Slumping Tack Fused Glass Stringers

After you have tack fused your stringers, you will have a fragile blank, about four stringer layers thick. This will need to be handled gently and watched carefully during slumping.

When you slump, you should fire very slowly. Although thermal shock is not likely, the thin pieces have a tendency to crack when they bend into the mould. A safe firing schedule would be to advance at no more than 150C/hr to 540C, then increase the temperature at about 55C per hour toward 677C. Keep watching until the piece slumps into the mould, then advance to the next segment of the programme and anneal as usual for 6mm thickness.

For your first attempts, it's a good idea to use a shallow mould. After you get a feel for the process, you can achieve deeper slumps.

Further information is available in the ebook: Low Temperature Kiln Forming.

Tack Fusing Glass Stringers

The most time consuming part of tack fusing stringers is laying the stringers out. Most stringer bowls start with around four layers of stringers. The stringers need to be arranged in rows. It is often necessary to use a small amount of glue to keep the stringers in place as they are arranged. Some people glue the stringers directly to a piece of paper (normal or thinfire) to make them easier to arrange.

Making an 450mm square piece will take around six tubes of stringers.

If you want a piece where the individual strands of stringers are visible rather than fully fused, you will need to fire to as low a fusing temperature as possible. The precise temperature will, of course, vary by kiln. Most kilns will achieve tack fusing results in the range from 700C to 730C. Fire as quickly as you would like to around 675C, then increase the temperature very slowly, 50C per hour or less. You need to watch closely as the temperature approaches 700C. When the top layer of stringers begin to sag, start cooling the kiln. Firing too high will lead to a flat piece with no feel of the individual stringers.

Further information is available in the ebook: Low Temperature Kiln Forming.

Temperature Equivalents of Orton Pyrometric Cones

The pyrometric cones used by ceramicists can be very useful for checking the temperatures within your kiln. Bullseye have a test described on their website for discovering the heat distribution in the kiln. The Orton cones can provide an alternate means of testing. This process will also test the accuracy of the temperature readings of you controller/output.

You need to place the cones on supports all around the kiln. Small cones, wich are most useful for this purpose have their own supports built in. The behaviour of the cones will indicate both the temperature achieved - if you fire them according to instructions - and where the hotter and cooler parts of your kiln are located.

You do need to make visual observations to determine when the cone has matured. So you begin checking about 20C - 15C below the indicated maturing temperature. What you will see is the point of the cone bending down. When the point of the cone is pointing directly down, the maturing temperature has been achieved.

You can now check the temperature that is recorded by your read out. Write that down some where. Switch the kiln off now, if you want to see what temperature differences there are within your kiln. You do not need to do any controlled cooling. When cool enough, you can open the kiln and observe where the temperature has differed, by the extent to which the cones are pointing down. If the cone has completely conformed to the edge of its support, it has been over fired. Those that do not point directly down, have not reached the maturing temperature.

The cone numbers that are useful for kiln forming are 022 - 011. Remember that to achieve the temperatures, the cones must be fired at the indicated rate. Any other firing rates will not give accurate temperatures, as the cones are measuring heat work.

Large Orton Cones fired at the rate of 60C/hr over the last 100C will give the following temperature equivalents:
019: 676
018: 712
017: 736
016: 769
015: 788
014: 807
013: 837
012: 858
011: 873

However if you fire large cones at 150C/hr over the last 100C, you will get the following temperature equivalents:
019: 693
018: 732
017: 761
016: 794
015: 816
014: 836
013: 859
012: 880
011: 892

You of course, get different temperatures for the small cones of the same numbers. The small cones must be fired at 300C/hr over the last 100C.
022: 630
021: 643
020: 666
019: 723
018: 752
017: 784
016: 825
015: 843
014: 870
013: 880
012: 900
011: 915

If you decide to use self supporting cones, the evidence you are looking for is slightly different. In this case, the cone has achieved the heat work when the point is level with the base. If you fire the self supporting cones at 60C/hr for the last 100C you will get the following temperature equivalents:
022: 586
021: 600
020: 626
019: 678
018: 715
017: 738
016: 772
015: 791
014: 807
013: 837
012: 861
011: 875

A wall chart is available from the manufacturer

Glueing Glass Pieces

The best solution is to avoid the use of glue completely. If you cannot, use as little as possible and make sure it burns out cleanly.

The glues to which kiln workers have normal access, do not survive to tack fusing temperatures. Therefore they can only be considered as a means to get the glass assembly to the kiln. The glue will not hold the pieces in place until the glass begins to stick, so the pieces must have a stable placement. If not, the pieces will slip, roll and move once the glue has burned out.

The second requirement of glues is that they burn out without leaving a residue.

Glues that have been used with little or no residue include:

Powdered CMC that can be disolved in warm water

-CMC (carbylmethylcellulose). It is a cellulose based binder used in a wide variety of industries, including food. For our purposes, it is also used in the ceramics industry and is often called glaze binder. It is a main constituent of "glastac" from Bullseye. This can be made up into a viscous solution to catch and hold frits and other sprinkled elements in place.

