Time and Temperature

credit: timeanddate.com

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“What are the pros and cons on turning up the max temperature slightly Vs. a longer hold time”? Lea Madsen

This is a difficult question to answer, because there are variables such as

the temperature range,

the ramp rates, and soaks,

the forces acting upon the glass at a given temperature, 

the process,

the desired outcome of the firing,

etc. 

When talking about temperature vs. time, it is heat work that we are considering.  In many processes time and temperature are interchangeable, although the temperature range is important too.  This is a brief discussion of heat work in various processes.

Slumps

Slumping temperature is generally in the range of 620˚C-680˚C/1150˚F -1255˚F *, which is below the devitrification range.  This allows the exchange of time for temperature without risk, allowing more time rather than more temperature.  Higher temperatures cause more marking from the mould since the bottom of the glass is softer than at lower ones.  Lower temperatures give higher viscosity, so the glass is stiffer, resisting marks.

Low temperature fuses

Sharp tack fusing, freeze and fuse, some pate de verre processes, and sintering occur in the 650˚C -720˚C /1150˚F - 1320˚F range, risking devitrification only at the upper end of this range.  Extending the time rather than the temperature is important to maintain detail in these processes.  Higher temperatures will smooth the surface, risking loss of detail.  

Rounded tack processes (720˚C – 760˚C /1320˚F - 1400˚F)

These are within the devitrification range making the choice between time and temperature a balance of risks.  In my experience, it takes about an hour for visible devitrification to develop.  This means that you can extend the time, if the total time between the end of the bubble squeeze and the working temperature, including the hold time, is less than an hour.  It has the advantage of a more secure attachment between the pieces of glass, without altering the surface much. 

But if extending the soak time increases the time in the devitrification zone to be more than an hour, it is advisable to increase the temperature, rather than time.  Devitrification develops in the presence of air, so reducing the time in that range reduces the risk of devitrification developing.  The glass is moving during rapid ramp rates, reducing the chance of devitrification.

Drops

This includes drapes, and other free forming processes.  Kilnformers will be observing the progress of these firings, making it easier to balance temperature and time.  There are already long holds scheduled for the processes, so it is a matter of getting the right temperature.  If, after half an hour at the scheduled top temperature, the glass has not moved much, it is time to increase the temperature by, say 10˚C/18˚F and observe after another half hour, repeating the temperature increase if necessary.   Be aware of thinning the glass at the shoulder by setting a high temperature.  Free drops may take as much as 6 – 8 hours, so patience and observation are important to get good results.

Full fuse

At full fuse try to get the work done in 10 minutes to avoid complications with devitrification.  So, increasing the temperature rather than the length of the soak seems best.

Flows

Whether frit stretching, making pattern bars, pressing, etc., low viscosity is important.  Viscosity is closely related to temperature, so increasing the temperature is the better choice.  Increasing time without increasing temperature does not change viscosity much.

Casting

Extending time at top temperature seems best for open face casting, as the temperature is already high.  A slow ramp rate to that top temperature may make adding time unnecessary, because the heat work will be increased by the slow rise.  Experience has shown that a rate of 200˚C/360˚F is enough to avoid devitrification.  With enclosed castings devitrification is not such a risk, so time can be added without concern.

 

Observation

In all these processes it is advisable to observe the progress of the firing by quick peeks to determine the effective combination of temperature and time.  Also note that heat work is cumulative, making for changes in profile with repeated firings. 

 

* The softening point of float glass is around 720°C/1328°F, so the slumping range is about 700°C/1292° to 750°C/1382°F.

Refractory Fibre on Top of Glass

“I've made this stencil out of rigidized SilkeMat. Can I fill it with powder and/or frit and leave the stencil on the glass when I fire it to a full fuse?“

SilkeMat is thicker, with greater insulating properties than shelf paper. The base glass will be insulated in two ways. The frit in the spaces of the stencil will insulate the heat from reaching the base under it. The SilkeMat will have even greater insulating effect on the glass it covers. So, the proposed layup would create three heat areas on the base layer - the area insulated by the SilkeMat, the portion under the frit, and the uncovered areas. These three heat areas are a big problem for the glass to cope with, as they each will heat differently.

