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.

Effects of Dam Materials on Scheduling

 I once made a statement about the effects of various dam materials on scheduling. This was based on my understanding of the density of three common refractory materials used in kilnforming – ceramic shelves, vermiculite board and fibre board. I decided to test these statements.  This showed I was wrong in my assumptions.

I set up a test of the heat gain and loss of the three materials. This was done without any glass involved to eliminate the influence of the glass on the behaviour of the dams. The dam materials were laid on the kiln shelf with thermocouples between. These were connected to a data logger to record the temperatures.

Test Setup

 The thicknesses of the dams may be relevant. The vermiculite and fibre boards were 25mm thick. The ceramic dam material was 13mm thick.

The schedule used was a slightly modified one for 6mm:

• 300°C/hr to 800°C for 10 minutes
• Full to 482°C for 60 minutes
• 83°C to 427, no soak
• 150°C to 370°C, no soak
• 400°C to 100°C, end

 

The data retrieved from the data recording is shown by the following graphs.

Temperature profile of the air, ceramic, fibre, and vermiculite during the firing.

Highlights:

• The dam materials all perform similarly.
• This graph shows the dams have significant differences from the air temperature – up to 190°C – during the first ramp of 300°C/hr. (in this case).
• There is the curious fall in the dams’ temperatures during the anneal soak. This was replicated in additional tests. I do not currently know the reasons for this.
• The dams remain cooler than the air temperature until midway during the second cool when (in this kiln) the natural cooling rate takes over.
• From the second cool to the finish, the dams remain hotter than the air temperature.

 Some more information is given by looking at the temperature differentials (ΔT) between the materials and the air. This graph is to assist in investigating how significantly different the materials are.

This graph is initially confusing as positive numbers indicate the temperature of the first is cooler than the material it is compared with, and hotter when in negative numbers.

 

A= air; C=ceramic; F=fibre board; V=vermiculite

Temperature variations between air and dams

 As an assistance to relating the ΔT to the air temperature some relevant data points are given. The data points relate to the numbers running along the bottom of the graph.

 Data Point       Event

• 1            Start of anneal soak.
• 30          Start of 1^st cool (482°C)
• 45          Start of 2^nd cool (427°C)
• 65          Start of final cool (370°C)
• 89          1^st 55°C of final cool (315°C)
• 306         100°C

 

At the data points:

• At the start of anneal soak the ΔT between the dams is 16°C with the ceramic shelf temperature being 18°C hotter than the air.
• At the end of the anneal soak of an hour, the air temperature is 20°C higher, although the ΔT between the dams has reduced to 12°C.
• At the end of the 1^st cool the ΔT between the dams has reduced to 9°C and the ΔT with the air is 3°C.
• At approximately 450°C the air temperature becomes less than the dams.
• At 370°C the hottest dams are approximately 17°C hotter than the air.  The ΔT between the dams is 10°C.

 More generally:

• The air temperature tends to be between 17°C hotter and 17°C cooler than the ceramic dams during the anneal soak and cool.  The difference gradually decreases to around 8°C at about 120°C.
• Ceramic and fibre dams loose heat after the annealing soak at similar rates – having a ΔT between 4°C and 1°C, with a peak difference of 9°C at the start of the second cool. This means the heat retention characteristics of ceramic strips and fibre board are very close.
• Between the annealing soak and about 300°C the vermiculite is between 12°C and 9°C hotter than the same thickness of fibre.  Vermiculite both gains and loses heat more slowly than the ceramic or fibre dams do. This means that vermiculite is the most heat retentive of the three materials.
• Vermiculite remains hotter than ceramic from the start of the second cool. This variance is up to 9°C and decreases to 3°C by 100°C.
• Fibre board is cooler than ceramic dams until the final cool starts, when there is little variance.  At the start of the second cool there is about 15°C between the two.
• Vermiculite remains cooler than fibre dams throughout the cooling process. This ranges from about 12°C at the start of the first cool to about 3°C at 100°C.

Since we cannot see more than the air temperature on our controllers it is useful to compare air and dam temperatures. The same data points apply as the graph comparing differences between materials.

 

Ceramic-Vermiculite; Ceramic-Fibre Board; Vermiculite-Fibre Board; Ceramic-Air Temperature
This graph shows the temperature differences throughout the cooling of various materials.

• During the annealing soak, the air temperature is greater than the dam temperatures. The fibre and vermiculite boards remain at similar temperatures and the ceramic dam is the coolest.
• The three dam materials even out with the air temperature at the start of the second cool.
• Through the second and final cools, vermiculite dams remain hotter than the air temperature – between about 24°C at start of the final cool and 9°C at 100°C.
• The ceramic and fibre dams are close in temperature difference to the air from the start of the final cool. Their ΔTs are 17°C at the start of the final cool and 6°C at 100°C.

