Fused Glass in Dishwashers

“Can glass be put into dishwashers?”

image credit: very.co.uk

There are many recommendations to avoid placing fused glass into a dishwasher.

The main reasons given are:

·        Corrosion

·        Devitrification

·        Etching and

·        Breaking.

There are distinct differences between these effects.

Corrosion

Glass corrosion generally comes from constant contact with moisture and has a greasy feel.  As experienced by weather or washing, the wetting of glass is not constant, and it dries between wettings.  No visible corrosion is present on window glass and, although float glass is a little different from fused glass, the same effect applies.

Devitrification

Devitrification occurs at much higher temperatures than those created in a dishwasher, and therefore is not a risk.

Etching

The main risk is etching from the washing process.  This can be mechanical or chemical, and dishwashers combine both. Over time, the glass will be etched just the way lead crystal is in a dishwasher.

Breaks

Glass breaks can occur in the dishwasher because of the shock of hot water.  Most dishwashers rinse while heating the water, so the glass experiences only slow rises in temperature.  Float glass of 4mm can withstand 140˚C differentials according to manufacturers.  Full and tack fused glass is not as homogenous as float glass and will be affected by smaller temperature differentials.  So, there is a small risk of breaks in dishwashers.

Additional risks relate to the layup of the glass. 

• ·   Tack fused glass has a variety of thicknesses that make it more prone to breaks from temperature differentials.
• ·   Contrasting colours can react differently and split at the contact lines.
• ·   Large internal bubbles can cause difficulties, which may arise from the insulating element of the contained air, or simply because of thickness.

 

Heat Work

“Heat work” is a term applied to help understand how the glass reacts to various ways of applying of heat to the glass. In its simple form, it is the amount of heat the glass has absorbed during the kiln forming heat-up process.

There is an relationship between how heat is applied and the temperature required to achieve the wanted result.  Heat can be put into the glass quickly, but to achieve the desired result, it will need a higher temperature. If you put the heat into the glass more slowly, the reverse applies.

For example, you may be able to achieve your desired result at 816C/1500F with a 400C/hr (720F/hr) rise and 10min soak. But you can also achieve the same result by using 790C/1454F with a 250C/hr (450F/hr) rise and 10min soak. The same amount of heat has gone into the glass, as evidenced by the same result, but with different schedules. This can be important with thick glass, or with slumps where you want the minimum of mould marks. Of course, you can achieve the same results with the a rise and a longer soak at the lower temperature, e.g. a 400C/hr (720F/hr) to 790C with a 30 min soak, but you will have more marking and difficulty with sticking separators.

In short, this means that heat work is a combination of time and temperature.  The same effect can be achieved with: 
- fast rates of advance and high temperatures.
- slow rates of advance and low temperatures.

- long soaks at low temperatures.

You obtain greater control over the processes when firing at slower rates with lower temperatures.  There is less marking of the back of the piece.  There is less sticking of the separators to the back and so less cleanup.  There is less needling with the lower temperature.  More information on heat work is here.

The adage “slow and low” comes from this concept of heat work. The best results come from lower temperature processing, rather than fast processing of the kiln forming.

More information is available in the eBook Low Temperature Kilnforming available from Bullseye and Etsy.

Revised 1.1.25

Slumping a Form Flat

There are a variety of reasons that you might want to make a formed piece flat again for another kind of slump or drape.  There are a variety of things to think about when preparing to make a shaped piece flat.  I am going to assume there are no large bubbles in the piece and there are posts on Large bubbles and Bubble at bottom  including the causes.

There are five groups of things to consider when contemplating flattening an already formed shape.

Shape/form

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

Orientation

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

Thickness

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

Temperatures

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

Separators

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

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

CoE as the Determinant of Temperature Characteristics

Credit:: Ryan Rutherford

Many people are under the impression that CoE can tell you a wide number of things about fusing glass. 

What does CoE really mean?

The first thing to note is the meaning of CoE.  Its proper name is the coefficient of linear expansion.  It tells you nothing certain about the expansion in volume, which can be as or more important than the horizontal expansion. 

