Wednesday, 12 August 2020

Anneal soaks

An odd concept was presented recently.  This in summary was that if you have long soaks on the way up to top temperature you do not need to have such a long anneal soak as normal.

This is a fundamental misunderstanding of the physics of glass.  As the glass temperature rises above the upper strain point (about 55°C above the annealing point), the molecules become disordered.  No amount of soaking at any temperature on the way up to the top temperature will change that. 

The glass (and the molecules of it) will need to be cooled relatively quickly from the top temperature to avoid crystallisation of the glass.  This is the reason for the fast cool to the annealing soak.  It is also a reason to avoid a soak at approximately 50°C above the annealing point – there is a slight risk that crystallisation could form.  This would appear as scum marks on the surface, rather than in the interior.

Whatever soaks you have performed on the way to top temperature, you will need the full length of soak for the full or tack fuse.  And you will need it for the slump too.

No amount of soaking on the way up to top temperature in kilnforming will have any effect on the requirements for the annealing soak at the cooling part of the schedule.  The soaks in the early part of the schedule, no matter how many or how long, do not change the annealing requirements.

Wednesday, 5 August 2020

Convex Shapes for Wall Hangings

The most common shapes for wall hangings seem to be the “S” or wave form in various sizes, and flat pieces of what ever outline supported by stand-offs.

There is another possibility.  You can produce a shallow domed shape which can work well for either landscapes or abstract pieces.  They will be best if circular, although rectangular forms can be used.

The usual resistance to doing this is that the surface will be marked, or that the tack fused surface will be flattened.

There is a way to do this without either effect.  Place the work upside down on a mould of appropriate diameter or dimensions and fire the piece slowly to a low temperature. 

Raise the temperature more slowly than you usually would for a slump in the normal way – top side up.  This allows both surfaces of the glass to be at the same temperature at the same time.  This equalisation of heat throughout the piece will protect against any breaks or splits on the underside of the glass – which will become the top surface.

Set the temperature for about 620C, depending on the span of the piece.  This temperature will be suitable for pieces of 300mm to 400mm and 6mm to 9mm thick.  Pieces with a smaller span will require higher temperatures or longer soaks.  Larger pieces will need a lower temperature.

You should set the soak at about 45 minutes. You will need to observe at intervals until you have the amount of depression you wish.  You will also need to know how to advance to the next segment of the schedule when that point is reached, so that you do not over slump the piece.

Since the piece only touches the mould at the rim, and you are not allowing much movement in the glass, you will not mark the glass with the mould. 

This process of making a domed wall piece will be unusual, although it will not be appropriate in all circumstances.

Wednesday, 29 July 2020

Measuring for Circle Cutting

Often there is uncertainty about which way the cutting head should be placed on the bar of the circle cutter to get the right diameter.  And the distance markings on the arm often get worn away.

It is for these two reasons that I have given up trying to get the right diameter circle from the measurement markings on the arm of the circle cutter.  Instead I measure and mark out the centre point and the radius of the circle directly onto the glass.  Only a few tools and supplies are needed.

Glass, measuring stick, marker pen, oil and circle cutter are all that are needed to measure the circle

First you need to decide on the centre point, leaving at least 2cm at the edge of the piece the circle is being cut from to allow a clean circle to be broken out.

The four black does are for measuring from the edge to the axis

Once you have done that, mark an axis at right angles at the centre point.

This shows the axis established and the radius marked out on the left.

Measure the radius from centre line , mark that on the line. 

Place suction cup at the centre of the axis. In the case of the cutter I use there are four markings to assist in the centering of cutter.

Move the cutting head along the arm until the wheel sets right on the radius mark.  I find that getting low helps a great deal in seeing the placement of the wheel.

Tighten the locking nut.

I put a drop of cutting oil on the wheel, so that in a preliminary run, I can both see where the scoring line will be and be sure everything is far enough away that the arm does not hit something on the way around.

