Home Made Devitrification sprays

You can buy a number of devitrification sprays. Some of them are lead bearing and will not be suitable for food and drink containers. Many times people apply them before firing the first time to prevent devitrification. More often these sprays are applied after a piece has become devitrified. However it is applied, these sprays are not cheap.

borax in powder form

It is possible to make your own devitrification solution. It is made from borax which you can buy from your local chemicals supplier, or sometimes as a washing powder – but make sure it has no additives! 

An example of a borax washing powder

 To make a solution, boil a few cups of water. Take the water off the boil and put in 4 – 5 tablespoons of borax. Stir and allow to stand until cool. Pour off the clear liquid and you have a saturated solution of borax. The sediment in the bottom can be added to more hot water to make more of the borax solution.  You will have to break up the remaining crystals of borax to enable suspension in the hot water.

Add a couple of drops of washing up liquid to the solution. This is enough to break the solution's surface tension. It helps to give an even distribution of the solution across the clean glass by reducing the surface tension and therefore, beading of the liquid that otherwise occurs.

You can spray this solution onto the glass, just as the commercial sprays.  Or you can brush it on as you do kiln wash on a shelf.  It requires an even application to ensure there are no streaks left on the finished glass. 

This works because borax is one of the fluxes used in glass making to reduce the melting temperature of glass batch and so serves to soften the surface of the glass enough to overcome mild devitrification.

Permanance 

Description of devitrification
Temperature range

https://glasstips.blogspot.com/2016/02/borax-characteristics.html

https://glasstips.blogspot.com/2009/06/borax-solutions.html

Revised June 2018

Kiln Wash

Kiln forming techniques require separators between the glass and the shelf or mould on which it rests during the heating process. These separators have different generic names – kiln wash and batt wash are two.

There are a number of brands of kiln wash. All of them contain two main ingredients – alumina hydrate (sometimes called slaked alumina) and kaolin (also called china clay). Different producers use these ingredients in various proportions. 

A number of makes also include a colourant that changes when fired above certain temperatures to indicate the wash has been fired.  It also distinguishes between the unfired and the already fired kiln washed shelves.

An important thing to remember is that the kaolin changes its composition once it is fired over 600C/1113F. This change of composition is completed by 900C/1620F.  The change is progressive.  It is so slow that slumping and draping moulds coated with kiln wash will last indefinitely. However this change is great enough by 770C/1419F that the kiln wash sticks to the glass on the next firing. Thus, it is essential to change the kiln wash after every firing that reaches tack fusing temperatures or higher.

It is possible to apply a fresh coat of kiln wash over the old one to save time. However, as soon as the kiln wash flakes you must scrape off all the old kiln wash and apply a new coat to the bare shelf or mould.

Some makers use much less of the binder (china clay) than others which makes them better for the popular casting moulds than those for shelves and slumping moulds as they can be brushed away without abrasion.


In addition, boron nitride is a suitable release from moulds.  It is very stable at reltively high temperatures and so can provide a smooth, "slippery" separator between the glass and its supports, whether shelves, moulds or kiln furniture.  It does seal porous surfaces, meaning that air cannot move through the treated surfaces.  It has to be removed with abrasion and so thought must be given to which surfaces it is applied.

Polarising Filters

Using polarized light filters to show stress works on the principle that stressed glass rotates the polarisation direction of the light as it comes through the glass. As polarized light filters placed at right angles do not allow any light through, only unstressed glass will continue to appear dark. 

If there is stress the light is rotated slightly and becomes visible through the filters.  

You can buy stress testing kits that incorporate a light source. You can also make your own. You need polarizing lighting gels. These come in sheets and are available from theatrical lighting sources. You will need to frame these in stiff card to keep them flat.