It is an average determined between 20°C and 300°C.  This is fine for materials that have a crystalline structure. Glass does not.  Glass behaves quite differently at higher temperatures. 

It may have an average expansion of 96 from 20°C-300°C – although there is no information on the variation within that range – but may have an expansion of 500 just above the annealing point. 

The critical temperatures for glass are between the annealing and strain points.  One curious aspect to the expansion of glass is that the rate of expansion decreases around the annealing point.  The amount of this change is variable from one glass composition to another.

credit: ScienceDirect

The CoE of a manufacturer’s glass is an average of the range which is produced.  Spectrum has stated that their CoE of their fusing compatible glass is a 10 point range.  Bullseye has indicated that their CoE range is up to 5 points. These kind of ranges can be expected in every manufacturer’s compatible glass.

CoE does not tell us anything about viscosity, which has a bigger influence on compatibility than CoE alone. 

Comparison of CoE and Temperature

Among the things people assume CoE determines is the critical temperatures of the strain, annealing and softening points of various glasses.

Unfortunately, CoE does not necessarily tell you fusing or annealing temperatures. 

“CoE 83”

Most float glass is assumed to be around CoE 83.  The characteristics depend on which company is making the glass and where it is being made.  These are the annealing points and softening points:

Pilkington Optiwhite              559ºC/1039ºF    720°C/1328°F

Pilkington Optifloat               548ºC/1019ºF    720°C/1328°F

USA float (typical)                548ºC/1019ºF    615°C/1139

Typical Australian float has a CoE of 84 and anneals in the range 505°C -525°C/941°F - 977°F.

“CoE 90”

Uroboros FX90 has an annealing point of 525°C compared to Bullseye at 482°C, and Wissmach 90 anneal of 510°C. 

Wissmach 90 has a full fuse temperature of 777°C compared to Bullseye's 804 - 816°C.   

There is a float glass with a CoE of 90 that anneals at 540°C and fuses at 835°C.

Bullseye has a slump temperature of 630°C-677°C and Wissmach’s 90 slumps between 649°C and 677°C, slightly higher.

“CoE 93”

Kokomo with an average CoE of 93 has an annealing range of 507°C to 477°C. Kokomo slumps around 565°C

“CoE 94”

Artista with a CoE of 94 has an annealing point of 535°C and a full

fuse of 835°C, almost the same as float with a Coe of 83. 

“CoE96”

Wissmach 96 anneals at 482°C with a full fuse of 777°C and a slump temperature of 688°C.

Spectrum96 and its successor Oceanside Compatible anneals at 510°C and full fuses at 796°C.

Conclusion

In short, CoE does not tell you the temperature characteristics of the glass. These are determined by several factors of which viscosity is the most important. More information can be gained from this post or from your own testing and observation as noted in this post.

Revised 23.12.24

The Importance of Viscosity in Slumping

 What is viscosity?

The official definition is that it is a measure of the resistance to flow, e.g., honey vs water, or hard vs soft glass.  Honey and hard glass have greater resistance to flow. 

Importance of viscosity

In slumping, large differences in viscosity of the combined glasses will have different rates of deformation across the piece.  There is the possibility of uneven slumps as a result.  The stresses between the different viscosities may cause breaks or splits with rapid temperature rises.  Combining large differences in viscosity requires more caution in ramp rates and in annealing and cooling.  Of course, unusual results can be obtained by manipulating time and temperature.

Effect of temperature

Viscosity is affected more directly by temperature than heat and time.

Credit: Bullseye Glass Company

There are frequent statements about viscosity such as dark glass is less viscous than light, or transparent is less viscous than opalescent.  Also, Bob Leatherbarrow ran some slumping testes showing thick glass slumped less at a given temperature than thin.  Further, Ted Sawyer mentioned to me that some opalescent is less viscous than some transparent glass.   My experience is different, so I wanted to test my assumptions against theirs.

Experiment setup

25mm/1" wide strips were suspended with a span of 20cm/8".  Weights were placed on ends to avoid any slipping.  

Does comparative viscosity vary with temperature?