Score the circle, making sure your fingers are only on the knob. If your fingers slip down, they can loosen the locking nut.  Some people score in an anti-clockwise direction to ensure they do not loosen the locking nut.  An anti-clockwise motion means that if your fingers do touch the nut, it will be tightened rather than loosened.

This photo shows the circle scored and to show the spacing between the edge of the glass and the score line.
Once you have set the cutting head on the arm of the cutter, you can cut as many circles as you wish of the same size without needing to do further measurements.
Further information on breaking out the circle is given in this blog post and a more comprehensive guide to measuring and placing all sorts of sizes is given in  Drilling Glass, guide no. 7.

Wednesday, 22 July 2020


Crazing appears as the multiple cracks similar to what is seen on ceramic glazes.  These occur when there is a great deal of incompatibility between the glaze and the clay body.  This can also be seen in glass.

Crazing as seen on a ceramic object

I have see crazing of glass in two circumstances.  It happens with severe devitrification, to a maximum extent of crumbling under light pressure.  This usually happens with glass not formulated for fusing, and especially on opalescent glass.

The more common occurrence is where the glass has stuck to the supporting structure.  This is frequently the case where the separator has not been sufficient to keep the glass from sticking to the shelf.  This will happen on kiln washed shelves when the coating of the separator has not been even, leaving areas with bare or very thin areas.

The standard of mixing kiln wash in the ratio of 1:5 parts by volume of powder to water is important.  The application should be with a wide soft brush such as a hake brush.  The kiln wash should be painted on in four coats, one in each direction of up, down, and the two diagonals.  A well coated shelf should have an even appearance of the coating.  Only an even film of separator is required to keep the glass from sticking to the shelf, mould or other kiln furniture.

Wednesday, 15 July 2020

Adjusting Cut Running Pliers

Typical cut running pliers

Cut running pliers are very useful tools if used correctly.  The pliers must have the curve in a “frown” rather than a “smile” to operate properly.  The knurled screw at the top and the scored line on the top jaw help place the pliers the right way up. They must be placed directly in line with the score. They should be only a centimetre or so onto the glass.  Holding them at the end of the handles, apply gently increasing pressure until you hear a click or see the score running.  If it does not run completely, turn the glass around and apply the pliers to the other end of the score.

Use of the Adjustment Screw

It is important to make use of the adjustment screw to get the best from the pliers.  If this is not adjusted properly, it is possible to crush the glass, or at the other extreme, not run the score at all.

The jaws need to be adjusted for the thickness of the glass.  The method I use for this is to place the edge of the runners on a corner of the glass to be scored.  

Loosen the screw until the glass is gripped by the jaws.  
Gradually tighten the screw until it resists your gentle pressure on the handles.  This gives you the correct opening of the jaws for that piece of glass.

When the pliers are properly adjusted to the thickness of the glass, you will not crush the glass and it is easy to use the pliers without cushions.

Wednesday, 8 July 2020

Containing Stress

People frequently report success in combining incompatible glass pieces with a larger, different base.

Questions arise.

Have the resulting pieces been tested for evidence of stress with polarised light filters?

Other destructive methods such as hot water, or placing in the freezer are not adequate measures of the long-term effects of incompatibility stress.  When you are doing something outside the accepted norms, then you must test for stress to be certain what you are producing remains sound before announcing success.

Why does glass with incompatible pieces survive?

Incompatible glass will show some stress when viewed through polarised filters. You will need to decide when it is excessive.  When viewed between polarised light filters high stress will be shown by a rainbow effect in the halo of light.  Lesser stress will be shown by pale light. The degree of stress will be shown by the amount of light.


There are some circumstances where the glass can contain the stress, and others where it cannot.

Generally, large mass pieces can contain the stress from small incompatible pieces of glass. 

Spherical objects can contain a lot of stress over a long period, which is why glass blowers and lamp workers are generally less concerned about incompatibility than kilnformers are.

Flat glass pieces behave a little differently.

