, but will be more textured underneath. A two hour soak at 650°C/1202°F will give a sugar grain appearance, but be weak. Extending the soak time to 4 hours will approximately double the strength. If you want to retain the sugar texture and have a strong piece, a long soak at a top temperature of about 650°C/1202°F for at least 3 hours will be needed.

Annealing

Sintered pieces need to be annealed for about 2.5 – 3 times longer than their thickness to be strong and the cool rates need to be related to the soak time to be as stress free as possible.

The e-book Low Temperature Kilnforming is available from:

Bullseye

Etsy

and me:  Stephen.Richard43@gmail.com

Observations on Some Suggestions about Annealing

There are writings from a teacher attempting to make glass fusing simple.  Unfortunately, glass physics and chemistry are very complicated.  Attempting to avoid these complications leads to failures and other difficulties as the practitioner progresses. 

Proper annealing is one of the fundamentals to achieving sound kilnforming results.  Some suggestions have been made by a widely followed person to “simplify” the understanding of the annealing process.  Discussion of the meaning and importance of annealing can be found in many places, including here.  

Annealing temperatures

It has been suggested that the annealing temperatures can be inferred from the CoE of the glass that is being used. Discussion of what CoE is and is not can be found here and here.

Annealing temperatures are not directly related to the expansion coefficient (CoE) of the glass.  This can be shown from the published annealing temperatures for different glasses organised by presumed CoE:

·        “CoE96”: Wisssmach 96 - anneal at 482°C;  Oceanside - anneal at 515°C

·        “COE94”: Artista - anneal at 535°C

·        “CoE 93”: Kokomo - anneal between 507°C and 477°C – average 492°C

·        “CoE 90”: Bullseye - anneal at 482°C; Wissmach90 - anneal at 482°C; Uroboros FX90 - anneal at 525°C

·        “CoE 83”:

o   Pilkington (UK) float - anneal at 540°C;

o   typical USA float - anneal at 548°C;

o   Typical Australian float - anneal between 505°C and 525°C, average 515°C

This shows there is no direct relationship between CoE and annealing temperature.  Do not be tempted to use a CoE number to indicate an annealing temperature.  Go to the manufacturer’s web site to get the correct information.

Temperature equalisation soak

Annealing for any glass can occur over a range of temperatures.  The annealing point is the temperature at which the glass can most quickly be annealed.  However, the glass cannot be annealed if it is not all at the same temperature throughout the substance of the glass.  It has been shown through research done at the Bullseye Glass Company that a temperature difference of more than 5°C will leave stress within the glass piece. To ensure good annealing, adequate time must be given to the temperature equalisation process (annealing). 

From the Bullseye research the following times are required for an adequate anneal soak:

6mm /   1/4"            60 minutes

[9mm /  3/8"           90 minutes]

12mm  / 1/2"          120 minutes

[15mm  /   5/8"       150 minutes]

19mm   / 3/4"         180 minutes

[ ] = interpolated from the Bullseye chart for annealing thick slabs

Anneal Cooling

There are suggestions that a “second anneal” can be used on important pieces.  Other than observing that all pieces are important to the maker, the suggestion should be investigated.  On looking into the idea, it is essentially a second soak at 425°C, which is slightly below the strain point, rather than controlled cool from the anneal soak temperature.

It is reported that the Corning Museum of Glass considers 450°C as the lower strain point – the temperature below which no further relief of strain is possible.  This means that any secondary soak must occur above 450°C rather than the suggested 425°C. Such a soak is unnecessary if the appropriate cooling rates are used. 

Cooling Rate

Except in special circumstances, the cooling rate needs to be controlled as part of the annealing process.  Soaking the glass at the anneal is not the completion of the annealing.  Most practitioners follow the practice of choosing a slow rate of cooling from the annealing soak to some point below the strain point rather than a rapid one with a soak at the strain point temperature.

Annealing is not just the soak time (which is there to equalise the temperature), it is about the rate of the annealing cool too. The rate at which you cool is dependent on the thickness of the glass piece and whether it is all of one thickness or of variable thicknesses.

