Showing posts with label Transparent glass. Show all posts
Showing posts with label Transparent glass. Show all posts

Wednesday 27 March 2024

Kilnforming Opalescent Stained Glass


The statement that a sheet of glass can be fused to itself is true in certain circumstances.  It applies to transparent and some streaky glasses best.  These forms of glass are more likely to fuse together successfully although not formulated for fusing.

Transparent and Streaky Glasses

Of course, the best practice is to test for compatibility.  I found in my early days of sticking stained glass together that it was beneficial to test. In doing so, I found Spectrum and Armstrong transparent and streaky glass to be largely consistent across many sheets.  I did not have access to much Kokomo or Wissmach.  I cannot comment on how their glass behaves in terms of compatibility across the production range.  Not all transparent and streaky glass remains stable at fusing temperatures. There are some glasses that opalise, some change colour, some devitrify. This variability makes compatibility testing important - even for the transparent form of stained glass.

Photo credit: Lead and Light


Wispy Glasses

The statement about fusing to itself is less applicable to wispy glass.  Not all the wispy stained glass from the same sheet can be fused.  It seems to be dependent on the amount of opalescence in any one area of the glass.  I found that it is possible - if you are very careful - to fuse certain Spectrum wispies with the clear fusing standard on top, but not on the bottom.  This should be applicable to other manufacturers’ wispy glass too.  There must be a marginal compatibility that is contained by the clear fusing glass on top, but I am not certain.

Photo credit: Lead and Light

Opalescent Glasses

The statement about fusing to itself is almost completely inapplicable to opalescent glass.  Stained glass opalescent glass does not have the compatibility requirements of fusing glasses.  They very often severely devitrify when taken to fusing temperatures.  This devitrification means that opalescent stained glass is often not compatible with itself.  So, no amount of twiddling with schedules will make stained glass opalescent glass fusible, even with itself.

Manufacturers have spent a lot of time and effort to produce fusing compatible opalescent glass.  It is as though there is a minor element of devitrification embodied in the opalising process.  Whether this is so, it becomes very apparent on doing compatibility testing that opalescent stained glass has severe devitrification at fusing temperatures.


Stock photo


Compatibility Testing

It is important to test for compatibility before committing to the main firing.  Some transparent and streaky glass changes colour, devitrifies, and some opalise at fusing temperatures. This applies with even more force to wispies.  They contain a significant proportion of opalescence within them.  Some opalescents are so unstable at fusing temperatures that the devitrification becomes so bad the glass crumbles.

The importance of testing pieces of the sheet for compatibility before committing to a firing is reinforced by these factors.

Slumping

Slumping temperatures are not so high as fusing, and it is often stated that single layers can be slumped.  Again, it is not always true.

Some glasses change colour at slumping temperatures.  A few opalise. It is not always certain what effect moderate temperatures will have on stained glass.  The compatibility testing will show.  Observe the test firing at slumping temperatures.  Also, you will learn if there are changes at moderate temperatures.

One element must be commented upon about slumping.  It is important to have the edges finished to the appearance that you want the final piece to have.  The regularity of the edges without bumps or divots, and the degree of polish need to be showing before the firing starts.  The slumping temperatures are not high enough to alter the shape or appearance of the edges.


Firing of stained glass to itself is normally a low risk activity, but with unpredictable results.  It can teach a lot about behaviour of glass at higher temperatures.  Slumping single layer pieces can give information about the way single layers of glass slump or drape.  But testing is important for fusing.  And can inform about how the glass will react at slumping temperatures too.

Wednesday 14 February 2024

Differential Cooling of Transparent and Opalescent Glass

A statement was made on a Facebook group that transparent glass absorbs more heat than opalescent glass. And it releases more heat during cooling. The poster may have meant that the transparent heats more quickly than the opalescent, and cools more quickly.

Yes, dark transparent glass absorbs heat quicker than most opalescent (marginally), and it releases the heat more quickly (again marginally) than opalescent. The colour and degree of transparency do not absorb any more or less heat, given appropriate rates. They gain the same heat and temperature, although at slightly different rates due to differences in viscosity.