Circular forms can contain stress more easily than other shapes.  Rectangular  shapes generally show the most stress at the corners.  Narrow or wedge-shaped pieces have the most difficulty in containing stress.  The stress is concentrated at the points.

The placing of the incompatible glass is also important to the survivability of the glass.  The further from the edge of the piece, the less likely there will be breaks. 

The smaller the pieces of incompatible glass in relation to the whole, the less risk of breaking. 

The more spread apart the pieces are, the greater the chances of survival for a while or long term.

The most essential piece of equipment for people starting out and those who are investigating new setups or working at the edges of accepted norms is a pair of light polarising filters to test for stress.

When combining incompatible glasses the general case is that the greater the mass of the whole object in relation to the incompatible glass, the greater the chance of survival. 

Wednesday, 1 July 2020

Rakes for Combing

It is of course, possible to buy commercial tools for combing hot glass.  But with a little ingenuity, you can make your own for a small amount of money and some effort.

My raking tool is a metre long round stainless steel rod, 8mm in diameter. I sharpened it on a  grinder for metal rather than my glass grinder. Then I bent a right angle to give me 75mm "hook". The handle is a piece of broom handle. I drilled an 8mm hole in the wood and hammered it on.

A longer metal and shorter wooden handle works better than the one I made with a long wooden handle, as there is no wood near enough the heat to burn. If you do have a long handle,  soak it in water to keep it from burning.

It is possible to make a rake using mild steel rod, but it is more likely to spall and drop flakes into the glass.  Both metals need to be kept cool.  Rest the rake in a bucket of water before the first pass at combing.  As the glass stiffens and you need to wait for the glass to come back to a combing temperature, put he rake back into water to cool it.  If you try to comb with a hot rake, it will stick to the glass.

It is important to have a handle made of an insulating material to avoid any possible electrical shocks.  It also makes for a more comfortable handle that does not heat up.

Safety gear is required to protect eyes and clothing from the heat.  It is not possible to have the kiln open at around 900C without it.  This is the face, hand and arm protection I use.  The coated visor protects your eyes against the infrared radiation from the kiln.  The gloves can be the aluminised silver colour ones or the kevlar ones.  The alminised ones are easier to manipulate things with.  The arm protectors are aluminised too. They are easy to put on and give additional safety to the body.

Natural fibres should be worn to avoid clothing bursting into fire.  I use a denim jacket reversed for additional chest protection.

Wednesday, 24 June 2020

Fibre Paper and Fibre Blanket

Refractory fibre is generally divided into paper and blanket.  There is a distinction between the two relating to binders and thickness.

Papers are those which have binders to keep the pressed fibres in a sheet or roll.  These binders burn off during the firing process

Blanket does not have binders and is much thicker.

These two forms of refractory fibre generally exhibit a difference in thickness.  

Papers range from cartridge paper thickness, to around 6mm thick.  

Blanket tends to be 25mm and thicker, although there is some 12mm I understand.

Wednesday, 17 June 2020

Firing Uneven Layers

There are a number of people firing stacked layers of glass in a pyramidical fashion to melt the layers down, in a nod to the 1950’s.

Annealing of a full fused platter of this nature is easier than a tack fused one. The degree of contour still in evidence will be important in determining what the annealing schedule should be.

Full or Tack Fuse

In this example shown by Vicki Urbich there are at least four layers - five if the base is two layers, although in this case there is only one.  A full fuse at 800°C will not be enough to give a flat piece for a trivet or platter.  You could extend the soak at 800°C but, it is better to go to 816°C for ten minutes rather than an extended soak at lower temperatures to avoid devitrification.

The edges of this piece will be wavy, unless dammed, because of the uneven layering.  Placing dams around will give crisp edges to the piece, even though the stacked pieces will round and spread. 