I fired samples at three temperatures and times

• 600C for 30 minutes
• 650C for 1 minute
• 690 for 1 minute

All at 150C/hr to top temperature.  The short soak time for the higher temperatures were because the glass deformed so quickly.

Results

Bullseye glass. Span of 20cm. Fired at 150C/hr to 600C for 30 minutes

            Code - name - deformation from horizontal

0126 Light Cyan              16mm

0243 Translucent White    20mm

0013 Opaque white         21mm

1101 Clear Tekta             21mm

0100 Black                     24mm

0141 Dark Forrest Green 24mm

1122 Red                       24mm

0161 Robbins egg blue    26mm

0137 French vanilla        27mm

1427 Light amber           27mm

1428 Light violet            29mm

0303 Dusky lilac            32mm

1125 Orange                 32mm

0147 Deep cobalt blue   33mm

0113 White  (.0038)      34mm

0126 Orange                 35mm

1246 Copper blue          37mm

1320 Marigold yellow     40mm

1341 Ruby pink sapphire 40mm  

(special production)

Most opals in this test were more viscous than the transparent glasses.  There are some exceptions such as Dusky lilac, Cobalt blue, Orange.  There were some exceptions too in the transparents: black, red, light amber.

Bullseye glass. Span of 20cm. Fired at 150C/hr to 650C for 1 minute

            Code - name - deformation from horizontal

0100 Black                    26mm

0013 Opaque white        30mm

1122 Red                      30mm

1428 Light violet           30mm

0243 Translucent white  31mm

0141 Dark forest green 31mm

0161 Robins egg blue    31mm

0147 Deep cobalt blue   32mm

0126 Orange opal          32mm

1101 Clear tekta           33mm

1125 Orange                34mm

0137 French vanilla       35mm

0216 Light Cyan            38mm

0303 Dusty lilac            38mm

1341 Ruby pink sapphire 39mm

1437 Light amber          41mm

1320 Marigold yellow     41mm

1246 Copper blue          43mm

0113 White  (.0038)      45mm

Some odd results appeared in this firing.  Black deformed least and white most. But in general, the opal was again more viscous than the transparent.  Exceptions were the red, and light violet in the transparents; and among the opalescents were the light cyan, dusty lilac and white.

Also of note is that the amount of deformation was very similar for the test at 600C for 30 minutes and the one at 650C for only 1 minute.  This re-inforces the concept that time and temperature are often interchangeable, so longer at a low temperature can equal the heat work effects of a shorter soak at a higher temperature.

Bullseye glass. Span of 20cm. Fired at 150C/hr to 690C for 1 minute

            Code - name - deformation from horizontal

0013 Opaque white        35mm

0141 Dark forest green   41mm

0137 French vanilla        44mm

1101 Clear                    49mm

1428 Light violet            52mm

0126 Orange                 53mm

0303 Dusty Lilac            54mm

1437 Light amber          54mm

0113 White   (.0038)     54mm

0243 Translucent white  55mm

1125 Orange                 56mm

1341 Ruby pink sapphire 59mm

1122 Red                      59mm

0161 Robins egg blue     60mm

0147 Deep Cobalt blue   62mm

1320 Marigold yellow     67mm

1246 Copper blue          90mm

The results of the higher temperature in this test showed variations in comparative viscosity.  Some opals (e.g., dark cobalt blue, robins egg blue) were less viscous than most transparents, but some transparents (e.g., light violet and light amber) were more viscous than most opals.

The test shows wide variability in the viscosity of transparent colours at a higher temperature.  It appears that hot and deep colours are the least viscous of the transparent colours in this test.  There are also significant differences in the viscosity of opalescent and transparent glasses of the same colour.  It is apparent that not all glasses have the same rate of viscosity change with the same rate of temperature change.

Summary

This test showed that in general, the opals in the test are stiffer than the transparent from 600C to 690C with some exceptions.  It appears transparent hot colours are less viscous than the light transparent colours.  This is not the same for opalescent colours which seem to have a wider range of viscosity at these temperatures.

The similar deformation of the test glasses at 600C for 30 minutes and at 650C for one minute, shows the possibility of using lower temperatures and longer times to achieve the same effects in slumping as at higher temperatures with shorter soaks.