Conclusions

• Dams will have little effect during the heat up of open face dammed glass.  The slight difference will be at the interface of the glass and the dams where there will be a slight cooling effect on the glass. Therefore, a slightly longer top soak or a slightly higher top temperature may be useful.
• The continued fall in the dams’ temperature during the anneal soak indicates that this soak should be extended to ensure heat is not being drained from the edges of the glass by the dams. There is the risk of creating unequal temperatures across the glass.
• The ability of ceramic and fibre dams to absorb and dissipate heat more quickly indicates that they are better materials for dams than vermiculite board. The slightly better retention of heat at the annealing soak, indicates that ceramic is a good choice when annealing is critical.        
• These tests were fired as for 6mm/0.25” glass and so show the greatest differences. Firing for thicker glass will use longer soaks and slower cool rates. These will allow the dams to perform more closely to the glass temperature during annealing and cool.

Based on these observations, I have come to some conclusions about the effect of dams on scheduling.

• There is no significant effect caused by dams during the heat up, so scheduling of the heat up can be as for the thickness of the glass.
• The lag in temperature rise of the dams indicates a slightly longer soak at the top temperature (with a minor risk of devitrification), or a higher temperature of, say 10°C, can be used.
• The (strange) continued cooling of the dams during the annealing soak indicates that extending the soak time to that for a piece 6mm thicker than actual is advisable.
• The cool rates can continue to be as for the actual thickness, as the dam temperatures follow the air temperature with little deviation below the end of the first cool.
• Ceramic dams of 13mm/ 0.5” perform better than 25mm/1.0” vermiculite and fibre board. 
• However, in further tests of 25mm/1.0” thick ceramic dams performed similarly to the same thickness of vermiculite. So, 25mm/1.0” fibre board the best when choosing between the three materials of the same thickness. But 25mm ceramic strips are not common, nor are they needed for strength or weight.
• The performance of the three dam materials tested do not show enough difference in temperature variation to have significant affects on the annealing and cooling at times and rates appropriate to the thickness of the glass.
• It is the thermal insulation properties of the dam material, rather than the density that has the greatest influence on performance as a dam material.

 

 

Effects of Dams on Scheduling

 I recently made a statement about the effects of various dam materials on the scheduling.  This was based on my understanding of the density of three common refractory materials used in kilnforming – ceramic shelves, vermiculite board and fibre board.  I decided to test these statements.  I found I was wrong.

I set up a test of the heat gain and loss of the three materials.  This was done without any glass involved to eliminate the influence of the glass on the behaviour of the dams.  The dam materials were laid on the kiln shelf with thermocouples between.  These were connected to a data logger to record the temperatures.

 

The schedule used was a slightly modified one for 6mm:

300°C/hr to 800°C for 10 minutes

Full to 482°C for 60 minutes

83°C to 427, no soak

150°C to 370°C, no soak

400°C to 100°C, end

 

The data retrieved from the data recording is shown by the following graphs.

 

Highlights:

·        The dam materials all perform similarly. 

·        This graph shows the dams have significant differences from the air temperature – up to 190°C – during the first ramp of 300°C/hr. (in this case). 

·        There is the curious fall in the dams’ temperatures during the anneal soak.  This was replicated in additional tests.  I do not currently know the reasons for this.

·        The dams remain cooler than the air temperature until midway during the second cool when (in this kiln) the natural cooling rate takes over.

·        From the second cool to the finish, the dams remain hotter than the air temperature.

 

Some more information is given by looking at the temperature differentials (ΔT) between the materials and the air.  This graph is to assist in investigating how significantly different the materials are. 

This graph is initially confusing as positive numbers indicate the temperature is cooler than the material being compared and hotter with negative numbers.

 

As an assistance to relating the ΔT to the air temperature some relevant data points are given.  The data points relate to the numbers running along the bottom of the graph.

Data Point   Event

    1                Start of anneal soak.

    30              Start of 1^st cool (482°C)

    45              Start of 2^nd cool (427°C)

    65              Start of final cool (370°C)

    89              1^st 55°C of final cool (315°C)

    306             100°C

 

At the data points:

·        At the start of anneal soak the ΔT between the dams is 16°C with the ceramic shelf temperature being 18°C hotter than the air.

·        At the end of the anneal soak of an hour, the air temperature is 20°C higher, although the ΔT between the dams has reduced to 12°C.