- PVA (Polyvinyl Acetate) is water-based glue. It is sometimes known as school glue. It can be diluted to about 10parts water to 1 part PVA. This is sufficient to hold the glass pieces together with only a drop for each piece of glass. It does not work so well for small sprinkled elements.

One of many brands of  Ethyl Cyanoacrylate
 glue

- Super glue burns off with no concerns about cyanide. It should be used sparingly and also works best for pieces of glass.

One of many hair lacquers in pump spray bottles

- Hair lacquer is normally applied as drops at the edges of the assembled pieces and so can be used to hold pieces of glass as well as sprinkled elements.  It is sometimes used as a mist over areas of frit to keep the particles moving while being placed in the kiln.

In all uses of glue the principles to remember are:
- Use the minimum to hold pieces together while getting the work into the kiln.
- Put the glue at the edges of the glass or where its combustion gasses can escape easily.
- And in all cases, you need to test to see if a residue is left on the glass at full fuse when using a new glue.

An alternative to glue is frit as described here.

Revised4.1.25

Effect of Heat on Sandblasted textures

This is based on Graham Stone’s work with float glass. The temperatures are applicable to float glass, and so need to be adjusted for other glasses, but illustrate the principle of how heating temperatures affect the glass.
Temperatures in degrees Celsius.

650 Blasted surface softened, evened, less "brutal".
690 Blasting still opaque but less "white"
700 Blasting becoming too sheeny but still okay for certain effects.
740 Blasting now subtle and glossy

Based on Firing Schedules for Glass; the Kiln Companion, by Graham Stone, Melbourne, 2000, ISBN 0-646-39733-8, p24

Approximate Temperature Characteristics of Various Glasses

Various glasses have different temperature characteristics. This listing is an attempt to indicate the differences between a variety of popular glasses used in kiln forming. They are not necessarily exact, but do give an indication of differences.

Bullseye Transparents
Full fusing 832C
Tack fusing 777C
Softening 677C
Annealing 532C
Strain point 493C

Bullseye Opalescents
Full fusing 843C
Tack fusing 788C
Softening 688C
Annealing 502C
Strain point 463C

Bullseye Gold Bearing Glasses
Full fusing 788C
Tack fusing 732C
Softening 632C
Annealing 472C
Strain point 438C

Desag GNA
Full fusing 857C
Tack fusing 802C
Softening 718C
Annealing 530C
Strain point 454C

Float Glass
Full fusing 835C
Tack fusing 760C
Softening 720C
Annealing 530C
Strain point 454C

Oceanside
Full fusing 788C
Tack fusing 718C
Softening 677C
Annealing 510C
Strain point 371C

Wasser
Full fusing 816C
Tack fusing 760C
Softening 670C
Annealing 510C
Strain point 343C

Wissmach 90
full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Wissmach 96
Full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Youghiogheny 96
Full fusing  773C
Tack fusing  725C
Softening  662C
Annealing  510C
Strain point

Polishing with Cerium Oxide

If you want to go to a clear polish, cerium oxide will give an optical polish.

You need to grind your glass at 400 or higher grit, followed by resin bound smoothing and polishing discs.  Any rougher surface will not enable the cerium polish to work.

Many do not like to use cerium oxide as it is messy.  Especially so on a wet belt sander as the speed is really too fast for the use of polishing pastes.  The speed sprays the slurry all over the place.  You need a felt wheel or belt to which you apply the cerium oxide.  First you mix the cerium with water to a yoghurt consistency and apply that to the wheel or belt.  Begin polishing and add more water and cerium paste as the polishing surface dries.  You will notice this as the glass will begin to drag. Do not delay, add more of the paste before continuing. Otherwise you will heat up the glass and risk breakage.

His Glassworks has good descriptions and videos on use of cerium oxide.

It is helpful to mark the glass with a paint or china marker before starting the polishing process to show the areas that are to be polished. This enables you to see what work has been done without completely drying the piece.

Revised 4.1.25

Viscosity Changes with Temperature

This image is taken from Pate de Verre and Kiln Casting of Glass, by Jim Kervin and Dan Fenton, Glass Wear Studios, 2002, p.27.

It shows in graphic form how the viscosity of glass decreases with increases in temperature. The temperatures are given in Fahrenheit.  

The coefficient of expansion also changes with temperature. 

This graph is also from Kervin and Fenton

 It is these two forces of viscosity and expansion that must be balanced around the annealing point to give a stable and compatible range of fusing glass.

Drop Rings

Mould

It is possible to purchase drop rings of various sizes. It is also easy to construct one from vermiculite board or ceramic fibre board. Merely cut a circle of the desired radius from the board. Leave at least 50mm of board outside the circle ( more for thinner boards).

Kiln wash the top and inner sides of the drop ring

Glass

The glass should be larger than the hole in the ring. This will vary by radius of the hole. The glass will need to be from 50mm larger diameter than the hole for smaller holes to 100mm larger diameter for holes over 300mm.