You use them over a light source. Place one filter down above the light source. Place the piece to be tested on top. Then orient the top filter so that the minimum amount of light shows through the filters. Any stress will show up as a light source.  The amount of light rotation depends on the stress direction, magnitude and light path length. The greater the intensity of the glow, the greater the stress the glass is exhibiting.   The amount light visible through the filters is wavelength dependent, as the filter transmits light with a particular polarisation direction. If there is large stress, different colours will be visible. 

This example shows extreme stress by the rainbow effect of light rotated in multiple directions

Note that the surface through which the light comes should be rigid, as any deformation of the surface will give a false reading.  The light filters through the slight curve and gives a stress reading, which may not be true at all.  Thus a firm flat surface is required, especially if you have a large light table for your light source.

Also note that the filters are normally on plastic sheets and easily scratched, so the glass should always be lifted and placed, rather than slid, to a new position.

A description of the compatibility test can be seen here.

revised June 2018

Cutting Lead Came

Cutting came is a gentle process rather than an abrupt chopping effort.
There are at least three kinds of implements in common use to cut lead came.

Lead nippers or lead dykes
Lead nippers/dykes are a kind of adapted side cutters, used for cutting wire and by electricians. But these have the bevel only on one side of the jaws, making them almost useless for anything other than cutting lead. This arrangement only crushes the lead on the cut-off side and also leaves a minimum of lead next to the back of the jaws.

In use, the jaws of the dykes are aligned in the same angle as the heart of the lead, cutting across the leaves of the lead. They do not cut from the top and bottom of the came. These are very quick for right angle or very oblique angles on the came. However they are of little use for acute angles.

Lead knives
For more acute angles, blades are more commonly used. These can be either straight edges or curved blades. The straight edge lead knives are essentially putty knives or stiff scrapers sharpened to an acute angle. This kind of knife is normally wiggled from side to side while applying pressure to work through the came.

Other knives are curved to make rocking back and forth easier. There are a variety of knives such as the Pro or Don Carlos. Some look more like a scimitar than a lead knife! These are used to rock along the line where you are cutting the came.

Whatever kind of knife you are using, be sure to be directly above the knife, looking along the blade to ensure vertical cuts.

Saws
Of course, saws are sometimes used. The blade needs to be coarse toothed to enable the soft lead to drop out of the teeth. These saws can be hand held or table saws. Normally, it is quicker to use lead dykes or knives. However, if you are in production mode, a powered table saw may be worthwhile.

Sal Ammoniac

There are sometimes concerns expressed about the use of sal ammoniac to clean the tips of soldering irons.  My conclusion is that there are no elements of the block that will affect the copper plating of the soldering iron bolt.  It is safe to use this as an occasional cleaning method of soldering iron bolts. This is based on the following information.

What it is

The common term, sal ammoniac, refers to the chemical ammonium chloride.  Sal ammoniac is the archaic name for it. The Romans named it from the ammonium chloride deposits that they collected from near the Temple of Jupiter Amun in ancient Libya.  It is found as encrustations around volcanic fumaroles, guano deposits and in burning coal seams. Notable occurrences include Tajikistan; Mount Vesuvius, Italy; and Parícutin, Michoacan, Mexico.

Wikipedia

Ammonium chloride is the product from the reaction of hydrochloric acid and ammonia.  Ammonium chloride is obtained as a by-product in different chemical processes.  It consists of white crystals that are also available in rods or lumps.  The substance changes directly from being solid to gas with no intermediate liquid state. The gas does not consist of ammonium chloride molecules but ammonia and hydrogen chloride. This shows that the salt decomposes easily. When stored, ammonia is continuously emitted and the substance gradually becomes more acidic.

https://www.fishersci.co.uk/shop/products/ammonium-chloride-99-6-analysis-acs-acros-organics-3/p-3586389

Safety

It is widely used in human medicines as an expectorant, diuretic, etc. and in veterinary medicines to reduce gallstones, so it is a relatively benign material in relation to human health. 