·        At the end of the 1^st cool the ΔT between the dams has reduced to 9°C and the ΔT with the air is 3°C.

·        At approximately 450°C the air temperature becomes less than the dams. 

·        At 370°C the hottest dams are approximately 17°C hotter than the air.  The ΔT between the dams is 10°C.

 

More generally:

·        The air temperature tends to be between 17°C hotter and 17°C cooler than the ceramic dams during the anneal soak and cool.  The difference gradually decreases to around 8°C at about 120°C.

·        Ceramic and fibre dams loose heat after annealing at similar rates – generally having a ΔT between 4°C and 1°C, with a peak difference of 9°C at the start of the second cool. This means the heat retention characteristics of ceramic strips and fibre board are very close.

·        Between the annealing soak and about 300°C the vermiculite is between 12°C and 9°C hotter than the same thickness of fibre.  Vermiculite both gains and loses heat more slowly than the ceramic or fibre dams do.  This means that vermiculite is the most heat retentive of the three materials.

Conclusions

·        Dams will have little effect during the heat up of open face dammed glass.  The slight difference will be at the interface of the glass and the dams where there will be a slight cooling effect on the glass.  Therefore, a slightly longer top soak or a slightly higher top temperature may be useful.

·        The continued fall in the dams’ temperature during the anneal soak indicates that this soak should be extended to ensure heat is not being drained from the glass by the dams to give unequal temperatures across the glass with the risk of inadequate annealing.  I suggest the soak should be extended to that for glass of 6mm thicker than actual to account for this.

·        The ability of ceramic and fibre dams to absorb and dissipate heat more quickly indicates that they are better materials for dams than vermiculite board.  The slightly better retention of heat at the annealing soak, indicates that ceramic is a good choice when annealing is critical.

Scheduling Effects 

Based on these observations, I have come to some conclusions about the effect of dams on scheduling.

·        There is no significant effect caused by dams during the heat up, so scheduling of the heat up can be as for the thickness of the glass.

·        The lag in temperature rise by the dams indicates a slightly longer soak at the top temperature (with a minor risk of devitrification), or a higher temperature of, say 10°C can be used.

·        The (strange) continued cooling of the dams during the annealing soak indicates that extending the soak time to that for a piece 6mm thicker than actual is advisable.

·        The cool rates can continue to be as for the actual thickness, as the dam temperatures follow the air temperature with little deviation below the end of the first cool. 

·        Ceramic dams perform the best of the three tested materials.

 

Limits to the “Low and Slow” Concept

I frequently advocate using slow rates of advance and low temperatures to achieve the results desired with a minimum of marking in forming, or a minimum of firing difficulties during the fusing part of kilnforming. 

But there are limits to this both in terms of physics and practicality.  There are temperatures below which no amount of slow heat input will affect the brittle nature of the glass, for example.  If your temperature is below the strain point of the glass, virtually no change will occur even with very long soaks.  The graph below shows the slumping range is from the annealing point (glass transition temperature) to about 180C above the annealing temperature.  After that temperature the glass is prone to devitrification (the beginnings of crystallisation). 

This shows the the slumping range of a specialised glass rather than the soda lime glass that kilnformers normally use.

In this graph, the glass has an annealing temperature of about 600C, which is higher than that for float glass and much higher than for kilnforming glasses.  The glass transition temperature range for existing fusing compatible glasses is around 510C (+/- ~10C).  Float glass has a higher annealing point of around 540C (+/- ~ 10C). Following the research behind this graph, stable slumping temperatures would be in the range of about 510C to 690C (+/- 10C).  

It is important to be aware that the annealing point is determined mathematically as the glass transition point.  This is the annealing point at which temperature the glass can be most quickly annealed. The practical research conducted by Bullseye has shown that a temperature equalisation soak in the lower part of the annealing range is a good solution to the the practicality of balancing adequate annealing with the use of the kiln time.  The annealing point temperature and that which you use to equalise the temperature within the glass may be quite different.

Even where it is possible to achieve an effect at a low temperature, it can take too long to be practical.  For example, I can bend float glass at 590°C in 20 minutes into a 1/3 cylinder.  I could also bend it at 550°C (just 10°C above the annealing point), but it would take more than 12 hours. This is not practical.

In addition to practicality, there is the physical limitation.  If you slump below the glass transition point, you will be unable to properly anneal the glass and therefore produce an unstable item.  It will contain stress from this inadequate annealing leading to an increased fragility.

The balance required between the rate of advance and top temperature means that you will need to do your own experiments to find where the practical limits to using heat work are for you. The more patient you are, the lower temperature you can use.

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