Bioactive glass

 A description of bioactive glass from Mo-Sci,Llc

Image credit: Mo=Sci, Llc

Bubbles on Single Layer Fusing

“I'm making 3mm French Vanilla sconce covers; …

·        [initially they were] fine, but now 1.5" bubbles form during the full fuse.

·        I pop the bubbles and fill the holes with frit and refire,

·        [The]… edges draw in and distort the design…

·        The shelf is flat,

·        I fire on Bullseye paper, and

·        the 13.5 hour long firing schedule [in F] is:

200 to 1150, hold 30 minutes.

50 to 1225, hold 30 minutes.

300 to 1490, hold 30 minutes.

9999 to 990, hold 60 minutes.

100 to 750, hold 1 minute.

Does anyone know what I can do to avoid the large bubbles? 

A critique of the schedule. 

 This is for a single sheet of 3mm glass, so the hold at 621˚C/1150˚F is unnecessary as is the slow rise to and hold at 663˚C/1225˚F, because it is a single sheet and does not need the traditional bubble squeeze. 

 The hold of 30 minutes at 810˚C/1490˚F is excessive. 

·        The temperature may be too high.

·        Ten minutes at top temperature is sufficient in most cases. 

·        A soak of 1 minute would be enough. 

·        The anneal soak at 990˚F is most probably a misprint for                          516˚C/960˚F. 

·        The anneal soak is longer than the half hour necessary, but not a             bubble creating problem.

 It means the schedule could have been:

111˚C/200˚F to 796˚C/1465˚F for 5 minutes

AFAP to 516˚C/960˚F for 30 minutes

83˚C/150F˚ to 370F˚/700F˚, 0 minutes

Off

 

Different firing strategies are possible.

•         Reduce the time at top temperature to no more than 10 minutes. 
•         Reduce top temperature by 55˚C/100˚F or more and extend the soak to 20 minutes, if necessary.  Peek frequently to see when the kiln work is complete.
•         Fire on fibre paper covered with Thinfire to allow air out from under the glass.

These strategies can be mixed as desired, and the reasoning for the strategies is:

• Excessive time at the top temperature allows the glass to thin as it migrates to form thicker areas/edges. This makes the glass too thin to resist the air pressure from below.
• Reducing the top temperature will increase the viscosity, so              resisting the migration of the glass, and maintain the original            thickness. 
• Also, single layers are prone to dog boning, but there are ways of reducing it.

Ways to reduce the risk of bubbles appearing in general are:

•    Reduce the time at the top temperature,
•    Reduce the top temperature,
•    Provide ways for the expanding air to migrate from under the glass.

Draping over steep moulds

 Draping over a narrow or small supporting ridge with large areas of glass is difficult.

One solution might be just to invert the whole piece and let the glass slide down into the mould. However, there rarely is enough height in a glass kiln for deep slumps, especially with a “V” shaped mould. It has to be high enough for the edges of the glass to be supported at its edges. You could also approach this by having a first mould with a shallower angle or broader support at its centre. Drape over this first, then use the steeper mould as the second draping mould. This makes the balance less critical.

The idea of supporting the glass is the key to doing this kind of slump that seems to require an impossible balancing act, if it is to be done in one go. Place kiln washed kiln furniture at the edges of the otherwise unsupported glass. Fire the kiln, but watch until the glass begins to slump. Then reach in with a wet stick and knock the kiln furniture aside to allow the glass to continue its slump and conform to the mould shape.

The lower temperature you use to do the draping and the slower your rate of increase is, the less the glass will be less marked by the mould. Frequent brief visual inspection during the drape is vital.

Also have a look at a suggestion for the kind of firing required for this here.

Dog Boning in Slumps

I have done a few experiments on rectangular moulds with 3mm and 6mm thickness. I could not eliminate dog boning with larger rims, slower rates, or lower temperatures in any combination - although they did reduce the effect.

Square single layers dog boned even with increased rim width, and reduction of slumping depth made little difference in the amount of dog boning. 

Rectangular single layers shapes persisted in dog boning on the long side regardless of the rim dimension, and exhibited more dog boning on the long side than in the equivalent single layer square.  Two layer slumping had a decrease in dog boning with increased rim width, but with less effect on the long side.

In general, glass slumped in rectangular moulds is more sensitive the shape of the rectangle than the size of the rim, and very sensitive to symmetrical placing on the mould.  The depth of the mould has less influence than the size of the rim, especially for single layers.  The wider the rim, the less dog boning, in general terms.

Deeper moulds, higher temperatures, longer holds, narrower rims, all increased the dog boning. I conclude slumped square glass looks better because the dog boning is symmetrical.

My solution is to make bigger rims and cut the piece square after slumping. This approach needs cold work to the edges, of course.

The reason rectangular slumps dog bone is because the glass at the sides is drawn into the mould more easily than the corners, because there is more glass to draw in, just as in flat dog boning.

An alternative to the cold working is to round the corners of the rectangles to reduce the amount to draw-in.  A 1cm/0.375” radius curve will reduce the extent of the dog boning, but does not eliminate the effect.

Pressing glass

I have been looking for a different way than flows or melts to mix colours and thought glass pressing might be a promising way to achieve what I wanted.

Weight vs Temperature

I conducted some experiments attempting to thin 1.25 kg/2.75 pounds of glass to 3-4mm.  One and then two 40x40cmx15mm thick shelves were placed on top of the glass cullet with 3mm spacers at the corners. The glass was fired at 220ºC/396ºF to 825ºC/1517ºF and initially held for 30 minutes, later extended to 90 minutes.  The thickness stubbornly remained between 5 and 7mm. 

A few other attempts with different times and temperatures gave inconsistent results.  Perhaps the uneven piling of cullet had an influence on the outcomes, but I was still looking for a flow and mixing of colours different to that obtained by melts.

Other experiments were being conducted in parallel, relating to viscosity. These indicated that glass became thinner than 6-7mm at higher temperatures without pressing.  These experiments lead me to think there are four elements controllable by kilnformers in pressing: size, weight, time, temperature.

The same weight of press with the same temperature and time will make small amounts thinner than large amounts, and this is not surprising.  More time with the same temperature, weight, and amount allows some slight decrease in thickness. 

Higher temperatures with the same weight, and time will allow thinner pressings of the same amount of glass.   Viscosity decreases with temperature, so higher temperatures make glass easier to thin.

More weight is required get the same thickness when pressing a greater volume of glass.  Of course, more time and temperature can be added to increase the effect of the weight.

However, the main factor in pressing large amounts of glass is higher temperatures, which results in reducing the viscosity and the resistance to thinning. 

 

Annealing and Cooling

An important aspect of pressing is the annealing requirements.  It is sensible to anneal for a longer time than normal for thick glass, because of the heat retention of the pressing weights. 

This image shows the stress in an 8mm/0.3” (or 5/16”) after annealing as for 16mm/0.63” (5/8”).  There is widespread low level stress with 30mm thick pressing weight.

Indications are that extending the annealing to at least 3 times the target thickness is a minimum annealing soak requirement.  Alternatively, if it is possible to remove some, or all, of the weight from the glass at the beginning of the anneal soak, the annealing time can be reduced.

 

Veiling

The stress picture above shows there is visual element too.  This veiling is most apparent in clear glass, and less obvious in coloured and opalescent glass.  Small volume stacks, which are pressed thin will exhibit less of the veiling.

 

 

Four factors that kilnformers can control in pressing glass to less than 6mm are weight, size, time, and temperature.  The main one is temperature.

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.