Slow and Low

Low and Slow Approach to Kilnforming

We are often impatient in firing our pieces and fire much more quickly than we need. After all, our computerised controllers will look after the firing overnight. So there is no need to hurry more than that.

The concept of heat work is essential to understanding why the slow and low method of firing works. Glass is a poor conductor of heat which leads to many of our problems with quick firings. The main one is stressing the glass so much by the temperature differential between the top and the bottom that the glass breaks. We need to get heat into the whole mass of the glass as evenly and with as smooth a temperature gradient as possible. If we can do that, the kiln forming processes work much better. If you add the heat to the glass quickly, you need to go to a higher temperature to achieve the desired result than if you add the heat more slowly to allow the heat to permeate the whole thickness of the piece.

Graphs of the difference (blue line) between upper and lower surfaces of glass of different thicknesses against cooling time

However, this slower heating means that the glass at the bottom has absorbed the required heat at a lower temperature than in a fast heat. This in turn means that you do not need to go to such a high heat. This has a significant advantage in forming the glass, as the lower temperature required to achieve the shape means that the bottom of the glass is less marked. The glass will have less chance of stress at the annealing stage of the kiln forming process as it will be of a more equal temperature even before the temperature equalisation process begins at the annealing soak temperature.

Applying the principles of low and slow means:

• heat is added evenly to the whole thickness of the piece
• there is a reduction in risk of thermal shock
• the glass will achieve the desired effect at a reduced temperature

The alternative - quick ramps with soaks – leads to a range of difficulties:

• The introduction of heat differentials within the glass. Bullseye research shows that on cooling, a heat difference of greater than 5ºC between the internal and external parts of glass lead to stresses that cannot be resolved without re-heating to above the annealing point with a significant soak to once again equalise the heat throughout the piece.
• It does not save much if any time, As the glass reacts better to a steady introduction of heat. Merely slowing the rate to occupy the same amount of time as the ramp and soak together occupy, will lead to fewer problems.
• It can soften some parts more quickly than others, e.g., edges soften and stick trapping air.
• Quick heating, with “catch up” soaks, of a piece with different types and colours of glass is more likely to cause problems of shock, bubbles, and uneven forming.
• Pieces with uneven thicknesses, such as those intended for tack fusing, will have significant differences in temperature at the bottom.
• Rapid heating with soaks during slumping and draping processes can cause uneven slumps through colour or thickness differences, or even a tear in the bottom because the top is so much more plastic than the bottom.

However there are occasions where soaks during the initial advance in heat are useful:

• for really thick glass,
• For multiple - 3 or more - layers of glass,
• for glass on difficult moulds,
• for glass supported at a single internal point with other glass free from contact with mould as on many drapes.

Of course, if you are doing small or jewellery scale work, then you can ignore these principles as the heat is gained relatively easily. It is only when you increase the scale that these principles will have an obvious effect.

Slow, gradual input of heat to glass leads to the ability to fire at lower temperatures to achieve the desired results, with less marking and less risk of breaking.

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

Maintaining a Single Colour on the Edge

The piece in the middle distance shows the different colours of the two layers

Keeping the edge one colour on a two or multiple colour piece can be done by cutting the upper layer larger than the lower one(s). If you are making a 6mm thick piece, the upper piece needs to be 3mm larger all around. So if you have a 300mm diameter base piece, the top will need to be 306mm in diameter. This allows a coloured top to bend over a clear base, giving the appearance of a single colour throughout.

However if you are building thicker than 6mm and are willing to allow the glass to flow, you do not need to add the full thickness of the glass to the size of the capping piece. I find that at 9mm thick, I need only 4mm all around to cover the layers. This may be because the outer edges are nearer 6mm than 9mm thick.

Of course, if the final piece needs to be a pre-determined size the lower layers can be cut 3mm smaller than the top all around (for 6mm thick pieces).  The top is cut to the final size of the piece.

Observation Ports for Kilns

Observation Ports for Kilns

When choosing a kiln, an often overlooked element is the observation ports. These openings in the side or top of the kiln enable you to observe the progress of your work during a firing without opening the kiln lid or door. They have ceramic or fibre plugs to keep the heat in the kiln when you are not using them to observe what is happening. 