There are some hazards though.  It can cause serious eye irritation on prolonged exposure, and is harmful if swallowed.  The precautions are to avoid eating, smoking, and drinking when using it.  Use gloves and eye protection if you are using it for extended periods. If it gets into your eyes, rinse with water for several minutes. https://www.fishersci.co.uk/shop/products/ammonium-chloride-99-6-analysis-acs-acros-organics-3/p-3586389

It is highly soluble in water, and forms a slightly acidic solution. Its main characteristic that you need to protect yourself against is that it vaporizes without melting at 340 °C to form equal volumes of ammonia and hydrogen chloride gas. https://www.britannica.com/science/ammonium-chloride

The amounts of the gas are small when used to clean soldering irons, but as the gas forms hydrochloric acid in contact with moisture, you should use dust masks rated for inorganic acids.  The amounts are small and generally only cause sneezing and coughing upon contact.

The primary hazard is the threat posed to the environment. Immediate steps should be taken to limit its spread to the environment.

https://pubchem.ncbi.nlm.nih.gov/compound/ammonium_chloride#section=Top

Uses

In addition to medicine, it is used to clean soldering irons. It has uses in jewellery-making and the refining of precious metals.  Sal ammoniac has also been used in the past in bakery products to give cookies a very crisp texture.  In some areas, particularly Nordic countries and the Netherlands, it is still widely used in the production of a salty licorice candy known as Salmiak, or Salmiakki.  Formerly it was used as the electrolyte in dry batteries.  It has uses in fertiliser as a source of nitrogen mostly for rice and wheat crops in Asia. It is also an ingredient in fireworks, safety matches, contact explosives, cosmetics and many other applications.

https://thechemco.com/chemical/ammonium-chloride/

Conclusion

Although there are some mild safety precautions that need to be followed, there is nothing in the sal ammoniac block that can harm the copper coating of the soldering iron tip.

Thermal Shocking Ceramics

When firing glass in ceramic moulds, and especially ceramic pots for pot melts, you should be aware of the temperatures at which the ceramic material quickly expands and contracts.

There are refractory ceramics which are not as sensitive as the kind of ceramics we are using in most kiln work.  The ceramics we use are not refractory materials and contain, among other things, quartz and crystobalite. These two elements are important, as they have considerable effect on the survival of the pot or mould during the firing.

The effects are called inversions.  This is because the rapid expansion experienced upon the heating is reversed as rapid contraction on the cooling of the ceramic.

The first element to be affected by the heat up is crystobalite.  This element has a sudden expansion of 2.5% at 226°C.  This does not seem to be much, but compare it to the expansion of glass at this temperature - .0085% - almost 300 times that of glass at the same temperature.  And of course, the ceramic contracts by that amount when it reaches 226°C on the cooling.

The second element affecting the heat up is quartz.  There is quite a bit of this in clay.  The critical temperature for this is in the 570°C to 580°C range.  The expansion and contraction is not so great here – only 1% - but it is still more than 100% that of the glass, and in a critical range for the glass on the cooling.   

More information on the quartz and crystobalite inversions are given here.

The importance of these inversions for us are to remind us to be careful at these temperatures of about 225°C and 570°C - 580°C to prolong the life of the ceramic pots and moulds that we use.  

It is probable that 150°C per hour is as quickly as we should increase the temperature when using ceramic moulds or pots.  Some thought should be given to the cooling of the moulds too.  They should not be taken from the kiln while hot nor subjected to draughts of relatively cold air.

Leading Small Circles

Putting came around small circles such as lenses and small bullions often leaves an irregular curve. There is a way to avoid this.

Use oval or round came to reduce the kinking of the leaves of the came. As there is less material at the edges of the leaves of oval came, there is less kinking than on flat came, where the thickness of the leaves is constant.

Begin to form the lead round the circle, about half way. Then take the circle out of the came and cut, at a right angle to the length of the lead, at an angle from top to bottom. The degree of the angle is not important at this stage, only that you can repeat the angle – so it must be fairly shallow and natural for you.

Put the circle back into the came and continue to form the came round it until you meet the angled cut at the beginning. Again at right angles to the length of the came, cut a repeat of the angle.