Even thickness

                                         Cooling rate

Dimension      time (mins)     to 427°C to 371°C

6mm              60                 83°C       150°C

9mm              90                 69°C       125°C

12mm            120                55°C       99°C

15mm            150                37°C       63°C

19mm            180                25°C       45°C

                                        Cooling rate

Dimension      time (mins)     to 800°F   to 700°F

0.25"              60                 150°F       270°F

0.375"            90                 124°F       225°F

0.5"               120                100°F       178°F

0.675"           150                67°F         114°F

0.75"             180                45°F         81°F

Tack fused/ uneven thickness

If your piece is tack fused, you need to treat the annealing rate and soak as though it were twice the actual total thickness. This gives the following times and rates:

Tack fused

Dimension (mm)                                Cooling rate

Actual     Calculated       time (mins)    to 427°C   to 371°C

6            12                 120                55°C       99°C

9            18                 150                25°C       45°C

12          25                 180                15°C       27°C

15          30                 300                9°C         18°C

18          38                 360                6.7°C       12°C

Dimension (inches)                                Cooling rate

Actual     Calculated       time (mins)    to 800°F   to 700°F

0.25          0.5                 120                100°F       180°F

0.375        0.75               150                45°F         81°F

0.5            1.0                180                27°F          497°F

0.675        1.25               300                16°F         36°F

0.75          1.5                360                12°F          22°F

Contour fusing requires firing as though the piece is 1.5 times thicker.  Sharp tack or laminating requires 2.5 times the the actual thickness.

Fusing on the floor of the kiln

There is a further possible complication if you are doing your fusing on the kiln floor, or a shelf resting on the floor of the kiln.  In this case you need to use the times and rates for glass that is at least 3mm thicker than the piece actually is. 

Thus, a flat 6mm piece on a shelf on the floor would use the times and rates for 9mm: anneal soak for 90 minutes, anneal cool at 69°C to 427°C and then at 124°C to 371°C.  It would be safest if you continued to control the cooling to room temperature at no more than 400°C per hour.

But if it were a tack fused piece of a total of 6mm you would use the times and rates for 18mm.  This is using the rates for twice the total thickness plus the additional 3mm for being on the base of the kiln.  This gives the times and rates as being an anneal soak of 360 minutes and cooling rates of 7°C to 427°C and 12°C to 370, followed by 40°C per hour to room temperature.  Any quicker rates should be tested for residual stress before use.

Source for the annealing and cooling of fused glass

These times and rates are based on the table derived from Bullseye research, which is published and available on the Bullseye site.   It is applicable to all fusing glass with adjustments for differing annealing soak temperatures.

Annealing over multiple firings

It has been recommended by a widely followed person that the annealing soak should be extended each time subsequent to the first firing.  I am uncertain about the reasoning behind this suggestion. But the reasons for discounting it are related to adequate annealing and what is done between firings.

If the annealing is adequate for the first firing, it will be adequate for subsequent firings unless you have made significant alterations to the piece.  If you have added another layer to a full fused piece, for example and are using a tack fuse, you will need to anneal for longer, because the style and thickness have been altered.  Not because it is a second firing.  If you are slumping a fired piece, the annealing does not need to be any different than the original firing.

The only time the annealing needs to be altered is when you have significantly changed the thickness of the piece, or the style of fusing (mainly tacking additional items to the full fused piece).  This is when you need to look at the schedules you are planning to use to ensure your heat up is slow enough, that your annealing soak is long enough, and the cool slow enough for the altered conditions.

Determining the annealing point of unknown glass

You don’t have to guess at the annealing temperature for an unknown glass.  You can test for it.  It is known as the slump point test.

This test gives the softening point of the glass and from that the annealing point can be calculated.  This test removes the guess work from choosing a temperature at which to perform the anneal soak. The anneal temperature is important to the result of the firing.  This alone makes this test to give certainty about the annealing temperature worthwhile.

You can anneal soak at the calculated temperature, or at 30°C below it to reduce the anneal cool time.  This is because the annealing can occur over a range of temperatures.  The annealing occurs slowly at the top and bottom of the range. But is at least risk of "fixing in" the stress of an uneven distribution of temperature during the cool when the annealing is done at the lower end of the range.

Do not be fooled into thinking that CoE determines annealing temperatures.  Use published tables, especially the Bullseye table Annealing for Thick Slabs to determine soak times and cooling rates.  Use the standard test for determining the softening and annealing points of unknown glasses.

Further information is available in the ebook Low Temperature Kiln Forming and in Annealing Concepts Principles and Practice 

Revised 14.10.25

Annealing a Stressed Piece

An stress test strip and annealing witness between polarised filters.

If an unbroken fired piece shows stress that is known not to be from incompatibility, it is possible to fire and anneal again to relieve the stress.  If the stress results from incompatibilities, annealing again will not change the compatibility.  The process for stress testing is here. 

Conditions for doing this re-firing are:

• Slower heat up rates than usual for this thickness and profile are required. The glass is more than usually fragile and needs gradual heating. This avoids creating additional stress that may cause a break.

• Take the temperature up to the lower end of slumping temperature range - say 600 - 620C (1100 - 1150F) - and soak for 10 – 30 minutes depending on profile and thickness.  This ensures any existing stress is relieved and the glass is ready for the annealing.