A kiln with a very large quartz observation panel

Some newer kilns are built with quartz observation panels in the kiln. These serve the same purpose as the ports, but without the (small) additional heat loss.

When doing any new work it is important to observe the progress of work, rather than just hope for the best and see what has happened after the whole process is finished. Observation can tell you when the piece has reached the desired stage and progress to the next part of the programme.

A port located too high to be of use for observation of the interior.  It is sealed with a ceramic fibre plug.

The location of the port is important. You need to be able to see the relevant part of the kiln or they are useless. 

This relatively large kiln has two ports, one at the center of the door, and one on top.  The top is mostly for ventilation.  The one in the door may be too high to observe work while firing unless the shelf is put up on tall kiln furniture

Although a small kiln, the observation port at the top is not so useful as one at the side.

A popular kiln with an appropriately placed observation port.  Often these have an additional one on the side opposite the controller. 

 Some kilns have multiple ports to make observation of various parts of the kiln easier.

There are a variety of shapes of these ports. The shape is not so important as the location and what can be viewed within the kiln through the ports.

A round port, but probably too low to be of much use

A rectangular port viewed from the inside showing the field of view that can be allowed

A kiln with multiple square ports

If your kiln has come without a port or one that is not placed where most suitable for your use, you can drill the casing and brick or fibre to provide another viewing port.  Make a ceramic plug or wad up ceramic fibre blanket to fill the hole when it is not in use.

Boiled Glass

This is a technique that will obtain a random, organic feel to glass that would otherwise be scrap (cullet) – remembering that you have to use compatible glass throughout. The principle is to take the temperature up high enough for the glass to begin to flow easily and bubbles to blow through and burst.

The results can be used as they come out, or they can be cut to provide points of interest in other work, or the glass pieces can be damed before firing to obtain thick pieces which can be cut into slices for other work. And I am sure, there are numerous other ways to use the resulting glass too.

The effects are rather like colourful molten rock with gases bubbling through. These bubbles mix the glass colours. So you need to be sure you do not use a wide variety of colours, or your result will be similar to the molten rock - muddy. Use a few contrasting colours, and ensure you include a significant proportion of white to maintain bright colours. Also remember that the hot colours – reds, yellows, oranges – opalise at high temperatures, so the transparents can be used as opals.

You can use whole sheets of glass or scraps. In either case, it is useful to start with a clear base to help avoid picking up kiln wash when the glass is moving about. The glass must be clean to reduce the incidence of devitrification. Stack you glass on top of the base glass in what ever order you like. Contrasting colours alternated give a strong result.

You can put shelf paper of 0.5 mm or thicker on the shelf or simply kiln wash the shelf with several layers of wash until the shelf surface is no longer visible through the wash. Use of thinfire is not recommended as the powder can be pulled into the glass.

If you do not dam the area to contain the glass calculate how far the glass will expand on the shelf, so that you do not put down too much glass and have it spill over the edge of the shelf.  

You can use bubble powder onto the base layer to promote the bubbling during the firing. However, if you are using cullet, you can just take the temperature up rapidly without a bubble squeeze, which will give you plenty of air pockets to burst through the layers of glass.

You can take the temperature up at about 300ºC per hour to 925ºC with no bubble squeeze and soak for 10 – 15 minutes. Then allow the kiln to drop the temperature as fast as possible to about 815 and soak there for around 30 minutes to allow the little bubbles to rise to the surface an burst too. Then reduce to the annealing temperature and soak for the thickness you calculated in preparation for the firing.

Precautions

You need to be careful in firing and annealing pieces using this glass. Any glass that has been fired to a high temperature tends to begin changing compatibility. So you need to be careful on your rates of advance, and on the annealing and cooling portions of the firing when using the glass in other projects. You may want to consider using a schedule for twice the thickness of the piece on subsequent firings.

There may be devitrification on the surface. You should sandblast or abrade away this devitrification in some way to be able to get a shining surface when you fire polish.