Annealing stacked pieces
Anneal this set-up for at least 9mm for a full fused piece. The pieces will spread and attempt to fill the gaps between the stacks.  Even with an 816°C fuse, the pieces will not be perfectly even in thickness.  To be safer, and account for the remaining unevenness, anneal as though it were 12mm thick.  Other lay-ups will have slightly different requirements.

If it is to be tack fused, you will need to anneal considering the different thicknesses across the piece.  You will have nearly 12mm thick at the thickest and only 3mm at the thinnest.  The generally accepted recommendations are to anneal for twice the thickest part - 24mm in this case.

The anneal is more than the length of soak. It is a combination of the soak and the rate at which you cool to at least 370°C. The cooling rate is directly related to the length of the soak.  If you require twice the length of soak at the temperature equalisation soak, you will require half the speed of anneal cool.  The Bullseye Chart forAnnealing Thick Slabs will give you the relevant rates regardless of the glass you are using. The temperature points will change for other glasses, of course, but the rates remain the same.

Rate of Advance
The earlier problem this lay-up gives you is the heat up to avoid thermal shock. 

The heat up of 4 layers of glass stacked on a single or even two-layer layer base is more difficult than for even layers across the whole piece. Each upper piece shades the heat from the lower ones, making for cool and hot areas next to each other.  With four layers, each layer needs to heat through to transfer its heat to the one below.  This means the bottom of the stack will take a long time to become as hot as the top layer.  Meanwhile, the uncovered glass is getting as hot as the top of the stack.  This often leads to the bottom layers breaking from the stress of the uneven heating.  

Graham Stone suggests 100°C (180°F) per hour for four, even layers. As this is four uneven layers, the rate of advance should be at least half that. This should be used all the way up to the bottom of the bubble squeeze to allow all the glass to heat at the same rate. Glass generally reacts better to a slow, steady contant rate of advance in heat, than faster rates with multiple soaks.

Bubble Squeeze
The bubble squeeze for this single layer base piece can be as quick as 50°C per hour over the 50°C range.  It does not need to be slower, as the weight of the stacks pushes the air out between layers more quickly than large, even and lighter layers.  A double layer base requires a slower bubble squeeze because the weight of the stacks will push the air out to be between the two base layers.  This means a rate of 30°C or even 25°C through the range.

Then you can go faster to the top temperature.

Firing uneven layers requires extreme care on the initial heat up to avoid thermal shock.  A high fusing temperature is needed to get an even thickness across the piece.  Annealing is easier to calculate for even pieces, but must be much more cautious for tack fused items.

Wednesday, 10 June 2020

Drying Kiln Washed Moulds

There seems to be a popular notion that newly kiln washed moulds must be cured before use.  I'm not sure where the information comes from, and no reasoning is given.  It is suggested that that quickly heating newly kiln washed moulds to 550°F (290°C) is important.

If you want to make sure the mould is dry, this may not be the best way to do it.  All ceramics have a quartz inversion at around 225°C.  This a very rapid increase in volume of 2.5% that often leads to cracks and breaks in ceramics when the rate of advance is quick.  The mould will react better and last longer if the rate of advance is slow until that inversion temperature is passed.

This is a reason to advance the temperature slowly when slumping or draping with a ceramic mould.  Another reason to heat slowly is to avoid steam formation within the ceramic body.  If the steam is created over a short time, the force can be great enough to break the ceramic.  To ensure the water evaporates, a soak at 95°C for a significant amount of time is a better, safer option.

But in addition to all these precautions, it simply is not necessary to cure kiln wash on slumping and draping moulds made of ceramics.  The glass does not begin to move until after 540°C (about 1000°F). Therefore, the kiln wash will be dry long before the glass gets near slumping temperatures.  Any vapor caused by evaporating water will escape through the vent holes in the mould or under the glass at the rim, as it will not form a seal until higher temperatures.

newly kiln washed mould beside others already fired

If you want to be sure your kiln wash is dry before you put the mould in the kiln, you can leave it in a warm ventilated space, or even on top of your kiln while it is being fired.  Using either drying method will dry the kiln wash sufficiently before the glass is placed on the slumping mould.

