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.”
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