Then fold this end toward the other end. Push the two angled ends together. If they slip up and down from each other, the came is too long. Open the came and cut a sliver off.

Try again until they meet with very little “slippage”.

Then the piece is ready to put into the panel. Place the join at a lead joint so you don't have an additional solder spot.

This technique can be used for small ovals too.

Tin Bloom

Using float glass sometimes produces partial clouding as though devitrification were present. Although float glass is prone to devitrification, not all the cloudy film on the surface is due to devitrification.

Float glass, which these days, is almost all clear smooth glass, gets its name from the process of floating the glass on molten tin. The tin in compression gives an apparent devitrification effect which is called tin bloom.

it is different from devitrification, to which float glass is particularly subject. Devitrification sprays and solutions will not have an effect on this surface defect called tin bloom. 

When the tin layer is stretched, it does not create a tin bloom on the surface.  Therefore, it is important to have a means to detect which is the tin surface.  Always fire the glass with the tin in the same relative location to each other.  I.e., on several layers of glass have all the tin side down or all up, but not mixed. 

This example of a test by Glass Art by Margot shows the tin bloom on the outer portions of the platter where the tin side was up, causing the tin too be compressed and show.  The flatter portion of the piece did not show this tin bloom as there was not the same extent of compression. You can visit the description of the experiment here.


When forming the glass (slumping, draping, kiln carving) make sure the tin sides will be stretched rather than compressed.  Of course, you can take advantage of the tin bloom by controlling the compression of the tin layers.

Leading Procedure

Cut the leads exactly as the cartoon indicates. In other words, where one line runs into another, that is generally a stopping/starting point for the came.

Always lead to the cartoon line, not the glass. This ensures accurate completion of the panel. If the glass is slightly too small, the cement will take up the gap (assuming the flange of the came covers the glass – if not, you need to cut another piece of glass that fits). If the glass overlaps the cut line, it needs to be reduced.  A description of the process is given here.

This shows the use of a guage to determine where to cut the horizontal lead came.

Cut the ends of the came shorter than the glass. The best way to determine this is to place a piece of came of the dimensions being used for the next edge on the cut line. Use it to determine the length and angle for the cut. The object is to have each piece of came butt squarely against the passing came, to make a strong panel and to make soldering easier.

Leading - Establishing the perimeter

The first thing to be established about the panel is the placing of the came that goes around the edge of the panel.

Fix your cut line cartoon to the work board.  Usually a long strip of masking tape on all the edges will be sufficient.  To establish the placing of the battens, which will form the frame for the leading process, you need to determine the spacing from the cut line.

This shows the initial battens in place and ready for the final two battens to be put in place before soldering.

To determine the size of the off-set of the battens you should cut a short piece of the came you will be using for the outside and use that as a guage.  Place the heart of the came on the outside cut line near one end and move the batten to the side of the came.  Nail that end of the came to the board.  Move the guage came to the other end of the cut line and do the same with the batten as you did for the other end.  Establish one other batten at right angles in the same way.  Then you are ready to place the cames.

Make a straight cut across the came to be used for the outside and put that trimmed end into the corner and along the vertical wood strip. The lead should extend beyond the cut line to accommodate the length of the upper horizontal came. The minimum length must be longer than the width of the perimeter came that will butt against it. If it is even longer, the extra can be trimmed off after the leading is complete or after soldering.


Next butt a trimmed piece of perimeter came along the horizontal wood strip. This one should be shorter than the cartoon. It should be half the width of the perimeter cames to allow the vertical came to butt against it. The reason for having the vertical cames running from bottom to top is that there is a fraction more strength in the heart of the came going all the way to the bottom of the panel, rather than resting on the flanges of the came.

This is how the finished perimeter cames will appear:

These perimeter cames should be held in place with horseshoe nails. Try placing the nails only where a lead line will be soldered in order to cover any nicks the nails might make. Alternatively, you can place the nails at the ends of the perimeter cames to keep them from sliding vertically or horizontally.