Kiln drying ceramic slumping and draping moulds is not necessary. It only adds another, unnecessary, step in kiln forming.  There are exceptionally good reasons to avoid rapid firing of damp moulds. The exceptions can occur with texture moulds and those intended for casting that do not have vents.

Wednesday, 3 June 2020

Large Bubbles

As you move up from smaller pieces to pieces that occupy most of the shelf, you sometimes begin to get large rounded bubbles at tack fuse and burst ones at full fuse.

Image from B Stiverson

You have to go back to basics to discover the cause.

It is not likely to be the schedule. It has worked for smaller items. But it is important to review the schedule.  Is it like others you have seen? Is it similar to what the glass manufacturer recommends?  Both these will reassure you that the schedule is OK, if not perfect, or to revise it.

Going back to the basics relates to the cleanliness of your kiln, among other things.  Even a small speck of material under the glass can result in a bubble. Although the grit lifts the glass off the shelf only a fraction, as it heats up the glass slumps around that and creates an air pocket.  That grows as the glass heats up and creates a large diameter bubble. If there is no grit in evidence, you need to check another element of your kilnforming practice.

The large bubble might often occur in the same relative place in the kiln, although different places on the glass pieces, depending where they are placed.  This is an indication that you may have a hollow in the shelf. It may not have been obvious with smaller pieces.  You need to check the shelf with a straight edge. If any light is seen between shelf and edge, you have a depression in the shelf.  It may only be a sliver of light, but that indicates a depression which is enough to create a large bubble. That must be fixed.

Image from Suze

There are temporary and permanent fixes for avoiding bubbles due to depressions in the shelf. 

The temporary fix is to use 1mm fibre paper on the shelf, to allow air out from under the glass.  This can be topped with Thinfire or Papyros. Alternatively, a thin layer of powdered kiln wash can be smoothed over the fibre paper to give the smoothest back possible in the circumstances. You can use a plasterer’s float, or simply a piece of float glass.

The permanent fix is to sand the shelf smooth and level.  A method for doing this is here.

Single Layer Bases
If you are firing with single layer bases, there may be nothing wrong with the shelf.  It is typical in tack fusing to use single layers with glass placed decoratively around the surface of the base.  This leaves gaps where the base glass is exposed.  Even though the whole piece may survive the differential heat up of the exposed base glass and the covered parts, there is the possibility of creating an air pocket under the exposed base.  This comes from the weight of the stacked glass pressing any air out to the side.  If the design is unable to provide a route out for the air, the possibility of creating an air bubble increases.

It is possible to create conditions to reduce the possibility of these large bubbles developing. 

One solution is to use a layer of fibre paper as for a shelf with slight depressions.  This allows air out from under the glass, even with a single layer layup.

The other solution is to change the rate and temperature of the firing.  By using the low and slow principle, you can reduce the risk of bubbles.  Use a much slower rate of advance to a lower temperature with a longer soak you can achieve the look you want without bubbles.  This utilises the concept of heat work.  It does require observation to determine when the effect you desire is achieved and then advance to the next segment.

Wednesday, 27 May 2020

Oxidisation of Foil

Often, life intervenes between foiling and soldering.  This frequently results in the foil not accepting the solder very well, because of the mild tarnish that has occurred in the meantime.

It is a good idea to clean the foil of any possible corrosion before soldering when there has been an interval between the two processes. It is enough to clean foil with a mild abrasive such as a foam-backed scrubber from the dish washing, or fine steel wool.  I prefer the scrubber as it does not introduce another metal.  

Some people prefer a vinegar and salt solution to apply to the tarnished foil.  I am concerned about the introduction of an acid into the process causing further problems later.  I don't recommend this method of cleaning.