An illustration of cutting holes in SilkeMat to form a stencil with thick edges
Credit: Katie Chapman (not the origin of the question)

At full fuse the SilkeMat or any other refractory fibre blanket will mark the glass and may stick in parts.  Adding weight will only increase the marking and sticking, as well as further insulate the covered area.  The most important problem with leaving refractory papers on top of the glass during a full fuse, is the insulating property of SilkeMat or any other refractory fibre paper, creating large temperature differentials, which require extremely slow heat ups, long anneal soaks, and very slow cooling.  If it were to be fired with refractory fibre paper on top, in spite of these warnings, I would fire it as though two inches thick.

A much better approach is to make a card stencil that is stiff enough to lift easily without spilling any excess powder as it is removed.  Apply multiple thin layers of powder, firing between each application.  Careful application and firing to a low temperature tack fuse each time will give a crisp edge to the powdered image.  This can be glossed with a full fuse firing after the last application.

 

A silicone rubber colour pusher or stiff brush can move the powder, either to the powder image or away and off the base to give a crisp edge to the image.

If you have not already built the piece in the kiln, powder and frit are heavy enough that they will not be disturbed easily on the way to the kiln.  An aerosol adhesive that drifts down onto the powder will be enough to hold it in place if concerned about movement.

 

But best of all, is to create a powder wafer where the thickness and crispness of the image can be controlled, and then placing it onto the base glass.  This avoids the risks of temperature differentials being created by the refractory paper

Reversibility of Boron Nitride

After using Zyp/MR97, can I sand it off and use kiln wash?

Some people are applying boron nitride to ceramic moulds for the "smoother" surface.  Boron nitride is an excellent separator for metal moulds and casting moulds whether metal or ceramic. But it has limitations, including the price and requirement for a repeated application at each firing.  Some are beginning to wonder if they can go back to kiln wash after having used the boron nitride. Some say you cannot unless you sand off the separator.

The general experience has been that you can't apply kiln wash on top of the boron nitride. It just beads up and flows off, because the boron nitride creates a non-wetting surface that survives relatively high temperatures.  The water in the kiln wash mixture merely beads up or washes away. This means the kiln wash in suspension has no opportunity to adhere to the mould.

The most accepted way to get rid of the boron nitride is by sandblasting. Then apply kiln wash as normal. The sandblasted ceramic mould previously coated accepts kiln wash with no difficulty. In the absence of a sandblaster, you can use a sanding pad. You do need to be cautious about taking the surface of the mould when using abrasive removal methods, as the ceramic is relatively soft in relation to the abrasive materials.

However, boron nitride is soluble in various alkaline chemicals, such as potassium hydroxide (caustic potash), sodium hydroxide (caustic soda or lye), and sodium nitrate (Chile saltpeter).  Soaking or washing the surface with one of these will dissolve the boron nitride, and should return a surface that will accept kiln wash.  Be cautious in the use of these chemicals as they are dangerous to the skin.

The difficulty of removal of the boron nitride means that you have to think carefully about which moulds you put it onto.  If the mould has delicate or fine detail, removing the boron nitride risks the removal of some of the detail.  This indicates that this kind of mould, once coated, should not be taken back to the bare mould to change the kind of separator.

Another use of boron nitride is to spray a very small amount on a fiber strip to be used for damming. This will give you fewer needles as it provides a non-wetting surface at relatively high temperatures. This allows the glass to slide down the fibre paper without hanging up and creating the needles.

One advantage of kiln wash over boron nitride is that you do not have to reapply every firing as with boron nitride. With the boron nitride it is recommended to apply before every firing.  It is best to use a paint brush to dispose of any lose material before giving a light re-coating. Not a whole lot is required on subsequent coatings.

If you are using boron nitride to get a smoother surface to the object, also consider using a lower slumping or draping temperature, as this will also minimise mould marks.  