If you want to have mitred corners, this post will show you the method.

The next stage of placing the first pieces of glass is shown here.

Leading acute angles

Most of us like flowing lines in leaded glass windows, but these often give very acute angles to be leaded up. One way is to avoid creating intersections by using passing cames.  

But, if the cartoon does not allow for passing cames in acute joints, you have to consider how to cut the came to butt well against the next came. The easiest, but most time-consuming method is as follows:

Determine what the length of the came must be to reach the end of the joint.

Mark your lead there.

Determine what the shortest part of the came will be at the joint and make a faint mark there too.

Cut the came at the first (longest) mark.

Use your lead dykes to cut the heart out of the lead, leaving only the flanges. This is done from the end to just beyond the faint mark you made to indicate the shortest part of the joint.

You then need to smooth the two flanges where the heart was. You can use a fid or your lead knife to draw over the rough interior of the flanges. This enables the flange to be inserted below the came already in place, or to slide the new came over the modified came.

You can trim the upper came flanges immediately to conform to the angle of the joint or do it when the whole panel is leaded. Make a mark with a nail or your lead knife along the edge of the un-modified came. Then raise the flange and use your lead dykes to cut the flange along the line. Fold the flange down to butt against the passing lead and it is ready to solder.

Element Coatings

You will notice that after the initial few firings of your new kiln that a grey residue forms on the elements.  This is a protective layer.  It is a surface oxidisation that protects the underlying metal from further corrosion. 

Kiln elements are generally made from Kanthal or Nichrome wire. 

Kanthal wire is an alloy of iron, chrome and aluminium.  The aluminium oxidises to provide a protective layer of aluminium oxide.

Nichrome wire is an alloy of nickel (the main element) and chromium in various proportions for different applications. It is the most common heating element for high temperatures. The chrome forms a protective layer of chromium oxide at red hot temperatures.  But once heated, it becomes brittle, so it can be manipulated only when hot.

This layer is not a chemical reaction to the things you put into your kiln.  It is the necessary protective layer to give long life elements. This coating should not fall from the elements unless it is disturbed by bending, abrasion or impact. If it does, check for damage to the elements and look closely for any break.

False Lines in Leaded Glass

False lines are used in leaded glass where the design calls for an angle that cannot be cut into the glass. This includes right angles and even more acute angles. E.g., the petals of a fuchsia flower. 

The design would call for an angle of about 60 degrees. This is impossible to achieve through hand cutting. So the glass is cut in a curve and the cames on the side and bottom of the petal have their hearts cut out so they overlap each other. 

In the example above, the red petal points would be cut rounded, so that the clear glass below can be rounded as well.  The came or foil is extended beyond the glass to give the visual points required.

The overlap is then trimmed to the shape of the outside of the petal. When soldered, the appearance is of the glass being cut at the angle required for the flower.

At other times, the requirement is for a line to go into a piece of glass, but not all the way across. As in this stained glass panel by Justin Behnke.  The hanging lines are those on the lower left of the panel, giving a great flow to the whole.

Again you cut the heart out of the came, and overlay the smoothed lead onto the glass. You can use just a little silicone to hold the lead in place until you finish cementing. After this you can lift the piece of came and use silicone or epoxy resin to firmly attach the came to the glass. You do not want to do this before cementing as any excess of glue will be made dirty by the cementing process and be very difficult to clean up.


There are also times when you may want to have a silhouette, you can cut it out of lead foil and solder it into place. This allows intricate shapes to be made when a dark representation of the shape is required. If the panel can be seen from both sides, the overlays should also be on both sides. These should be glued to the glass just as for cames.

Further information on removing the heart of lead came are given in this post on leading of acute angles.

These principles can be applied to copper foil too.

Slumping a Form Flat

There are a variety of reasons that you might want to make a formed piece flat again for another kind of slump or drape.

