Characteristics of Some Glasses

This information has been taken from various sources. Some manufacturers may change the composition of their glasses or the published information about them from time to time. Therefore, this information can only be used as a guide. If the information about strain, annealing, and softening points is important, contact the manufacturer for the most accurate information.

The temperature information is given in Celsius.

Strain point – the temperature below which no annealing can be done.

Annealing point – the temperature at which the equalisation soak should be done before the annealing cool.

Softening point – the temperature at which slumping can most quickly occur.

Armstrong – Now made by Kokomo

Typical Borosilicate – nominal CoE 32

Strain point – 510 - 535C / 951 - 996F

Annealing point – ca. 560C/1041F
Softening point - ca. 820C/1509F

Blackwood OZ Lead – nominal CoE 92

Annealing point - 440C/825F

Blenko – nominal CoE 110

Annealing point – 495C/924F

Bullseye – nominal CoE 90

Transparents

Strain point - 493C/920F

Annealing point - (532C)  Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 677C/1252F

Opalescents

Strain point - 463C/866F

Annealing point – (501C)  Note that Bullseye has changed this to 482C900F for thick items

Softening point - 688C/1272F

Gold Bearing

Strain point - 438C/821F

Annealing point - (472)   Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 638C/1182F

Chicago – nominal CoE 92

Desag  Note that this glass is no longer produced

Artista – nominal CoE 94

Strain point – 480 - 510C / 897 - 951F

Annealing point – 515 - 535C / 960 - 996F

Softening point – 705 – 735C / 1302 - 1356F

Fusing range – 805 – 835C / 1482 - 1537

Float Glass (Pilkington UK)

Optiwhite

Strain point – 525 - 530C / 978 - 987F

Annealing point – 559C/1039F

Softening point – 725C/1338F

Optifloat

Strain point – 525 - 530C / 978 - 987F

Annealing point – 548C/1019F

Softening point – 725C/1338F

Float Glass (typical for USA) nominal CoE 83

Strain point - 511C/953F

Annealing point - 548C/1019F

Softening point – 715C/1320F

Float Glass (typical for Australia) nominal CoE 84

Strain point - 505-525C / 942 - 978F 

Annealing point – 540 -560C / 1005 - 1041F

HiGlass “GIN” range – nominal CoE 90

Annealing point - 535C/996F

Gaffer colour rod – nominal CoE 88

Gaffer NZ Lead – nominal CoE 92

Annealing point - 440C/825F

HiGlass

Annealing point - 495C/924F

Kokomo – nominal CoE 92 - 94

Cathedrals

Strain point - 467C/873F

Annealing point - 507C/946F

Softening point - ca. 565C/ca.1050F

Opal Dense

Strain point - 445C/834F

Annealing point - 477C/891F

Softening point – ca. 565C/1050F

Opal Medium

Strain point - 455C/834F

Annealing point - 490C/915F

Softening point – ca.565C/1050F

Opal Medium Light

Strain point - 461C/863F

Annealing point - 499C/931F

Softening point – ca.565C/1050F

Opal Light

Strain point - 464C868F

Annealing point - 502C/937F

Softening point – ca.565C/1050F

Kugler 

Clear – nominal CoE 96 +/- 2  (94-98)

softening point: - 694C/1281F

Annealing point: - 508C/946F

Strain point: - 485C/904F

Colours - nominal CoE 96 +/- 4 (92-100)

Annealing point: - 500C/932F

Strain point: - 460C-500C/860 -879F

Typical lead glass – nominal CoE 91

Lenox Lead – nominal CoE 94

Annealing point – 440C/825F

Merry Go Round – nominal CoE 92

Moretti/Effetre – nominal CoE 104

Strain Point: 448C/839F

Annealing Range: 493C – 498C / 920F - 929F

Softening Point: 565C/1050F

Pemco Pb83 – nominal CoE 108

Annealing point – 415C/780F

Reichenbach - 

nominal CoE 96 +/-2 (94 -98)

Annealing range; - 470C-530F/878F-986F; Ave 510C/950F

nominal CoE 104  no further information at present.