Cutting thin strips

Cutting thin strips of glass such as used in Mission Style patterns and precision fusing projects requires skill and assistance. For transparent and translucent glass you can arrange a right angle guide on a board and tape a piece of lined notepaper to the jig. Use a cutting square and move it right along the lines on the note paper making four or six scores at a time and then breaking on the last score first and then every other score, and then each one in half.

Another method is to use the edge of the bench or work board as a guide. With a small adjustable carpenter’s square, you hammer in nails at the predetermined width (plus half the thickness of the cutter head). Align the glass to the edge of the bench between the nails. Place a straight edge against the nails and score. This gives strips of the same width every time, but works best with strips of 10mm (3/8”) or more. This is illustrated in the processes section.

The thinner the strips are to be cut, the more important it is to make the scores and then divide the sheet in half - the two halves in half each - the 4 quarters in to halves, etc, until you are down to the piece that only needs to be divided in two.

The thinnest strip that can be cut is a fraction wider than the thickness of the glass.  This is because the glass will always break toward the weakest area.  If the strip is thinner than the glass is thick, it will break within the strip.  The narrowest strip that can practically be cut is at least one or two millimetre wider than the glass is thick.  So, if you have 3mm glass, the narrowest you can cut is 4 or 5 mm.  Four millimetre wide strips can only be cut from really smooth consistent thickness of glass sheets.  It is much more practical with decorative glass to limit the width to twice the thickness of the glass you are cutting.

Dividing two thin strips

Cutting from Cartoon

The cutting of glass directly over the cartoon without patterns is variously called trace cutting, English or European method. The advantages include a more direct process with fewer operations, making for less chance of inaccuracies.  These make this method quicker than using patterns or templates to draw or cut around. The disadvantages are that you still need to make patterns for opalescent or very dark glass, and there is no pattern to guide any grinding required.

To employ the method, use a strong contrast ink for drawing the lines of the cartoon. Draw these in the appropriate width - for copper foil (ca. 0.8mm).  This can be accomplished with a ball point pen or fine felt tip.  For lead, a thicker line (ca. 1.6mm) is required - a bullet tipped felt pen is usually appropriate, if it is not worn.

leadlightdesign.com

To cut, place the glass over the appropriate part of the cartoon and cut at the inside edge of the line. It is best to cut and break one line at a time.  Re-set the newly cut edge along the cutline and score the next line.  Break it and repeat the number of times required to cut out the whole shape.  Which line should be the first to be scored and broken is described here. 

Although the glass is normally only three millimetres thick, there is some possibility of a refraction of the light if you look from the side of the cutter.  As described elsewhere, you should be holding your cutter upright in the left to right plane and angled slightly back toward you so that you can look with your dominant eye along the barrel of the cutter and head.  This ensures you are directly above the cut line and do not have any refraction caused by the glass. 

Translucent glass may, and opalescent glass will, need a light box to assist in the scoring.  This increases the light coming through the cartoon and glass to enable a sharp image of the lines to be seen.  This means that if you intend using opalescent glass, your cartoon should be done on translucent paper to allow the maximum light through.

freepatternsforstainedglass.com

For black, dense and strongly opalescent glass, pattern pieces will need to be cut, as the insufficient light will pass through the glass to be able to see the cartoon accurately.

The reduction in the number of operations to guide the cutting of the glass also reduces the occasions for small errors to creep in.  It does increase the accuracy of cutting and speed of building a panel.

Diagnosis of Cutting

If your scoring and breaking of your glass is not going well, you need to diagnose the reasons.  There are always a lot of suggestions that warming the glass will solve the problem. Yes, warming glass may help. A discussion of the effect is here. But it will not overcome any faults in the basic skills of scoring.

A lot of images, shown on the internet, of straight line scores failing to break along the score, indicate some possible elements in scoring that lead to these unwanted break-outs. 