Glass should be at least 6mm thick for the first 100mm of drop and an additional 3mm for each 50mm more. So, a drop of 200mm would require glass of 12mm thick.  A more accurate method of determining the thickness of glass in relation to hole diameter and length of drop is given by Frank van den Ham.


Temperatures

The temperature rise should be no more than 150C per hour to about 675C for 6mm glass and less for thicker glass. Remember the glass is much closer to the elements than normal and it is easy to thermal shock the glass.

With close inspection you can see that the edge of the glass rises from the mould as it sinks in the middle.

The outside edges of the glass rise from the mould as the centre begins to drop in the centre.  As the glass gets hotter, this raised edge settles back on to the mould.  If the glass is really near the elements, there is a small risk the glass will touch the elements.  No harm will be done to the kiln, but the glass edge may have some needles.

The rate and amount of slumping is controlled by temperature, span (the width of unsupported glass on the mould) and time. The higher the temperature the faster a piece will slump and the thinner the walls will be. However you can slump at lower temperatures by holding the temperature for a longer time to reduce the thinning of the sides.

Also note that the wider the span, the faster the glass slumps.

If you slump at high temperatures with a drop ring the sides of the bowl tend to be straight and steep. The strain is limited to the region immediately inside the rim. Therefore the glass tends to thin next to the rim and the colours are diluted. If you slump at a lower temperature for a longer period of time the strain is distributed over the entire unsupported area. This results in a more rounded shape for the bowl and even thickness of the glass across the bottom of the bowl.


Experiment

Finding the right combination of time and temperature requires a bit of experience and guess work. If you want a rounded bottom, heat the glass to the point that it starts to bend on the mould and wait for 30 minutes. If it has slumped about 1 inch in that time wait another 30 minutes. You are looking for a slumping rate that is acceptable. If it hasn't moved very much then increase the temperature 15C and check again in 15 minutes. Keep moving temp up and waiting for 15 minutes until the piece has completely slumped. This might take several hours.

If you want straight sides keep heating the piece rapidly.

Stopping
When the piece has slumped to the desired shape, flash cool the kiln to about 30C above the annealing point to stop movement in the glass. Extend the annealing soak and increase the length of the annealing cool time (reduce the rate of temperature fall) over normal slump firings of the same thickness.

Glass falls through drop rings in relation to the size of the glass on the drop ring, the size of the opening, the temperature rise rate and to some extent the colours and amount of opalescent glass used. 

There is an introduction to aperture drops here, that also links to many other elements of the subject.

Revised 5.1.25

Mould Cleaning

There are a variety of moulds available to kilnformers – slumping/draping, texture and casting are currently popular ones.  Each has a slightly different maintenance regime.

Slumping and Draping

Slumping and Draping moulds are the easiest to maintain, as they are not taken to high temperatures. Normally one application of kiln wash will last very many firings.  The kiln wash needs to be renewed when bare spots appear on the mould.  Some people immerse their moulds in water to wash off the old kiln wash.  This is excessive and requires a long slow drying time for ceramic before you can re-apply the separator.  

Normally, you only need light abrasion such as with a green washing up scrubby to clean off the old kiln wash. You can also use a nylon bristled brush to take off the old kiln wash and prepare it for a new application.

If you are using boron nitride on your slumping or draping mould, you need to brush off the old separator each time you fire the mould according to the manufacturers, although many so not. Certainly renew the boron nitride surface when any defects appear in the surface to ensure the glass does not stick.

Texture Moulds

Texture moulds require cleaning before applying any additional separator to avoid blurring or obscuring the textures of the mould.  It is best to use a kiln wash that does not have a lot of china clay in it, such as Primo, to allow easy brushing of the separator off.  If you use a kiln wash with little or no china clay, you will need to clean and re-apply each time you prepare to fire the mould.  Boron nitride works well for texture moulds, but also needs to be carefully brushed off the mould before re-applying the separator in preparation for the next firing.  This is both to avoid blurring the texture and to ensure there is sufficient separator to avoid sticking.

Casting

Casting moulds that are intended to be re-used multiple times are best coated with boron nitride.  The boron nitride should be lightly brushed off after each use to ensure the detail is retained, and then re-coated.  A nylon brush is good for this.

The materials and purposes of moulds have an effect on the separators used and the methods of maintaining them clean.

Revised 5.1.25

Wire for Fusing

Although there are other ways to combine wire with glass, one popular method involves fusing wire inside the glass. This technique generally fuses and seals the wire between two layers of glass, so it is important to select a wire with the right characteristics. The main characteristics are:

1. The wire must be capable of withstanding the heat of the kiln.

2. The wire must emerge from the kiln in a relatively pristine condition, or at least can be easily cleaned.

3. The wire must also retain the desired flexibility and pliability. If it's too soft or brittle it may not support the piece.

4. The wire must not react with or contaminate the glass. In most cases colour changes and metal flakes are not desirable.

5. The wire must be of a small enough diameter to avoid causing excessive stress within the glass.

6. It is a bonus if the wire is reasonably priced or even inexpensive.

This post gives the characteristics of some types of wire for fusing.