Schott Borosilicate (8330) nominal CoE 32

Annealing point - 530C/987F

Schott “F2” Lead – nominal CoE 92

Annealing point - 440C/825F

Schott “H” & “R6” rods - nominal CoE 90

Annealing point – 530C/987F

Schott “W” colour rod – nominal CoE 98

St Just

MNA

Strain point - ca.450C/843F

Annealing point – ca. 532C/ca. 991F

Spectrum

System 96 – nominal CoE 96

Transparents

Strain point – 476C  +/- 6C  /  890F +/- 11F

Annealing point – 513 +/- 6C  /  956C +/- 11F

Softening point – 680 +/- 6C  /  1257F +/- 11F

Opalescents

Annealing point – 505 -515C  /  942 - 960F

Spruce Pine 87 – nominal CoE 96

Annealing point – 480C/897F

Uroboros system 96 – nominal CoE 96

Transparents

Strain point - 481C/899F

Annealing point - 517C/964F

Opalescents
Strain point - 457C/855F
Annealing point - 501C/935F

Uroboros - nominal CoE 90

Transparents

Strain point - 488C/911F

Annealing point - 525C/978F

Opalescents

Strain point - 468C/875F

Annealing point - 512C/955C

Wasser - nominal CoE 89

Annealing point – 490C/915F

Wissmach
Wissmach 90
Annealing point - 483C/900F
Softening point - 688C/1272F
Full Fuse - 777+

Wissmach 96
Annealing point - 483C/900F

Softening point - 688C/1272F

Full Fuse - 777+ / 1432+

• 
  

Solder Alloys, 2

This is an updated version of a table on various possibly useful solders.

Solder Alloy	Composition	Solidus	Liquidus	Uses
25/75	Sn/Pb	183C	266C	general plumbing, car radiators
30/70	Sn/Pb	183C	256C	general plumbing, car radiators
30/50/20	Sn/Pb/Zn	177C	288C	economical solder for aluminium, Zinc and Cast iron
40/60	Sn/Pb	183C	238C	brass, plumbing, car radiators
50/50	Sn/Pb	183C	216C	general purpose, plumbing, not for gold, silver
50/48.5/1.5	Sn/Pb/Cu	183C	215C	reduces copper erosion on irons
60/40	Sn/Pb	183C	190C	electronics, good wetting, duller surface than 63/37
63/37	Sn/Pb	183C	183C	eutetic, electronics, stainless steel, bright joints
62/37/1	Sn/Pb/Cu	183C	183C	similar to 63/37 and reduces erosion on irons
90/10	Sn/Pb	183C	213C
95/5	Sn/Pb	238C	238C	plumbing and heating
96.5/3/0.5	Sn/Ag/Cu	217C	220C	recommended lead free for electronics
95.8/3.5/0.7	Sn/Ag/Cu	217C	218C	wave and dip soldering
95.6/3.5/0.9	Sn/Ag/Cu	217C	217C	eutectic
95.5/3.8/0.7	Sn/Ag/Cu	217C	217C	European preference for wave and dip soldering
96.5/3.5	Sn/Ag	221C	221C	wide use, poor wetting, strong lead free joints, stainless steel
95/5	Sn/Ag	221C	254C	strong, ductile joints on copper, stainless steel
94/6	Sn/Ag	221C	279C	strong, ductile joints on copper, stainless steel
93/7	Sn/Ag	221C	302C	strong, ductile joints on copper, stainless steel

Ag = Silver
Cd = Cadmium
Cu =Copper
PB = Lead
Sn = Tin
Sb = Antimony

Common stained glass solder compositions

Cutter Wheel Angles, 1

The Effects of Wheel Angles on Glass Cutting

The wheel of a glass cutter does not “cut” the glass. The objective is to create a crack or "fissure" along which we expect the glass to break when we bend it. The idea is to produce a fissure which is continuous, and of uniform depth, without creating a flaky score line full of loose glass chips. While the wheel angle is only one of several variables which influence the quality of the fissure, it is the best place to start. The other main variables are wheel diameter and cutting pressure.

The angle of a wheel is identified as the included angle to which the apex is honed. This means it is measured from one beveled face of the wheel around through the wheel to the other face. Thus the angle between the wheel and the glass on a 150° wheel will be 15° on each side.
When downward pressure is exerted on the wheel rolling along the glass, forces are created which radiate down and to the side trying to shear or separate the glass along the surface. These forces are in a downward direction with little angle to the side when an appropriate angled wheel is used. If these forces are great enough to overcome the inherent compressive conditions near the surface, a crack or fissure will be generated along the path of the wheel. The direction of these shearing forces is determined by the wheel angle.

A wheel with a large or blunt angle produces shearing forces that tend to be directed downward more than to the side. It would require a great deal more cutter pressure to create enough lateral force to overcome the compression in glass. This explains why a cutter requires more pressure as it gets older. The apex tends to flatten so its effective angle becomes greater.