One possibility is you are using too much pressure. A discussion of the amount of pressure required is here.  You should be scoring to the pressure required, rather than any sound that may come from scoring.  This is emphasised when cutting opalescent glass.  The correct scoring pressure makes almost no sound or only a gentle rumble as it cutter moves over the undulations of the glass.  The most frequent reason for more difficulty in breaking opalescent glass is excessive pressure while attempting to get the same sound as from transparent glass.  There are even a few transparent glasses that make little or no sound when being scored with the correct pressure.

Another common problem in scoring is keeping an even pressure throughout the score.  It can be difficult to keep the pressure even on complicated cuts.  When the cartoon has multiple curves or deep concave lines, it can be difficult to keep the pressure even as you move your body around to follow the line.  One piece of advice I received early on in my learning was to rehearse the score allowing the cutter wheel to move along the score line with virtually no pressure.  This shows how the piece of glass needs to be oriented to ease your movement around the glass to make the score.

Slowing the cutting speed can help to keep the pressure evenly distributed along the score.  Straight lines are often scored quickly.  But, even on straight lines, slowing the speed can make the pressure more even throughout the score.  It can also avoid variable speed during the scoring, which leads to different forces being placed on the glass.  The pressure may be consistent, but the effective pressure is greater when slow than when fast scoring is used.  If the speed is variable, the effective pressure differs along the score line.

A fourth thing that may be happening on straight lines is that the cutter wheel is at an oblique angle to the direction of the score.  This will often be heard as a scratching sound as you move along the score line.  This can be overcome by a gentle pressure against the straight edge you are using to align your score.  Of course, the straight edge needs to be held firmly to avoid having it move.  Allowing the head of the cutter to have a little freedom of movement also helps keep it parallel to the straight edge.

All this is merely speculation about your scoring practice.

You need to get someone to observe you scoring.  They do not need to be experts, nor other glass artists.  They just need to be observant. Tell them what you are looking for in each of the four elements of scoring and have them observe only one thing at a time.

First get scales that you can zero when you have a small piece of glass on it. Score without touching the glass. Have the observer tell you if the pressure was consistent throughout the score, and if you are in USA, whether the pressure was above 7 pounds or below 4 pounds. (For the rest of the world 3kg to 1.8kg). Practice until you can score consistently at about 2.2kg (ca. 5 pounds).

Second, have the observer stand a little distance from you. Score toward the observer. They need to observe whether your cutter is perpendicular to the glass while scoring and if there is any variation.

Next, they need to tell you if your head was directly above the cutter all the way through the score. They will be able to see whether your eye is directly above the cutter

Is your body behind the cutter, or do you use your arm to direct the cutter?  The observer will be able to tell that when you are scoring curves. The most consistent speed and pressure is delivered when the cutter is steered from your torso, rather than your arm and wrist.  It slows the scoring action, gives smoother curves, and more even pressure.

The last element, you can do yourself.  Once you are doing all the things above, you will be able to hear any scratching noise, rather than the gentle creaking noise of an even score with adequate pressure.  If the scratching noise is intermittent or only at one point, the likelihood is that you are twisting the cutter head, so the wheel is not in line with the score line.

Adjusting Cut Runners

There are a number of types of cut running pliers.  These photos show some of them. 

The apparently most popular is this:

Cushions

It is frequently difficult to find replacements for the plastic cushions that come with a new pair of cut runners.  People resort to a number of means to provide a substitute.  Some wrap electrical tape around the jaws, others use fabric bandages (Elastoplast/band aids).  I have even used the liquid plastic that is designed for coating tool handles.

However, if you adjust the cut runners appropriately, you can use them to run your scores even without cushions.  The purpose of these cushions is only to compensate for too much pressure in running the score.

Use without covers

You can run the score without cushions by using the adjustment screw on the top jaw of the tool. Yes, it does tell you which is the top jaw without having to check the end of the runners, but it has a more important use.  It is not just a pretty cool way to tell which is up. 

Its purpose is to adjust the width of the opening so that it provides the appropriate amount of bending force no matter how much pressure you exert at the handles.  If you are running scores in three-millimetre glass, set the jaws to that width by turning the screw until the jaws are that width apart.

