Stress Analysis of Broken Glass

Will stress still show with polarised filters on cracked and broken glass?

It's not a straightforward answer.

I was looking at some broken fused float glass a few years ago.  I had always subscribed to the idea that a fracture relieves the stress. Not always. The broken float glass had been slumped, and the pieces still showed stress.  This turned out to be a compatibility problem, although both layers were float.  

The stress of inadequately annealed glass is likely to remain visible through the filters, because inadequately annealed glass will have stress distributed across the whole piece.  But glass that has been cooled too quickly and suffered thermal shock, is more likely to show minimum stress because the break relieved most of it.

It is likely stress will show on the tree piece pictured because it has not completely broken a[art. And even when it does break, it may still show a residue of stress.

It is sensible when trying to diagnose the problem to perform a strip test of the glasses for compatibility of the glasses concerned to be sure what is happening. If no stress shows on the test strip, the stress showing on the cracked piece is unlikely to be from incompatible glass, and other factors need to be considered.

Photo credit:  Debi Frock-Lyons 

Rapid Ramp Rates with Soaks

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

250°C/450°F to 200°C/482°F, soak for 10 (or 20 or 30) minutes

250°C/450°F to 500°C/933°F, soak for 10 (or 20 or 30) minutes

300°C/540°F to 595°C/1100°F, soak for 10 (or 20 or 30) minutes

300°C/540°F to 677°C/1250°F, soak for 10 (or 20 or 30) minutes

330°C/600°F 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, and therefore a poor transmitter of heat.  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 not relieved.  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°C/450°F to 200°C/482°F, soak for 30 minutes

250°C/450°F to 500°C/933°F, soak for 30 minutes

This part of the schedule will take three hours.  You can achieve the same heat work by going at 167°C/300°F per hour to 500°C/933°F.  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°C/540°F, the appropriate heat up rate is one third of that or 100°C/180°F. 

This “one third 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/100°F above the annealing point.

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

100°C/180°F to 540°C, no soak

225°C/405°F to bubble squeeze, soak

330°C/600°F 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 first ramp rates  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 that periodic soaks in combination with gradual increases in the rates of advance are important, because it is more successful. 

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

100°C/180°F to 100°C/212°F, soak 20 minutes

125°C/225°F to 200°C/392°F, soak 20 minutes

150°C/270°F to 400°C/753°F, soak 20 minutes

200°C/360°F to draping temperature

Call me inconsistent, but this has proved to be more effective than dramatically slowing the rates of advance.  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 also applies to situations where you need a burn out, of for example vegetable matter at around 500°C/933°F for several hours.

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

Revised 23.2.25

Cordierite/Mullite vs. pizza stones or tiles

Description of the materials

Cordierite refractory shelves are generally combined with mullite to achieve low expansion rates.  These are most often manufactured as solid slabs, although there is an extruded version with hollow channels along the length, given the trade name corelite.

Cordierite is magnesium, iron and aluminium in a cyclosilicate form (or rings of tetrahedra).  It is named after its discoverer, Louis Cordier, who identified it in 1813.

cordierite/mullite shelves

Mullite is combined with cordierite in small amounts to increase strength and reduce the amount of expansion. It does this through the formation of needle shapes that interlock and resist thermal shock. It also provides mechanical strength.

Mullite was first described in 1924 and named for an occurrence on the Isle of Mull, Scotland, although it occurs elsewhere, usually in conjunction with volcanic deposits.   

Pizza Stones and Tiles

Pizza stones are a variant of baking stones where the food is placed on (sometimes heated) stones.  Baking stones are a variation on hot stone cooking, one of the oldest cooking techniques. The stones are normally unglazed tiles of varying thicknesses.  What is said of pizza stones also applies to tiles.

Characteristics

Pizza stones  

Ceramic tiles and pizza stones are essentially the same things.  Some tiles may be thinner, especially if they are not large. In both cases, the ceramic is a poor heat conductor and the thermal mass means care needs to be taken in rapid heating and cooling of tiles and of baking stones. These are dry pressed which give a coarser surface texture than cast shelves.  All these ceramics are generally fired at about 1100C, so they can withstand kiln forming temperatures.  They are adequate as small shelves, but will deform over larger areas over time.

Cordierite-Mullite kiln shelves and furniture.