There may also be a number of pin hole sized bubbles at or just below the surface. These will close with a fire polish also.

Lead Corrosion in Acids

Lead forms a protective film, which if undisturbed preserves the metal below this layer.

The corrosion resistance of lead is based on its ability to readily form a tenacious coating of a reaction product. This then becomes a protective coating. Protective coatings on lead may form as the result of exposure to sulphates, oxides, carbonates, chromates, or chemical complexes.

Handbook of Corrosion Data, by Bruce D Craig, p26

Lead is resistant to corrosion especially “with solutions containing sulphate ions, such as sulphuric acid.”

However, the new or bright metal reacts quickly with a variety of alkalis and many organic (although not most inorganic) acids.  “...Lead is not stable in nitric and acetic acids, nor in alkalis. The metal does not resist nitric acid. Lead corrodes rapidly in acetic and formic acids.” (Handbook of Corrosion Data, by Bruce D Craig, p.29)

Lead has very limited resistance to acetic acid.... Dilute [acetic acid], even at room temperature attacks lead at rates exceeding 1.3mm/year. These rates increase rapidly with increasing aeration and velocity However … acetic acid … has little effect at strengths of 52% to 70%.

The corrosion rate in acid increases rapidly in the presence of oxygen and also in oxygen in combination with soft waters such as rain and distilled water. Corrosion increases at the rate approximately proportional to the oxygen content of the water.”

Handbook of Corrosion Data, by Bruce D Craig, p.26, 29

This another good reason to avoid vinegar as a cleaning agent for leaded windows.

Lead dissolves in organic acids (in the presence of oxygen). Lead also dissolves in quite concentrated alkalis (≥10%) because of the characteristic of the lead salts that can act as either an acid or an alkali. These salts are soluble in the presence of water and oxygen.

Alkali salts are soluble hydroxides of alkali metals and alkali earth metals, of which common examples are:

• Sodium hydroxide (often called "caustic soda")
• Potassium hydroxide (commonly called "caustic potash")
• lye (generic term, for either of the previous two, or even for a mixture)
• Calcium hydroxide (saturated solution known as "limewater")
• Magnesium hydroxide is an example of an atypical alkali since it has low solubility in water (although the dissolved portion is considered a strong base due to complete dissociation of its ions).

Although this has been a rather technical posting, these data show that lead is subject to rapid attack by both organic (and some inorganic) acids and alkalis in relatively low concentrations when in the presence of aerated water. However in normal environmental conditions the protective reaction layer avoids much of this vulnerability.

Tack Fusing Considerations

1 – Initial Rate of Advance

Tack fuses look easier than full fusing, but they are really one of the most difficult types of kiln forming. Tack fusing requires much more care than full fusing.

On heat up, the pieces on top shade the heat from the base glass leading to uneven heating. So you need a slower heat up. You can get some assistance in determining this by looking at what the annealing cool rate for the piece is. A very conservative approach is needed when you have a number of pieces stacked over the base layer.  One way of thinking about this is to set your initial rate of advance at approximately twice the anneal cool rate. More information on this is given in this entry. 



2 – Annealing 

The tacked glass can be considered to be laminated rather than fully formed together. This means the glass sheets are still able, partially, to act  as separate entities. So excellent annealing is required.

Glass contracts when it's cooling, and so tends to pull into itself. In a flat, symmetrical fuse this isn't much of a problem. In tack fuses where the glass components are still distinct from their neighbours, they will try to shrink into themselves and away from each other. If there is not enough time for the glass to settle into balance, a lot of stress will be locked into the piece that either cause it to crack on cool down or to be remarkably fragile after firing. In addition, in tack fusing there are very uneven thicknesses meaning it is hard to maintain equal temperatures across the glass. The tack fused pieces shield the heat from the base, leading to localised hot spots on cool down.

On very difficult tack fuses it's not unusual to anneal for a thickness of four to six times greater than the actual maximum thickness of the glass. That extended cool helps ensure that the glass has time to shift and relax as it's becoming stiffer, and also helps keep the temperature more even throughout.