Place the jaws in line with the score, aligning the mark on the top jaw with the score line and squeeze the pliers.  As you squeeze, the curved jaws provide enough bending force to run the score without over stressing the glass.  It is the adjustment screw that limits the over-stressing of the glass during the running of the score. Yes, you may not be able to run the whole length of the score this way, but you can repeat from the other end and that is usually enough to complete the running of the score.

You can continue to use cushions of various sorts with this adjustment for thickness, but I found that these were not necessary when the runners were properly adjusted.  In fact, I found that soft cushioning made more difficulties than using them with the bare metal.  I discovered this during the period of using the liquid plastic coating as used for tool handles.  I dipped the jaws multiple times to give a cushioning effect and it worked fine.  The cut runners continued to work even after the coating had worn off.  It was then that I realised I could control the running pressure more directly than by having a cushion between the glass and the jaws.

Setting the spacing

An easy way to set the correct opening of the jaws is to test against the glass you are about to score and break.  Place one side of the jaws against the edge of the glass. Slide that corner just a few millimetres over the glass.  Turn the set screw on the top of the jaws anticlockwise until they are fitting the glass snugly.  Back off a half turn (clockwise) so the jaws move easily along the edge.  This is now set to run the score on this glass. 

Open the jaws and place the centre mark in line with the score.  Close them gently and you can observe the arc of the jaws above the score line. Squeeze the handles and the score will run along the line away from the cut runners.  As you have adjusted the opening, no matter how hard you squeeze the cut runners, you cannot add more pressure.  This means you avoid crushing the glass.

The principles

The curve of the jaws is designed to provide the bending force required to run the score.  The radius of the curve has been designed to provide the correct bending pressure for differing sizes of glass.  The most common ones are useful for glass up to, but not including, 6mm glass.  The screw adjustment provides compensation for differing thicknesses of glass.  Setting the width of the gap to match the thickness of the glass prevents the application of too much pressure.

Thicker glass

For thicker glass you need cut runners with wider jaws.  These usually are fitted with three points to apply the breaking pressure - one under the score and one each side of the score on the top.  Again, these are adjusted to be just less than snug to the glass before applying the pressure.

One example of  cut runners for thick glass.  There are a variety of others.

Lead Corrosion

There are three important versions of lead corrosion: Red, Brown and White.  In addition, there are other factors that can weaken the lead came.

Red lead is a corrosion product that appears as a bright red surface, is dangerous, and requires water, air and often wood, to form. Sometimes water in the manufacturing process can develop red lead.   The chemical composition of red lead (Lead (II, IV) or triplumbic tetroxide is Pb₃O₄ or 2(PbO^.PbO₂).  It is a bright red or orange crystalline or amorphous colour.

Red lead is virtually insoluble in water or in ethanol. But, it is soluble in hydrochloric acid as is present in the stomach.  When ingested, it is dissolved in the stomach’s gastric acid and absorbed, leading to lead poisoning. It also dissolves in undiluted acetic acid, as well as in a dilute mixture of nitric acid and hydrogen peroxide.

When inhaled, lead (II,IV) oxide irritates the lungs. In the case of a high exposure, the victim experiences a metallic taste, chest pain, and abdominal pain.

High concentrations can be absorbed through skin as well, and it is important to follow safety precautions when working with lead-based paint.

This means that anyone dealing with read lead needs protection against skin contact, and breathing protection.  Methods need to be implemented to ensure no dust is raised, or that the area is thoroughly cleaned after dealing with red lead. Clothing should be discarded or washed separately from all others.

White lead corrosion, Lead(II) carbonate, is the chemical compound PbCO₃. It occurs naturally as the mineral cerussite.  It is a curious compound, as it is soluble in both acid and alkali.  It is possible, but rare, for excess whiting left on the lead to give rise to this form of corrosion. Generally, it will be neutralised by the weather.

Brown lead corrosion appears as a brown to dull red colour. 