An occasional table


The rate of heating and cooling is important in maintaining an equal rate of absorption of heat. The temperature of both styles can become the same if appropriate lengths of heating, annealing, and cooling are used. The slightly different rates of heat gain can give a difference in viscosity and therefore expansion.  This slight mismatch during rapid ramp rates, might set up stresses great enough to break the glass. This can occur on the quick heat up of glass during the brittle phase (approximately up to 540ºC/1005ºF). In fact, most heat-up breaks occur below 300ºC/540ºF.

The main impact of differential heat gain/loss is during cooling. Annealing of sufficient length eliminates the problem of differential contraction through achieving and maintaining the Delta T = 5C or less (ΔT≤5C). It is during the cooling that the rates of heat loss may have an effect. The marginally quicker heat loss of many transparents over most  opalescent glass exhibits different viscosities and rates of contraction. The stresses created are temporary. But they might be great enough to cause breaks during the cooling. Slow cooling related to the thickness and nature of the glass takes care of the differential contraction rates by maintaining small temperature differentials.

Significance of Differential Heat Gain/Loss

Uneven thicknesses and the tack fusing profile both have much greater effects than the differential cooling rates of transparent and opalescent glass. It may be that strongly contrasting colours (such as purple and white) are also more important factors in heat gain and loss than transparent and opalescent combinations.  Cooling at an appropriate rate to room temperature for these factors will be sufficient to remove any risk of differential contraction between transparent and opalescent glasses.

Friday 27 August 2021

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 – nominal CoE
Annealing point - 470C/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: 493 – 498C / 920 - 929F
Softening Point: 565C/1050F

Pemco Pb83 – nominal CoE 108
Annealing point – 415C/780F

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+


Wednesday 21 July 2021

Viscosity of Colours

“I have been advised in the past, that blue fires quicker. I was told this by a Master glass maker.”

Viscosity has some relation to colour and intensity.  But you should note black & stiff black are both of the same intensity, and are fusing compatible, but have different viscosities.  This shows that colour is not the only determinant of viscosity, as the stiff black shows the viscosity can be adjusted within the same colour.  The quotation above indicates that the reasons behind any declarative statements need to be investigated.

Some factors in viscosity
Opalescent colours tend to be more viscous than their transparent counterparts.

It is the metals that develop the colours that produce much of the difference in viscosity.  The same metal can produce different colours in different furnace conditions, so viscosity cannot be assumed to be directly related to colour. 

Some people in the past have done their own tests on viscosity and colour relationships, but I have no access to them.  More recently Bob Leatherbarrow shows (Firing Schedules for Kilnformed Glass, 2018, chapter 7.2.5, p.88) some slumping tests done with opalescent glass. It shows how much less viscous black is than white, and that white is the most viscous.  The other results show red a little less viscous than white, then some greens, yellows and oranges, other greens, purple, pinks (in that order) and of course, the least viscous is black.


Transparent glasses tend to be less viscous than opalescent glasses.


How does this information relate to kilnforming practices?  It indicates that a piece with the less viscous glasses requires lower temperatures or less heat work to complete the forming of the glass than more viscous glasses.

When you have a combination of more and less viscous glasses in a piece you need to fire more slowly to ensure all the glass is thoroughly heated through and will deform equally.  You will need to observe and be prepared to move the piece on the mould to straighten it up.

Do your own viscosity tests
You can do your own tests for viscosity differences by arranging 10mm wide strips all the same length (about 30cm) of different colours. These should be placed on a kiln washed pair of narrow batts set parallel to each other 25cm apart and about 15cm high.  Fire at about 150°C per hour to about 650°C, setting the soak to 30 minutes.  Observe at intervals from 620°C.  Stop the firing when the least viscous has almost touched the floor of the kiln. When fired all together at the same time you can see the relative viscosity of the colours tested.  You can label these and store them, or tack fuse these labelled curves to a piece of base glass for future reference.