There are lots things you think about when preparing to make a shaped piece flat.

I am going to assume there are no large bubbles in the piece.  You can see the posts  Large bubbles and Bubble at bottom  on the causes.

The following comments are things in five groups to consider when contemplating flattening an already formed shape.

Shape/form

• ·         Shallow forms with no angles have the fewest difficulties.  Take it out of the mould, put it on the prepared shelf and fire to the slump temperature.  Observe when it is flat and proceed to the annealing.
• ·         Forms with angles or multiple curves are a little more difficult.  If the piece has stretched in some areas to conform to the mould, you will have some distortion in the pattern and possibly some thinner areas.  It should be easy to flatten pieces on a prepared shelf with the same schedule, but a slightly higher top temperature as used to slump it.
• ·         Forms where the sides have pulled in will become flat, but continue to have curved sides.
• ·         Deep forms are possibly the most difficult.  The glass may have stretched, giving thin areas.  It may be that the process of flattening the glass will cause a rippled effect as the perimeter of the piece is a smaller size than the original footprint.  These deep forms are the least likely to flatten successfully.

Orientation

• ·         Which way up? Upside down or right side up?  Shallow forms are easiest to flatten by placing them right side up on a prepared shelf.  For deep or highly formed pieces, it may be best to put it upside down to allow the now higher parts to push the perimeter out if it is necessary.

Thickness

• ·         Thick glass will flatten more quickly than thin glass, so you need to keep a watch on the progress of the work to avoid excess marking of the surface of the glass.
• ·         Very thin pieces are likely to develop wrinkles as they flatten.  Even if they do not, there will be thick and thin areas which might cause difficulty in subsequent slumping.
• ·         Tack fused pieces are likely to tend to flatten at different places and times due to the differences in thickness and therefore weight. This makes shallow forms easier to flatten.

Temperatures

• ·         In all these processes, you should use the lowest practical temperature to flatten.  This means that you will need to peek at intervals to see when it is flat.
• ·         Your starting point for the top temperature to use will be about 10°C lower than that at which the original was slumped, normally.  The amount of time may need to be extended significantly. The reason for this is to avoid as much marking on the finished side as possible.
• ·         Shallow forms and thick pieces will flatten more quickly than others, so a lower temperature can be used.  You will still need to observe the progress of the flattening.
• ·         Angled shapes and deep forms will need more heat and time than the shallower ones. 
• ·         Thin pieces may require more time than thick pieces.
• ·         Tack fused pieces need more attention and slow rates of advance to compensate for the differences in thicknesses.

Separators

• ·         Kiln washed shelves are usually adequate for flattening.
• ·         Thinfire or Papyros are needed when flattening upside down to ease any sliding necessary.
• ·         Powdered kiln wash or aluminium hydrate can be dusted over the kiln washed shelf when it is felt the form will need to slide on the shelf while flattening.

It may be that after all this, you feel it is not worth it to flatten.  It certainly is worth the effort, if only to learn about the characteristics of the form and its behaviour in reversing the slump or drape.

Thinfire* and Devitrification

There are reports that Thinfire causes devitrification by rising over the edges of the piece.  There as many saying they have no difficulties with the Thinfire curling.  This indicates there are several factors that may be at work.

If the Thinfire curls over the edge of the glass while firing, it will deposit a fine powder on the edge and perimeter of the piece.  This gives an ideal condition for devitrification to form.

Bullseye recommends placing dams or other kiln furniture on the edges of the paper to resist any tendency for the paper to curl.  Of course, if the paper is put upside down, it is much more likely to rise over the edge.  The smoothest surface should face upwards. Now that Bullseye prints their logo on the bottom, this is unlikely to be a problem.

Cutting the paper to the size of the piece is initially an attractive idea.  However, it does not account for the expansion beyond the initial footprint that glass goes through while heating to the working temperature, and before it contracts to its final size.  The Thinfire must be cut larger than the piece. The amount depends on the thickness of the piece.  6mm larger may be adequate for a 6mm thick piece.