So in general, tack fused pieces should be annealed as though they are thicker pieces. Recommendations range from the rate for glass that is one thickness greater to at least twice the maximum thickness – including the tacked elements – of the whole item. Where there are right angles - squares, rectangles - or more acutely angled shapes, even more time in the annealing cool is required, possibly up to 5 times the total thickness of the piece.

It must be remembered especially in tack fusing, that annealing is much more than the annealing soak. The soak is to ensure all the glass is at the same temperature. The anneal cool over the next 110ºC is to ensure this piece of different thicknesses will all react together. That means tack fusing takes a lot longer than regular fussing.

3 – Effects of thicknesses, shapes, degree of tack

The more rectangular or pointed the pieces there are in the piece, the greater the care in annealing is required. How you decide on the schedule to use varies. Some go up two or even four times the total thickness of the piece to choose a firing schedule.

A simplistic estimation of the schedule required is to subtract the difference between the thickest and the thinnest part of the piece and add that number to the thickest part. If you have a 3mm section and a 12mm section, the difference is 9mm. So add 9 to 12 and get 17mm that needs to be annealed for. This thickness applies to the heat up section as well.

Another way to estimate the schedule required is to increase the length the annealing schedule for any and each of the following factors:

·         Tack fusing of a single additional layer on a six millimetre base

·         Rectangular pieces to be tack fused

·         Sharp, pointed pieces to be tack fused

·         Multiple layers to be tack fused

·         Degree of tack – the closer to lamination, the more time required

The annealing schedule to be considered is the one for at least the next step up in thickness for each of the factors. If you have all five factors the annealing schedule that should be used is one for at least 21mm thick pieces according to this way of thinking about the firing.

4 – Testing/Experimentation

The only way you will have certainty about which to schedule to choose is to make up a piece of the configuration you intend, but in clear. You can then check for the stresses. If you have chosen twice the thickness, and stress is showing, you need to try 3 times the thickness, etc. So your annealing soak needs to be longer, if stress shows. You can speed things by having your annealing soak at the lower end of the annealing range (for Bullseye this is 482C, rather than 516C).

You will need to do some experimentation on what works best for you. You also need to have a pair of polarisation filters to help you with determining whether you have any stress in your piece or not. If your piece is to be in opaque glasses, you need to do a mock up in clear.

Supporting Overhangs on Moulds

In general, the blank should be no larger than the thickness of the glass over the mould. So a 6mm blank would have no more than 6mm overhang.

In the case of steep sided moulds, the glass should be entirely within the mould to avoid any hangup on the edge, leading to uneven slumps and needling on the edges.

But, if you need the glass to be the size of the mould, you can make a collar to go around the mould, which will support the glass while it begins to slump into the mould.

Make a donut shape that will fit around the mould (whether round, oval or rectangular) and extend beyond. Support the collar on kiln furniture to be as high or slightly higher than the top of the rim of the mould. This makes a kind of drop out ring, allowing the glass to drop into the mould.

Donut ring suitable for placing around a circular mould

This arrangement is suitable for placing around a mould of the same diameter as the interior of the ring

Make sure that the collar is well covered with kiln wash to ensure the glass can move along the fibre board. This includes both the surface and vertical edges of the collar.

As the glass softens and begins to fall into the mould, the glass at the edge does not have the weight to bend down and so raises off the collar and begins to slip into the mould.

And finally, you need to ensure that the mould is not so steep as to trap the glass inside. This is more of a concern on steel with its greater expansion and contraction than ceramic.

A steel mould likely to trap the glass inside with its vertical sides

Super Glue

Super Glue

by Morganica

There are multiple cyanoacrylates (superglues) on the market, and they will give very different results. Gel superglue formulations usually have some type of rubber or fumed silica additive to make them thicker, and the additive usually doesn't burn out. That's probably where the "superglue leaves a mark" originates. Usually the cheapest possible superglue is best for temporary glass holds because it'll mostly be additive-free.

The glue will burn out around 700F or so, so it shouldn't be used to position the glass against gravity. I disagree, however, that it should never be used. I buy cheap superglue by the carton and use it in everything from temporary casting assemblages to making glass boxes for frit panels to tack-fusing. It is the best way I know to hold wobbly pieces in place until you can assemble the rest of the glass around it.