Viscosity and expansion rate are roughly related at lower temperatures, but both change rapidly above the softening point.  This experiment demonstrates that expansion rates vary within a single fusing compatible range of glass.  Also, glass with significantly different viscosities can be compatible, since this was all Bullseye fusing compatible glass.

It is apparent from this unscientific experiment that when preparing for slumping an important piece that combines different colours and styles, testing for relative viscosity is a good idea to determine if there are widely different viscosities.  This knowledge will enable an accommodation to be made in scheduling.

Tom Sawyer comments on the subject of viscosity:

“Viscosity is not always lower for transparent glasses than for opalescent glasses.  Opalescent glasses will begin to move more at temperatures of 538ºC/1000ºF than will transparent glasses, and even at 677ºC/1250ºF, there are still some opalescent glasses that move more than many transparent glasses.  It is only when we get to fusing temperatures that we begin to see the majority of transparent glasses moving more than the majority of opalescent glasses.  In general, it is correct that darker glasses will move more than lighter glasses – both because of their chemistries and because of their greater propensity to absorb infrared energy.”

More information on the effects of viscosity in kilnforming can be found in the ebook Low Temperature Kilnforming.

Sample Tiles

credit: Tia Murphy

There are advocates for making tiles as references for future work.  

• They show the profiles achieved at different temperatures.  
• They can be stored for easy visual reference when planning a firing.  
• It is a useful practice for any kiln new to the user.  

These tiles are assembled in identical ways to enable comparisons.  They should include black and white, iridised pieces- up and down, transparent and opal, and optionally stringers, confetti, millefiori, frit and enamels.  

The tiles are fired at different top temperatures with the same heat up schedule with the top temperature of each at about 10C or 20F intervals.  These show what effect different temperatures give.  Start the temperature intervals at about 720C or 1330F.

This is a good practice, even if time consuming.  It gets you familiar with your kiln and its operation.  It gives a reference for the profiles that are achieved with different temperatures at the rates used.

Ramp rate and time

But, as with many things in kilnforming, it is a little more complicated.  The effect you achieve is affected by rate and time used as well as the temperature.

The firing rate is almost as important as the temperature.  

• A slow rate to the same top temperature will give a different result than a fast rate.  
• The amount of heat work put into the glass will affect the temperature required.  
• Slow rates increase the time available for the glass to absorb the heat.  
• Glass absorbs heat slowly, so the longer the time used by slower rates, the rounder the profile will be.

Since time is a significant factor in achieving a given profile, any soaks/holds in the schedule will affect the profile at a set temperature.  A schedule without a bubble squeeze will give a different result than one with a bubble squeeze at the same temperature.

To help achieve knowledge of the rate/time effect, make some further test tiles.  Use different rates and soaks for the test tiles of the same nature as the first temperature tests. But vary only one of those factors at a time. Consider the results of these tests when writing the schedule for more complex or thicker layups. 

Mass

Also be aware that more mass takes longer to achieve the same profile.  Slower rates and longer times will help to achieve the desired profile at a lower temperature.  It is probably not practical to make a whole series of test tiles for thicker items.  But, a sample or two of different thicknesses and mass will be helpful to give a guide to the amount of adjustment required to achieve the desired outcome.

The results of sample tiles are due to more than just temperature.  They are a combination of rate, time, and temperature (and sometimes mass).  These factors need to be considered when devising or evaluating a schedule, because without considering those factors, it is not possible to accurately evaluate the relevance of a suggested top temperature.

See also: Low Temperature Kilnforming, available from Bullseye and Etsy

Longer Soak or Higher Temperature?

 ‘Is it better to extend the soak or add more firing time when the firing program isn’t quite enough? What are the meanings of “soak,” “hold,” “ramp,” “working temperature” and “top temperature”?’  

Let’s start with some of the terms.

“Soak” and “hold” have the same meaning in scheduling.  Schedules are made up of a series of linked segments.  Each segment contains a rate, temperature, and time.  The time is often called a “hold” in the schedules.  That time can have several effects.  It can allow enough time for a process, such as slumping, to be accomplished.