This formulation of materials has an extremely low coefficient of thermal expansion that explains the outstanding thermal shock resistance of these kiln furniture materials. They are also strong although heavy. Cordierite/mullite shelves are sintered, to allow the mullite needles to form, and fired at 1400C+, higher than tiles.

This material can be cast, dry pressed or extruded.  Cast shelves are the cheapest of the methods and provides a smooth surface.  These are used for kilnforming glass, and low temperature ceramic firing. 

Dry pressed shelves have a higher temperature resistance than cast. For this reason, these are often marketed as ceramic shelves, even though the cast shelves are fine for smaller areas.  These are more expensive than the cast shelves.

Corelite, a brand name for extruded shelves with hollow channels, is often used where larger shelves are required, as the weight is less than the solid cordierite. Extruded shelves are ground smooth after forming.

pizza stones

Preparation

Pizza Stones and Tiles

Due to the thermal mass of pizza stones and the material's property as a poor heat conductor, care must be taken when firing.  Firing quickly can break the stone or tile.  The stone or tile should be fired slowly to just under the boiling point and soaked for a couple of hours to eliminate any dampness in the material.  This probably should be done each time kiln wash is applied.  Because it is porous, a baking stone or tile will absorb any liquid applied, including detergent. They should be cleaned with a dry brush and then plain water if further cleaning is necessary.

Pizza stones and tiles should be checked for having straight and level surfaces. It is not a priority for these to have flat surfaces as for glass and ceramics shelves.  If by placing a straight edge on the surface you can see slivers of light, the shelf needs to be smoothed.  You can do this by grinding two of the proposed shelves together with a bit of coarse grit between.  This best done wet to avoid the dust getting into the air.

Cordierite

Cordierite/mullite shelves do not need this level of preparation, unless they have been stored outside.  It is possible to kiln wash and air dry for a few hours before placing glass on the shelf and firing.  This difference is the low rate of expansion (CoLE 19, if you are interested).

corelite shelves

Corelite

The extruded corelite shelves are made with cordierite/mullite, but are more delicate due to the hollow channels along their length.  They should be fired slowly to just under the boiling point of water - to eliminate the moisture - then continue the slow rise to at least 260C/500F to avoid the crystobalite inversion.  It should be fired to 540C with a pause before going to the top temperature.  The shelf should be supported at 30cm intervals under the shelf to minimise breakage.  The whole surface of the shelf should be filled rather than having just one heavy piece; again this is to minimise breakage.

Revised 23.2.25

Flat Kiln Shelves

A question has been asked about using tiles in addition to standard kiln shelves to fire glass upon.  Yes, you can use the unglazed backs to fire on, assuming they are not ridged or in other ways not a regular surface.

It is important to have flat shelves, as ones with even small shallow depressions can promote bubbles at higher temperatures. Tiles for walls and floors do not need to be flat to do their intended job and so are not checked for be flatness.

A magnified view of a shelf surface that is not perfectly even

You can do a quick check for flatness, by placing a ruler on edge across the tile or shelf to see if any light comes through underneath the ruler.  The light areas are the places where the surface is lower than the rest.  A more certain way to determine flatness is to sprinkle a black powder on the shelf.  Then draw a straight edge across the shelf to reveal any black areas.  These are the places that the kiln has a depression.  

If the depressions are few and small you can make corrections in the surface of the tile by grinding.  Put two tiles back to back and grind them together. The initial grind will show you the high spots as they will have the grinding marks there. 

You can eliminate these higher areas by rubbing the tiles together with a coarse grit (ca. 80) between the tiles to speed the grinding. If you are concerned about the dust or don’t have good ventilation, you can make a slurry of the grit by adding water. When the whole surface has the same marks, both will be flat. To double check, sprinkle black powder on the shelf and repeat the test for flatness with a straight edge.  If it is not fully flat, repeat the grinding process and checking until the tile or shelf is flat.

This sounds time consuming and lots of effort, but you will be surprised at how quickly you can achieve flat smooth surfaces even on larger tiles.  This also works for larger kiln shelves.

Revised 23.2.25

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.  

However the frequent use of organic materials - binders, plants, organic glues, etc - can degrade the protective oxide coating.  After prolonged use, it is advisable to clean the kiln of all dusts and fire empty to rebuild the oxide layer.

Revised 19.2.25

Time and Temperature

credit: timeanddate.com

Credit: Shutterstock

“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, and 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.