• Reduce the temperature as fast as possible to the existing or new annealing temperature.

• Anneal for longer than previously. This can be for a greater thicknesses than the thickness and profile used for the stressed piece.  Most importantly, the anneal soak for the combination of profile and thickness needs to be followed.

• My experimentation has shown that the profile determines the additional amount of thickness that needs to be allowed for a sound anneal is as follows:

• Full flat fuse - fire for the thickness (i.e. times 1)
• Contour fuse -  fire for 1.5 times the thickest part
• Rounded tack fuse - fire for 2 times the thickest part
• Sharp tack/sinter - fire for 2.5 times the thickest part.

• Use the cool rates related to the anneal soak time. These are available from the Bullseye site for Celsius and Fahrenheit.  Too rapid a cool can induce temporary stress from differential contraction of the glass that is great enough to cause breaks, so follow the rates determined for this thickness and profile. 

• These rates are scientifically determined for all glass and especially for fusing glass and are inversely related to the anneal soak.  That means the longer the anneal soak, the slower the cooling rates need to be, and directly related to the soak length.  It does not matter which manufacturer's glass is being used, all the target times and temperatures should be followed, except the annealing temperature.

More information is available in my e-book Annealing Concepts, Principles, and Practice available from Bullseye, Etsy, and stephen.richard43@gmail.com

Cast Iron Mould Risks

There is a lot of concern about the safety of many products used in kilnforming, and much of it is based on hearsay. The best source for understanding the health and safety risks is Gregorie Rawls website.

Another, but more difficult to interpret, source is the SDS for each product.

Cast iron composition and safety

In this case the investigation is cast iron used as moulds. The first element is to know what cast iron is:

Cast iron is a class of iron-carbon alloys having a carbon content of more than 2% and silicon content around 1–3% with a melting point of 1,539°C (2,802°F). [Wikipedia]

The SDS gives the following information on Gray Cast Iron, the material of the cookware commonly used in the kiln:

• This material is rated as NOT HAZARDOUS by OSHA

• Appearance and Odor: Solid Mass, No Odor

• Specific Gravity: 7.86

• Boiling Point: 5000F

• 5 mg/m³ is the Time Weighted Average (TWA*) for fumes over an eight hour day. https://www.cdc.gov/niosh/idlh/1309371.html

These indicate there is no risk from fumes during casting firings as melting point will not be reached and the boiling point of cast iron is much higher than kilnforming kilns can reach.

The real risks are at room temperature, and are from the powder that may be created while grinding or smoothing the metal surface. The TWA* for cast iron dust is 10 mg/m³ over 8 hours. There are two alloy elements that also may be of concern – nickel and chromium. The amounts are low – chrome is from 0.5% to 2.5%, and even less nickel. The amounts are very low, giving little possible exposure.

The health concerns about using cast iron as a mould seems to be one of the misapprehensions of the amount of exposure, and therefore risk, that are common. The precautions are to have ventilation at source, use eye protection, and wear a N95 respirator.

The use of cast iron as a mould material

“Cast iron is a poor heat conductor compared to copper and aluminium, and this can result in uneven heating if a cast-iron pan is heated too quickly or… [unevenly].  Cast iron …[is] capable of storing more heat longer than... stainless steel pans. Slow heating... can lead to a more even temperature distribution. Due to the thermal mass of cast-iron utensils… they can retain heat for a long time...” Wikipedia.

This indicates that slower than usual ramp rates are advisable during the heat up to avoid breaking the cast iron through uneven heating.

Another thing to note is that the expansion rate [CoE] of between 106 and 114. The mould will contract more than glass, so preparing the mould with smooth sides and a sufficient draft is important to being able to remove the glass from the mould.

*Time Weighted Average (TWA) example:

"Rarely is exposure consistent throughout the day. Let’s say you are working in your studio for 8 hours grinding glass and exposure varies throughout the day… [Exposure varies in amounts]. The exposures throughout the day are averaged and the Time Weighted Average is determined. [In the example cited], … the OEL = 10 mg/m3 and the Time Weighted Average is 3.2 mg/m3, so actual exposure is below OEL (Occupational Exposure Limit)."    https://gregorieglass.com/general-information

Concealing Layers at the Edge

Credit: Sheri Coughlin

At tack fusing temperatures the layers of glass making up the piece are evident at the edge with two layer bases. 

 It is possible to conceal the layers at rounded tack fuses with a simple layup. If you cut the top piece 6mm larger than the bottom and place it carefully and evenly over the bottom, the top will form over the bottom, concealing the join of the two layers. This will work at all except sharp tack fusing levels.