I then coat the exposed foil with a film of paste flux to protect from further tarnish. This acts better than any loose covering of cloth or plastic to protect from oxidation.  The purpose of flux is to both provide a "wetting" agent for the foil to accept the solder,  and to prevent oxidisation. Liquid flux cannot provide protection, once dry,  from the copper tarnishing.  I prefer the use of paste flux to reduce boiling of flux and to keep the copper free from corrosion.  The paste flux will not indefinitely prevent oxidisation, but will do so for a week or two.

Wednesday, 20 May 2020

Pin holes in melts

Pin holes in screen and pot melts are the result of very small bubbles rising to the surface.  These bubbles are sometimes within the glass melted, but more often come from small amounts of air trapped within the flowing glass.  These are perceived to be unsightly, or make it impractical to make a functional piece from the melt.

There are ways to minimise bubble formation or to deal with the formed bubbles.

Bubble Formation
In pot and screen melts, the glass spirals as it touches down onto the shelf. This spiralling action can trap small amounts of air as each successive spiral forms beside the previous one. Efforts at prevention of tiny bubbles in the final piece need to concentrate on this fact.

A preliminary element in bubble prevention is to have a long bubble squeeze to allow the glass to settle in the pot or on the screen so that the rest of the process can proceed with a minimum amount of air trapped within the flowing glass.

Two-Stage Drop
In some cases. it is possible to have the glass flow from the pot onto an angled shelf where the spiralling glass has to flow from the initial touch down to the edge and then flow onto the shelf.  This allows any tiny bubbles initially trapped to escape before the final drop onto the shelf.  This provides two mixing processes and means that a lot of clear glass needs to be included to avoid a complete mix of the colours.  It requires careful selection of the original colours to avoid a brown or black result.  It also requires a big kiln with sufficient height for a two stage drop.

This two-stage drop is of course, not suitable for a screen melt where you wish the glass strands to remain.  Nor is it suitable when you wish to have many “pools” of colour mix in the final piece.

Where the two-stage drop is not practical or suitable other methods can be used.  These relate to scheduling, cold working the surface and re-firing the piece.

Scheduling relates to using a soak at full fuse temperature before proceeding to the anneal.  The melt will occur at 850°C to 950°C.  You can cool as fast as possible to a full fuse temperature of about 810°C and soak there for an hour or more.  This allows the small bubbles to surface, break and heal.  Schedule the rapid cool to the annealing soak, once the high temperature soak is complete.  This will eliminate lots of the bubbles, but not all.

A sample friring schedule from bubble squeeze upwards and then down to a high temperature bubble reduction soak

Cold Work
Cold working the melt is about abrading the surface to open the bubbles that are just emerging to form a small dome at the surface.  Sand blasting is a common form, as usually kiln wash or fibre needs to be removed from the bottom of the melt, and some devitrification from the surface.  It would be possible to continue to grind the surface of the glass to eliminate the small depression in the glass caused by the now opened bubble, but this is likely to expose more bubbles that were at a slightly deeper level. What next?

As you will need to do a fire polish firing after blasting or grinding the surface, you can use a full fuse temperature to allow the surface to become plastic enough to fill the bubble holes.  Remember to schedule the firing as though the piece were at least 12mm thick.  You may find that more bubbles are exposed in addition to the ones healed at the conclusion of this second firing.

An alternative is to fire upside down.  You will have noted that there are no bubbles on the bottom of the melt.  This is because the bubbles have risen through the heated glass.  This physical fact can be used in the second firing.  Fire with the melt surface to the shelf.  It is best to have a clean and newly kiln washed shelf, or fibre paper (not Thinfire or Papyros) under the glass. Fire the glass to a full fuse or high temperature tack fuse with a significant length of soak to allow the bubbles near the original surface to move toward the interior of the glass.  After firing, the glass will need thorough cleaning before being fire polished. This should leave you with a pin hole free piece.

Achieving a pin hole free pot or screen melt requires several stages of coldworking and firing.  This makes melts inexpensive in materials (it is scrap of course) but expensive in time and firings.