Bullseye does not recommend using Thinfire under multiple small pieces of glass because the paper can shrink and move, disrupting the glass placement on the kiln shelf.  Instead using kiln wash as the separator may be better in these circumstances.

There are other things that can affect the deposit of the separator powder from the Thinfire onto the glass.

Venting – It seems to be good practice to open the peep holes or leave the door/lid slightly ajar during the heat up.  These should be carefully closed once the smell of the binder burning out disappears.  This is usually around 500°C.  The idea here is that the combustion products from the binders are allowed out of the kiln without settling on the glass.  I do not find this necessary, but many do, so it is worthwhile trying it out.  When the smell of the burnout of the binders ceases close the lid slowly and place the bungs gently into the peep holes to avoid disturbing any dust within the kiln.

Opening the kiln or ports - Opening or closing the kiln above ca. 500°C, if done quickly, will create a draft that will distribute the powder around the kiln.  Some of this will land on the surface of the glass. Other parts of the Thinfire will be moved up onto the edges of the piece.  This dust and the pieces of Thinfire will create nucleation points for devitrification.  Always open or close any part of the kiln slowly when there are powders or anything else which can be disturbed by a gentle waft of air.

Over firing - Another element that can bring Thinfire onto your pieces are a too hot a firing.  During high temperature firings, the glass will expand and thin more than usual.  During the cooling phase, the glass will draw back to being 6-7mm thick. This means the glass will have expanded over the Thinfire and drawn some of it back onto the edges as it thickens and retreats.  The solution for this is to reduce the top temperature and possibly lengthen the soak time, but do not do both at the same time.  First see what a lower temperature with a 10-minute soak will do.

Of course, if you are not having problems with Thinfire or Papyros, continue your practice as normal.

*I have used the term “Thinfire” almost exclusively throughout, but remember all these notes apply to Papyros too.

Rates of Advance with Soaks

I’m sure I have written about this before, but a repetition will not hurt.

I have seen many schedules with initial rates of advance interrupted by soaks.  These kinds of schedules that are written something like this:

250 degrees C to 200C, soak for 10 (or 20 or 30) minutes

250 degrees C to 500C, soak for 10 (or 20 or 30) minutes

300 degrees C to 1100C, soak for 10 (or 20 or 30) minutes

300 degrees C to 1250C, soak for 10 (or 20 or 30) minutes

600 degrees C to working temperature (1450, 1500 etc.)

When I have asked, I’m usually told that these are catch up pauses to allow all the glass to have an even temperature.  There are occasions when that may be a good idea, but I will come to those later.  For normal fusing, draping and slumping these soaks are not needed.

To understand why, needs a little information on the characteristics of glass.  Glass is a good insulator.  It is a poor transmitter of heat.  Therefore, glass behaves better with a moderate steady input of heat to ensure it is distributed evenly throughout the glass.  To advance the temperature quickly during the initial heat up stages where the glass is brittle risks thermal shock. 

The soaks at intervals do not protect against a too rapid increase in temperature.  It is the rate of heat input that causes thermal shock.  Rapid heat inputs cause uneven temperatures through and across the glass.  When these temperatures are more than 5°C different across the glass, stress is induced.  As the temperature differential increases, so does the stress until the glass is not strong enough to contain those stresses and breaks.  At higher temperatures these stresses do not exist as the glass is less viscous.

If, as is common and illustrated in the schedule above, you advance at the same rate on both sides of the soak, the soak really does not serve any purpose – other than to make writing schedules more complicated.  If the glass survived the rate of heat input between the soaks, it will survive without the soaks.