Some tips for using superglue:

• You are more likely to get whitish residues if you let moisture get to the superglue while it's drying, so keep the glass surfaces as dry as possible and don't put a superglue-assembled piece on a wet kiln shelf.

• Always try to put the glue under opaque or dark glasses, just in case something goes wrong.

• Use the smallest amount possible. Don't flood an area with glue and lay the glass on top - that will almost always leave too much glue on the glass. Instead, I assemble the glass and put a drop of glue right where the two glasses join. Capillary action sucks just the right amount of glue into the joint.

• If you wipe excess glue away with acetone, be careful about which acetone you're using. Some types (such as nail polish remover) can have additives that leave residues on the glass and make the problem worse. If the glue is in a readily accessible area, it is usually better to wait for it to dry, then peel it off the glass with a razor blade. Only use acetone where there's texture or something else that makes the glue difficult to remove. And in any case, don't worry much about removing superglue right on the surface--it will burn off.

• Superglue joints will NOT support the weight of your glass, i.e., never, ever lift your assemblage by a superglued-on piece of glass. Common superglue is actually a lousy glue for glass--which is why it works as a temporary hold.

Disguising Joints in Fusing

One advantage of fusing over leading or copper foiling is that shapes impossible to cut as a single piece can be made from multiple pieces. However these joints often show up in the finished work.

You are always more likely to have the joints show when the cut coloured glass is on the bottom. The infra-red heat of the kiln elements goes through the clear glass to the coloured below, allowing it to soften first. As the glass underneath softens and pulls in, it allows the top glass to sink into the space. Upon cooling the seam is kept open even sometimes showing a clear line at the joints.

Putting the clear as the base and the jointed pieces on the top has a better chance of having the joints fully fuse together. There is no glass above to spread the pieces apart.

When you need the joints to be concealed, you can put a line of powder the same colour of glass over the joint. This line should be slightly rounded above the surface along the joint to account for the reduction in volume as it fuses. When it is two colours meeting, using powder of the same colour as the darker glass is most successful.

Fusing to a contour fuse for 10 minutes is normally hot enough, but taking the piece to a flat fuse – again for 10 mins - will certainly be enough to fully melt the powder into the joint.

Brushes for Painting

A quality paint brush will have hairs that form a point and have a good spring to them - they bend while painting but return quickly to their original shape. A good brush will also hold lots of paint and deliver that paint evenly throughout the stroke. Brushes usually have a number to indicate their size - the larger the number, the larger the paintbrush. The larger the brush the wider the line that can be produced, although with a light touch a fine long line can be made because of the pointed nature of the brush.

The best brushes are made from natural hairs, although there are brushes made from a combination of natural and synthetic materials which are adequate.

Sable hair brushes are considered to be the best for painting. The hair comes from a variety of pine martin and the Kolinsky sable from Siberia is considered the best. These brushes are more expensive than others, but are soft and flexible, hold their paint well and can make an expressive thick to thin line.

Ox hairs are normally used for making rigger brushes. This is a round brush with long hairs, said to be used to paint the lines of ships' rigging in the past. The hair is strong and springy making it useful for long lines and thicker paints.

Squirrel hair brushes are useful for applying paint in broad, thin layers for matting.

Goat hair brushes are normally known as hake brushes. These are a traditional, oriental style brush. It lacks spring, but forms a good point and so is useful to cover larger areas quickly with a gentle touch.

Pony hair is made into short round brushes used as soft stipplers.

Hog hairs are made into hard, very economical brushes. They come in flat and round shapes. They are most used for stippling and can be trimmed, shaped, used, and abused for years.

Badger hairs are thicker at the end and thinner at the root, creating a conical shape. These soft brushes are used to blend paint once it has been spread on the glass. The brush is swept across the surface of the paint to blend or move paint and remove stroke lines.

Glass Shifting on Mould

There are a number of things to investigate if your blank is shifting on the mould during firing.

Is there a heat differential?