Lead(IV) oxide, commonly called lead dioxide or plumbic oxide or anhydrous plumbic acid …, is a chemical compound with the formula PbO₂. … It is of an intermediate bond type, displaying both ionic (a lattice structure) and covalent (e.g. its low melting point and insolubility in water) properties. It is an odourless dark-brown crystalline powder which is nearly insoluble in water. …. Lead dioxide is a strong oxidizing agent which is used in the manufacture of matches, pyrotechnics, dyes and other chemicals. It also has several important applications [e.g.,] in the positive plates of lead acid batteries.    Source: wikipedia

Air, water and salt are needed to form brown lead. This means coastal areas and those with driving rain are prone to this kind of oxidisation. Lead dioxide also forms on pure lead, in dilute sulfuric acid.  So, with the acid rain that we are all subject to, it can form in almost any situation, but will be more obvious on areas exposed to the prevailing wind.  The corrosion is soluble in strong acetic acid.

Tin corrosion also has a brown, almost copper appearance, very similar to brown lead.  The tin corrosion will be confined to the solder joint and surrounding area rather than all along the length of the came. 

Corrosion resistant lead

The ideal composition of lead to resist corrosion is 98.5% lead with up to 1% tin. This, combined with fractions of a percent of antimony and traces of silver, bismuth and copper provides a combination of metals and trace elements to resist corrosion of the lead as well as stiffening it.  Conservators indicate that, for whatever reason, cast lead incorporating trace elements is the most resistant to corrosion.  This is evidenced by the longevity of medieval lead cames.

Solder composition

Conservators also indicate that the higher the lead content of solder, and the better the match it is to the lead came, especially the almost pure lead came, the more resistant it is to lead came fracture at the margins of the solder joints.

Stretching the lead came, rather than simply straightening it, not only weakens the lead, it leaves very small pits in the surface of the lead.   These small pits allow the elements of the environment to penetrate the lead’s surface and act as sites for the beginning of corrosion.

Stretching also causes stress points near the solder joint.  The stretching creates stress along the length of the lead.  When the lead is heated in the soldering process the molecules of lead sort themselves into a stress-free arrangement.  As heat does not travel far or fast in lead, there is a stress point formed a short distance from the soldered lead joint where the already stressed and the stress-free lead meet.

Conclusion

Clearly there are a range of factors that relate to the resilience of lead came.  98.5% lead with trace elements including tin and antimony provides the greatest strength and resistance to corrosion.  Stretching the came can lead to general weakness and introduce pits into the surface forming sites for corrosion. Stretching can also lead to stress points near the solder joints.

All these indicate that resilient leaded glass windows can be produced by:

the use of lead came with 1.5% of trace elements,

the use of high lead content solders, and

the straightening (rather than stretching) of the came.

Tapping Glass Scores

Many people tap the underside of the glass after scoring.  The purpose of this is to run the score.

However, this tapping is often unnecessary.  Running the score can be done in a variety of ways, some more suitable for one kind of score line than another.

Straight score lines can be run in several ways.

•       Move the score line to the edge of the bench or cutting surface and use a controlled downward force on the glass off the edge while holding the remainder firm.  Works best if at least a third is being broken off.
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•      You can place a small object, such as the end of your cutter or a match stick, directly under the score and place your hands on either side and press firmly, but not sharply, down on each side at the same time.  This is good for breaking pieces off from half to a quarter of the full sheet.
•      Make your hands into fists with the thumbs on top of the glass and the fingers below.  Turn your wrists outwards to run the score. Works best if the glass is approximately half to be kept and half to be broken off.
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•      Take the glass off the cutting surface, hold in front of your knee at about 45 degrees and raise you knee quickly to the glass.  This will break the glass cleanly, but is only useful for moderate sized sheets and where you are breaking off about half of the sheet.
•      Use cut running pliers to run the score.  Be sure the jaws are adjusted for the thickness of the glass, and do not apply excessive pressure.  If the score does not run all the way, turn the glass around and run the score from the opposite end. Best where there are approximately equal thin parts to be broken away from each other and when the score line is no less than an oblique angle to the edge. It does not work very well for thin pieces or acute angles.
  