But you may wish to be a little more careful. The same heating effect can be achieved by slowing the rate of advance.  Just consider the time used in the soak and then slow the rate by the appropriate amount.  Take the example above using 30-minute soaks:

250 degrees C to 200C, soak for 30 minutes

250 degrees C to 500C, soak for 30 minutes

This part of the schedule will take three hours.  You can achieve the same heat work by going at 167 degrees C per hour to 500 degrees C.  This will add the heat to the glass in a steady manner and the result will be rather like the hare and tortoise.  If you have to pause periodically because you have gone too quickly, you can reach the same end point by steady but slower input of heat without the pauses.

But, you may argue, “the periodic soaks on the way up have always worked for me.”  As you work with thicker than 6mm glass, this “quick heat, soak; quick heat, soak” cycle will not continue to work.  Each layer insulates the lower layer from the heat above.  As the number of layers increase, the greater the risk of thermal shock. Enough time needs to be given for the heat to gradually penetrate from the top to the bottom layer and across the whole area in a steady manner.

To be safest in the initial rate of advance, you should put heat into the glass in a moderate, controlled fashion.  This means a steady input of heat with no quick changes in temperature.  How do you calculate that rate?  Contrary as it may seem, start by writing out your cooling phases of the schedule.  The cooling rate to room temperature is the safe cooling rate for the final and now thicker piece.  If that final cool rate is 300 degrees, the appropriate heat up rate is half of that or 150.  If you are in the habit of turning off the kiln at 370°C, you can use the cooling rate that is scheduled to get you there.  Normally, you would double the rate you used to get to 370°C as the rate to room temperature.  So, the rate to 370°C is the same as half the final cooling rate.

This “half speed” rate of advance will allow the heat to penetrate the layers in an even manner during the brittle phase of the glass.  This rate needs to be maintained until the upper end of the annealing range is passed.  This is normally around 55°C (110°F) above the annealing point.

Then you can begin to write the rate of advance portion of your schedule.  It could be something like:

150°C to 540°C, no soak

225°C to bubble squeeze, soak

300°C to working temperature, soak 10 minutes

Proceed to cool segments 

I like simple schedules, so I normally stick to one rate of advance all the way to the bubble squeeze.  This could be at the softening point of the glass or start at 50°C below with a one hour rise to the softening point with a 30-minute soak there before proceeding more quickly to the working temperature.

Exceptions.

I did say I would come back to an exception about soaks on the rate of advance segment of the schedules.  When the glass is supported – usually in a drape – with a lot of the glass unsupported you do need to have soaks.  The kind of suspension is when draping over a cylinder or doing a handkerchief drop.  This is where a small portion of the glass is supported by a point or a long line while the rest of the glass is suspended in the air.  It also occurs when supported by steel or thick ceramic.

The soaks are not to equalise the temperature in the glass primarily.  They are to equalise the temperature between the supports and the glass.  A thick ceramic form supporting glass takes longer to heat up than the glass.  The steel of a cocktail shaker takes the heat away from the glass as it heats faster. 

The second element in this may already be obvious.  The glass in the air on a ceramic mould can heat faster than that on the mould.  The glass on a steel mould can heat faster over the steel than the suspended glass.  Both these cases mean that you need to be careful with the temperature rises.

Now, according to my arguments above, you should be able to slow the rate of advance enough to avoid breakage.  However, my experience has shown me that periodic soaks in combination with gradual increases in the rates of advance are important, because more successful. 

An example of my rates of advance for 6mm glass supported on a steel cylinder is:

100°C to 100°C, soak 20 minutes

125°C to 200°C, soak 20 minutes

150°C to 400°C, soak 20 minutes

200°C to draping temperature

Call me inconsistent, but this has proved to be more effective than dramatically slowing the rates of advance. 

Note:

This exception does not apply to slumps where the glass is supported all around by the edge of a circular or oval mould, or where it is supported at the corners of a rectangular or square one.

Another exception is where you have a lot of moisture in a mould, for example. You need to soak just under the boiling point of water to dry the mould or drive out water from other elements of your work before proceeding.  This applies to situations where you need a burn out, of for example vegetable matter at around 500C for several hours.

In both these cases, these are about the materials holding or contained in the glass, rather than the glass itself.