Although “soak” is entered into the schedules in the same way as a hold, it has a different concept behind it.  The hold when used as a soak allows the set temperature to permeate the whole thickness of the glass.  An example is in annealing. An annealing hold/soak is set.  This is to allow the glass to become the same temperature throughout. 

The ramp is the rate at which the controller is set to increase/decrease the temperature.  This is normally the first element in the segment.

“Top” and “Working” temperature are the same thing.  It is the temperature at which the desired effect is achieved.  They have slightly different nuances.  Top temperature is normally considered as a point where the desired profile will be achieved in a few minutes.  The working temperature is also that, but includes the idea that it will take time for the effect to be achieved.

Which should you alter first – soak time or temperature?

Most important is that you alter only one at a time.  If you alter the two elements at the same time, you do not know which was the cause of the result.

In general, you lengthen the soak if the effect is not achieved at the temperature and in the time set.  There are two reasons for this.  Glass has fewer problems at lower temperatures.  Secondly, the controllers are set up in such a way that it is easy to extend the time. Check your manual for the key sequence to extend the time.  It is more difficult to alter the temperature during a firing. 

To determine if you need more time, you peek into the kiln as the kiln approaches the top temperature.  If the profile has not been achieved by the time set at your working temperature, you enter the combination of keys to keep the kiln at the top temperature until you see the effect you want.  Then enter the combination of keys to skip to the next segment.

Whether you alter time or temperature, depends on what you are doing.  Soak plus temperature equal heat work.  With heat work you can accomplish the same effect at lower temperatures.  It may be that taking more time (usually slower ramp rates) to get to the same or lower temperature, will give the results desired.

For slumping, draping and other low temperature processes extending the hold/soak is appropriate. It reduces the amount of marking that is created by the mould or surface supporting the glass.

When tack, contour, or full fusing, you should be aiming to finish the work in about 10 minutes. Soaking/holding significantly longer increases the risk of devitrification.

For high temperature processes such as pot and screen melts and some flows, increasing the temperature is probably the right thing to do, to avoid the devitrification possibilities of long holds of open face high temperature work.

These can only be guidelines.  Your instincts and experience will help you determine which is the right thing to do in the circumstances.

 

Placing of Pieces in the Kiln

 Distance from Sides of Kiln

 

"Is there a rule of thumb for interior size of kilns and piece size? (i.e., “allow for X inches between the piece and kiln walls on all sides”).  I’m thinking about how to determine piece size limitations when shopping for a kiln."

I don’t know of a formula, or rule of thumb, to determine the amount of space required between the glass and the kiln walls.

I have only been able to determine the spacing required after I have purchased the kiln.  Each kiln has different characteristics. 

The most obvious is whether the kiln is fired from the side or from the top.  More space is required with side fired kilns.  The radiant heat from the elements tends to heat the edges of the glass before the centre becomes equally hot. This requires more space or baffles between the elements and the glass.

Top fired,  with enough distance to get even distribution of heat

Side fired. Red arrows indicate the important infrared heating.
Blue arrows indicate the less effective ambient heat.

There is less concern about uneven heating with top fired kilns.  But as each kiln is different, you must test the heat distribution around the kiln.  Bullseye Tech Note #1 has a good method.  This will show where the temperature is less than the rest of the shelf.

In general, rectangular kilns are cooler in the corners.  Round kilns do not have the same characteristic, but may still have uneven temperatures, due to the configuration of the elements.  Smaller kilns seem to have more even temperatures than large kilns, which tend to be cooler along the sides.  Kilns with a ring element below the shelf seem to have the most even distribution of temperature.

I had a large kiln 2 metres by 1 metre which had a requirement of 50mm/2” from the edge to even the temperature.  A recently purchased 50cm square kiln has almost perfectly even temperatures across the whole shelf.

[The illustration is taken from the ebook Low Temperature Kilnforming, available from Bullseye and Etsy.]

The required glass distance from the side will depend on side or top elements and size but no formula is available.  Testing for heat distribution is necessary once you have the kiln.