  
  
•      Use two grozing pliers nose to nose and flat side up at the score line and bend them down and away.  This works best on thin and or pointed pieces.
•      Breaking pliers can be used at intervals along the score. This is most useful on long thin pieces.

Curved score lines, of course require a bit more care but generally employ the same methods.

•     Gentle curves can be dealt with as though they are straight lines, although the breaking at the edge of the cutting surface is a bit risky. This means the two-fist, running pliers, two grozing pliers and breaking plier methods can be used.
•     Lines with multiple curves usually require cut running pliers to start the run at each end of the score.
•     Deep curved scores may require the running pliers whose angle can be adjusted to be at right angles to the score.  The ones I know are Silberschnitt, made by Bohle, although the ring pliers by Glastar work in the same way. This usually requires that the edge of the glass is not more than 5 cm from the score.  This blog gives information on a variety of cut running pliers. 

Tapping

After trying all these methods to run the score, sometimes the score is so complicated or deep into the glass that you cannot simply run the score.  Tapping may then be required, but it is a last resort.

Tapping, to be effective, must be accurately directed to places directly under the score line.  The tapping cannot be at random places under the glass. Each tap must be controlled – to be direct and to be firm. 

The impact needs to be directly under the score. 

•     Taps that are either side of the line will either not be effective, or will promote breakage other than along the score line. 
•     Tapping to either side of the score also promotes shells to either side of the score line.  These are not only dangerous when handling, but also require further work to remove these ledges of glass.

The impact also needs to be firm. Random impacts to the glass promotes breakage other than along the score line.

•     The taps need to be firm – neither light nor hard.
•     Each tap should be at the end of the run begun by the previous one.  This promotes a smoother run of the score with less opportunity to start a run off the score line. 
•     To avoid the incomplete running of the score that leaves parts of the score untouched you need care. As the glass begins to break along the score line, place the next impact at the end of that start to continue the run. 

Tapping the glass under the score should be a last rather than first resort in running a score.

Break Diagnosis in Slumping

The usual advice in looking at the reasons for breaks in your pieces must be considered in relation to the process being used.  Breaks during slumping need to be considered differently to those occurring during fusing.  

The advice normally is that if the edges are sharp, the break occurred on the way down in temperature. Therefore, the glass must have an annealing fracture or a compatibility break.  It continues on to say if the edges are rounded it occurred on the heat up, as it broke while brittle and then rounded with the additional heat.

This is true, but only on rounded tack and fused pieces.

When the process is a slump, there is not enough heat to round the edges.  So, the edges will be sharp whether the break was on the heat up or the cool down.

How can you tell in a slump process when the break occurred?

The first, but not obvious, way to tell if the break is on the heat up is to peek at about 260C/500F as most heat up breaks occur around that temperature, and again at the strain point, about 540C or 1000F.  These are the two critical heat up temperatures that will give the knowledge of when the break occurred.  If at either of these peeks, the glass has broken, the firing can be abandoned.  If the break is at the higher temperature, it needs to be annealed though.

 

The other way to tell if the break occurred on the cool down is that if you can put the pieces of the slump back together and they fit perfectly, the break was on the cool down, as the piece was already fully formed at the time of the break.

If the pieces do not fit together perfectly, the break was on the heat up.  This is because the break occurred, and then the two (or more) pieces slumped independently, thus leaving slightly different shapes at the break line.

There is a special case here, of course.  Sometimes the break is only a split in the bottom, that does not come all the way to the top of the piece. This split (or splits) occur when the heat up is too fast.  The theory goes that the top became plastic while the bottom was still brittle/stiff.  The weight of the hotter, more pliable glass overcomes the strength of the cooler and heat stressed bottom, causing it to split.  

Another theory is that the layers of glass were not fully fused together, so forming a thermal break between the upper and lower pieces, again allowing the top to heat more quickly, but causing thermal shock to the lower part.

More information is given in the blog post Diagnosis of Breaks, and a full discussion in the ebook Kilnforming Principles and Practices available from Bullseye.

There is also extensive information on diagnosis of breaks in this blog entry on slumping cracks.