With a very sharp wheel angle, the shear forces are directed more parallel to the surface of the glass. This might suggest it is easier to produce a fissure with a sharp wheel than a dull one. The shear forces are directly opposing the compressive condition near the surface of the glass therefore, requiring less downward pressure to make a crack. But a sharp wheel tends to cause chips and a flaky score. Also, when the shear forces run close to the surface of the glass they are more likely to cause a lateral crack which then breaks out to the surface, creating a chip. You can see these chips leap out of the glass a short time after scoring. Again, the compressive condition of glass near the surface literally squeezes the fissure closed, spitting out loose chips. They can be seen lying on top of the glass.

Part 2

Based on information from the Fletcher Terry Company.

See also wheel angles

Revised 23.12.24

Cutter Wheel Angles, 3

The effect of glass thickness on cutting

Most of the thicker glass being used today is produced by the "float" method. In this process the glass travels horizontally from the furnace, through a molten tin bath, through annealing lehrs, then continues on rollers where it is inspected, scored and broken into the sizes required. The thickness generally dictates how fast the ribbon of glass moves. The thicker the glass, the slower it is processed and the more effective the annealing. This applies to thicker art glass too.

The key to subsequent cutting float glass is the annealing cycle. Thicker glass tends to have less compression at the surface and tension in the interior. As a result, the glass cutting wheel encounters less resistance to producing a fissure with the shearing forces. However, this means the glass surface will chip more readily. Therefore, a larger wheel angle is required to prevent chipping. It is also common practice to use a larger diameter wheel and larger angle so the fissure can be driven deeper without chipping.

Part 1

Part 2

Prepared from information provided by the Fletcher-Terry company.

Cutter Wheel Angles, 2

Effect of wheel angles on the cut edges of glass

Another factor to consider in selecting the proper wheel angle is the "edge". The objective of good glass cutting is to produce an edge which is flat and relatively free of irregularities such as "shark teeth".

Shark teeth are the occasional deep spikes in the edge and are accompanied with flakes or tiny chips on the surface. A three mm thick glass scored with a sharp wheel (114°) will produce this effect. This edge irregularity may lead to failure during the life of a window.

A three mm thick glass scored with a proper angle (134°) of wheel, will produce a fissure that is made up of individual "hackles" which overlap one another. They have a unique semi-circular shape and indicate the direction of the cutting wheel. With proper pressure the edge will be relatively free of irregularities and without shark teeth.

Part 3
Part 1

Prepared from information supplied by the Fletcher-Terry company

revised 23.12.24

Cutting concave curves

There are several methods that can be used to break out extreme inside curves. In all the cases you should retain a significant amount of glass around the edges of the curve. You should make this most difficult cut the first on the piece. If it fails, you may be able to move the glass a little and score again, without loosing too much glass.

To accomplish inside cuts by using the hand breaking method and/or pliers method, you must first score according to the cartoon line. Then you can make a series of concentric scores. Gently run the primary score line so any break does not run beyond this. Remove the graduated concentric scores in sequence.

In this example the glass is placed over the cartoon and scored directly over it.

You can also accomplish this type of cut by using the criss-cross pattern of score lines instead of concentric scores. First you must run the score of the curve to avoid the criss-cross lines from running beyond the curve. Then you begin to take out the little pieces from the waste area.


Another method is to score and run the curve (1), and then score a number of small crescents in the waste area, looking like fish scales or the fan type of paving seen in some European cities. Pull out each small crescent working toward the main curve (2,3,4).

Deep inside cuts with turntable

Deep inside cuts can be assisted by using a lazy susan – a turntable affair, similar to a cake decorating turntable.

image credit: Amazon

The first question you have to ask yourself is whether you should make such deep inside cuts or redesign the piece to avoid creating such fragile shapes.

OK. You have decided to go ahead with your plan in spite of good advice. Put your cartoon onto the turntable and the glass over it. If the glass is too dark or opalescent, make a template and mark the glass. Adjust the starting point, put one hand on the glass and cartoon, and turn the glass instead of yourself to get round the score with ease.

You still have the task of breaking out the glass from the score line. This is the subject of another tip on concave curves.

revised 23.23.24

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

Fixing the Perimeter Cames

When all interior leads are in place, the top edge came is cut to butt against the vertical side edge, using a small piece of lead the same size to act as a gauge. These edge cames should butt up to the came ends in the interior.

Place a narrow strip of wood against the top outside came and hold it in place with horseshoe nails. Check with a square or by measuring to be sure the just placed came is at right angles to the left side of the panel. Do the same with the other side.

If adjustment is necessary, firmly tap the wood batten with the hammer end of your lead knife to get it into position. Place nails to hold the cames in position and get ready to solder.