A Sintering Project

Ready for firing

The project is to fire 6mm “balls” stacked 3 high onto a single sheet of clear glass without significant alteration to the base sheet or to the stacked balls. This creates a total thickness of 21mm. The proposal is to sinter the whole in one firing.

Scheduling for a sinter firing needs to be done as though 2.5 times the thickest part – in this case 52mm, or 2 inches

It is slightly more risky to do this in two firings, than one, in my opinion. A suggested schedule for sintering frit using Bullseye was:

• 100ºC /180ºF — 482ºC /900ºF, 60'  =5.8 hrs

• 40ºC /72ºF — 593ºC/1100ºF,10'      =2.8

• 20ºC /36ºF — 665ºC /1230ºF,30     =4.1

• Skip to anneal temperature, soak for 6 hours =6.5

• 6.7ºC /12ºF — 427ºC /800ºF,0'       =8.2

• 12ºC /22ºF — 371ºC /700ºF,0'        =4.7

• 40ºC /72ºF — room temperature,0’ =8.8

• Off                       =40.9 hours total or 1.7 days

This was annealing as for 38mm/1.5 inches thick. Annealing for 50mm/2” thick would need about 112 hours or 4.6 days.

However this schedule was not successful – the pieces were only lightly stuck together. Thinking about why, led to the proposal that the soak time and temperature were not long or high enough to give adhesion between the pieces.

A second attempt used a faster ramp rates to higher temperatures.

• 200°C /360°F – 540°C /1004ºF, 30’ =3.2 hrs

• 60°C /108°F -625°C /1157ºF, 30’     =1.92

• 30ºC /54ºF - 685ºC /1265ºF, 120’    =4.0

• skip to anneal temperature and soak/hold for 4 hours (as for 25mm/1”)

• 15ºC /27ºF – 427ºC /800ºF, 0’        =3.67

• 27ºC /49ºF – 370ºC /700ºF, 0’        =2.11

• 90ºC /162ºF – 50ºC /122ºF, 0’       =3.56

• Off

•  = a minimum total of 18.5 hours plus natural cooling of the kiln

This schedule used a:

• faster first ramp to a higher (540ºC /1004ºF) first soak

• a faster (60ºC /108ºF, which is 150% of the previous) rate to the lower slump temperature (625ºC /1157ºF)

• the same relative reduction (50%) in rate to a higher temperature (685ºC /1265ºF)

• a shorter (120’) anneal soak

• and consequently faster cooling rates, which showed no stress after firing

The whole structure held together and was sound. There was no apparent change in the size of the individual 6mm balls.

This difference in scheduling is an illustration of how time and temperature can be interchanged.

It also shows that size matters when sintering pieces together. Higher temperatures and more time are required for dots and balls than for frit.

More information is available in my e-book Low Temperature Kilnforming, available from Bullseye, Etsy and stephen.richard43@gmail.com

Elevation of Moulds

Is it necessary to elevate slumping moulds above the shelf? 

I first heard of the need to elevate moulds from a Bhole representative about 2007. I ignored it, but didn't get around to testing until working on my e-book Low Temperature Kilnforming.

That work showed there is a larger difference in air temperature above and below the unsupported mould than the supported one. But that difference is much smaller than between the air temperature and the glass.

At 150°C/270°F per hour the maximum difference in the temperature under the mould between the elevated and on-the-shelf mould at top temperature was 41°C/74°F while the air temperature difference was 126°C/227°F higher than under the elevated mould.  Many of the tests showed less difference than the maximums given here.

By reducing the ramp rate from 150°C/270°F per hour to 120°C/216°F, the under mould to above mould differential was reduced by a quarter. I didn't test beyond that. But it would appear that slower rates of 100°C/180°F and less will reduce that differential.

The graph also shows that there is a large difference between what the pyrometer reads than the mould temperature of the slump. Slower ramp rates produce an air temperature much closer to the mould temperatures.

Shortly into the rapid cool towards anneal soak and cool only minor temperature difference showed between elevated and on-the-shelf moulds throughout the anneal soak and anneal cool.

These details make it clear to me that elevating moulds is completely unnecessary with slow ramp rates. This of course, fits with the low and slow mantra that many of us promote. However elevating the mould will not harm the slump.

One caution, though. Damp. Wet, or heavy moulds must be supported to avoid breaking the shelf. So I advocate placing these moulds on the floor of the kiln with 2cm posts, rather than on the shelf. I don't know if it is necessary. I haven't tested it. But I do know that moulds in this condition will break the shelf without significant separation between the two.

Low Temperature Kilnforming e-book is available from Bullseye  and Etsy and is applicable to all fusing glasses.