Glass absorbs heat at different rates depending on colour and type meaning that one part may begin to move before another. The solution to this is to slow down the rate of advance to allow all the glass to gain heat at the same speed. It may also be useful to slump at a lower temperature.

There also may be a heat differential within the kiln. You need to run a check on the heat distribution of your kiln to be sure where the (relatively) hot and cold areas of your kiln are. Bullseye published Tech Note no.1 on how to do this.

Not perfectly balanced on the mould?

Glass can be placed just off square or level and that can allow it to start slumping unevenly. Measurements and observation can help to get the glass placed squarely on the mould. Also a small spirit level placed on the glass can tell you if the glass is level within the mould.

The mould may not be level.

The kiln, shelf and mould should each be checked for level in all directions. The kiln level can be established and can be assumed to be level until it is moved. The shelf level should be checked each time it is moved. The mould level should be checked each time it is used.

Is the glass overhanging the mould?

Glass overhanging the mould rim can hang up on some of the edges more than others. Check the rim of the mould for any rough areas and smooth them. If you do have glass overhanging, you should slow the rate of advance to allow the edge of the glass to tip up and begin to slide down into the mould. If the problem persists, make the glass blank smaller, or support the overhanging glass with a collar.

Is the glass heavier on one side?

The glass may be uneven thickness and so heavier on one side. The heavier area of the glass will begin to slump first and so promote movement of the whole glass in an asymmetrical manner. The solution to this is to fire slower and to a lower temperature.

Do you have a wonky mould?

The mould can be imperfect. So you need to check the mould for accuracy. I have a slumper that has one side lower than the other three. Being aware of this, I can place the glass so that it is still useable. Measuring the mould in all directions will help determine its symmetry.

If all these things have been investigated and the solution not found, it is possible to create a bevel on the bottom edge of the glass so that the edge sits in the mould at the same angle as the mould. This provides a larger contact point for the glass and mould than just a thin edge. This appears to allow the glass to move evenly during the slump.

Of course, a major solution is to observe the slump.  Peeking into the kiln at the beginning of the slump soak and frequent intervals after that will show if the piece is slumping evenly or not.  If it is uneven, you can put on the appropriate protective gear and with gloves on your hands, shift the glass to be set evenly in the mould.

The major solutions to avoid uneven slumping are:

• Avoiding the hot and cool parts of the kiln
• Making everything level
• Careful placement on the mould
• Slower rates of advance
• Lower slumping temperatures
• Observation

Using Space on Shelves

Often there is unused space on the kiln shelves when you are firing a project. With a bit of planning, you can make use of the spaces for a variety of things.

Frits fired on fibre paper

Bowl made from frit balls

You can place piece of frit in the clear areas to make frit balls.

You can make colour tests on plaques of glass to see the results of strikers, powder combinations or results of various depths of colour.

Compatibility tests can be done with pieces of glass of which you are not certain.

simple stress testing set-up

Strip of fired glass samples for testing

Results - those with halo are stressed

In the same way, annealing tests can be conducted.

You can fire small pieces of jewellery at the same time as your larger pieces.

You can also prepare elements for incorporation into other fusing projects and lay them out in the open spaces on the shelf.  Your use of the spare space is related both to your imagination and to your future projects.

Cleaning Blending Brushes

Cleaning badger brushes just before use, is easy. Flick, gently and rapidly, the very ends of the brush hairs against the side of your hand – but use respiratory protection and be careful not to inhale any dust. If you notice flecks of dust in your paint when you create a grisaille you’ll know it’s time for a thorough and wet cleaning again.

After each use, rinse out the brush tips in cool water. Gently rub the tips of the brush hairs to loosen any extra paint. Grasp he hairs above the tips to keep the water from the main part of the brush. Then wet the exposed ends of the hairs and rub them gently until the water runs clear. 

If you use a blender for oil, you will need to use a small amount of natural soap, if so, thoroughly rinse.

Flick the brush to remove excess water, smooth the hairs into shape and allow to completely dry by hanging the brush with the hairs pointing downward – this avoids water flowing into the brush base where the hairs are attached. If you have round-handled brushes, you can twirl the brush between your hands to remove excess water.