Cementing Brushes

image credit: stainedglasscraft.co.uk

Use stiff, but not hard bristle brushes for cementing. Nylon scrubbing brushes have a good stiffness without being too hard. Some natural bristle brushes are very hard and scratch the came excessively. In general, moderately stiff brushes with about 1 1/2" bristles are fine for cementing. There should be space between the bristle bunches to aid cleaning.  As they do not last very long, they should be cheap, but with firmly attached bristle bunches.

Cleaning the brushes is very simple. The action of rubbing the cement under the leads with whiting causes a natural cleaning action to take place. As the bristles flex back and forward over the came, the cement is forced upward toward the handle, and then outward between the bristle bunches. Only a little effort is required to finish the cleaning: push a rounded stick between the bunches to move out the remaining cement. You now have a clean brush for the next job.

The alternative is keeping the brush in water, but this presents the problem of getting rid of the water (oil and water do not mix) before beginning to cement. As the water will emulsify with the linseed oil, it will be carried into the putty, leaving gaps in the cement when the water eventually evaporates. The cement will eventually harden, even though in water, as linseed oil cures by creating an organic polymer through oxidisation. It can also rot the wood handles.

Keeping the brush in mineral spirits does keep the brush flexible but requires drying/evaporating the spirit before beginning the cementing to avoid the residue of the spirit creating cement that is too thin at the start. This can be a really messy problem!

If you choose the “dry” method, it is important to keep the brushes free of hardened cement as it will scratch the leads badly, if not the glass too. Most brushes will only last 5-10 uses, and as they are not expensive, should be easy to throw away.

Refractory fibres

 

There is a lot of imprecise terminology for refractory fibre paper and blanket.  My interpretation:

Shelf paper is a very thin - like cartridge paper - material held together with organic binders, and often containing fibreglass particles.  Thinfire and Papyros are two brand names.

Thinfire. photo credit Warm Glass

Fibre paper is rougher than shelf papers. The fibres are longer and not compressed so much. They seem to be available in 2-6mm thicknesses and are held together with organic or chemical binders.

Refractory fibre paper.  Photo credit Laurel Refractory

Fibre blanket tends to be uncompressed fibre from 12-75mm thick.  It relies on the interconnected fibres rather than binders to keep its thickness.

Refractory fibre blanket. photo credit Amazon

More information in this blog post. 

Lead Testing Kits Evaluation

An example of a lead testing kit from Amazon

There is legitimate concern about lead content of some glass intendended for culinary use.  Surface lead testing kits have become popular and indicate the presence of lead on many glasses.  It seemed to me that some evaluation of home lead test kits was in order.  I looked at some sites for scientific evaluations and some reviews of testing kits and found these results. 

Public Lab, whose mission is “Pursuing environmental justice through community science and open technology”, reports in the paper, “Evaluating Low-cost Lead Screening Products”, by Read Holman that “There are two evaluated [surface lead] test kits, the remaining three for surfaces have not been scientifically evaluated.” The report states that the tests for

“Paint/Surfaces...

• ESCA Tech, Inc. D-Lead Paint Test Kit. This product was "EPA-recognized" in 2010, but for negative results only; the rate of finding false-positives is 16% (Source PDF))
• 3M LeadCheck Swabs. This product was "EPA-Recognized" in 2010, but for negative results only; the rate of finding false-positives is 98% (Source PDF)). This is an extremely high false-positive rate.”

Source

There are seven other scientifically evaluated tests for dust and water, which are not applicable to glass surfaces.

 

The conclusion of a report for the US Dept of Commerce states:

“Currently available spot test kits cannot be used to determine lead-based paint, which is defined as a paint having lead at levels equal to, or greater than, 1mg/cm2 [the allowable level]. This finding was consistent with conclusions from several previously published field studies. As was found in the field studies, the spot test kits in this controlled laboratory study generally gave relatively high percents of false positives at the lead-based paint level of 1 mg/cm². That is, the spot test kits were generally sensitive to lead in paint at much lower levels” (p61)

Source 

The experience of people using these tests (reviews on Amazon) show that almost all surfaces show traces of lead, but at much lower concentrations than the allowable levels. 

A sample review:

“We got a heart attack because what we wanted to test turned positive, we proceeded to then test other stuff as a control, and guess what? All positive.  We got suspicious and started testing random objects that couldn’t possibly contain lead. They also turned positive!”  JSP Lead Test Kit

 

The high levels of false positives (up to 98%) leads me to question their value or accuracy.  Although I am not going to spend money on any of these tests, I suspect the test kits will show lead on clear glass too.

My conclusion is that these tests are not reliable indicators of risky levels of lead presence on the surface of glass artifacts.  Any concern needs a much more reliable test than the currently available surface lead test kits.

 

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.