Sharp points on rectangles

At the conclusion of firing pieces with right angles or sharper shapes you often find very sharp needle points at the corners.

This is a result of the expansion of the glass as it heats up.  At top temperature, the glass piece is larger on the shelf than when you put it in cold.  The amount of this expansion is related to the thickness of the piece and the temperature it has been fired at.

As the glass cools, it contracts.  The contraction at corners and points has greater effects on the glass than at the sides.  The corners are contracting from two sides rather than only one.  This makes them a little more resistant to contract and often leaves a little of the glass stuck at the furthermost point of expansion as it contracts.

I have found the best prevention of sharp points on the corners of rectangular pieces, and those with even sharper angles, is to nip off the tiniest bit of the corners. This very slight blunting of the corners allows the glass to expand and then retract without the corner or point catching on the separator and so creating the sharp needles.

Further information is available in the e-book: Low Temperature Kilnforming.

Soaks Below the Softening Point

There are frequent suggestions that holds in the rise of temperature for glass are required.  Various justifications are given.  A few notes before getting to the explanation of why they are uncessary.

A note is required about the softening point sometimes called the upper strain point. There is a reasonable amount of discussion about the lower strain point.  So much that it is often simply referred to as the strain point.    Below the lower strain point, the glass becomes so stiff and brittle that no further annealing can occur.  Thermal shock can happen though, so the cooling needs to be controlled.

There also is an upper point at which the behaviour of the glass is different.  Above this temperature, no annealing can occur either, because the glass has become plastic and the molecules randomly arranged.  It is only just pliable, of course, but its molecules are no longer strongly bound to one another.  This is the temperature at which much of slumping is done.

It is disputed whether such a point exists.  Still, in practical terms it is where the glass becomes so plastic that it cannot be temperature shocked.  The temperature of this “point” is approximately 45°C above the annealing point, rather than the temperature equalisation soak. 

Note that the temperature at which Bullseye recommends that the annealing soak should occur is a temperature equalisation point, which is about 33°C below the glass transition temperature - the point at which glass can be most quickly annealed.  The average glass transition point for Bullseye is 516°C.  Most other fusing glasses use the glass transition (Tg) point as the annealing temperature for the soak.  They or you could employ the Bullseye technique on thicker slabs of the glass by setting the temperature equalisation point 33°C below the annealing point, and soaking for the same kinds of time used in the Bullseye chart for annealing thick slabs.  In fact, this is what Wissmach has recently done with its W90 and W96 fusing glass ranges.  They now recommend 482C (900F) as the anneal soak temperature.

Now to the point of the post.

The soaks that are often put into schedules on the rise in temperature are justified as allowing the glass to equalise in temperature.  Glass in its brittle phase is an excellent insulator.  This means that heat does not travel quickly through the glass.  Consequently glass behaves best with steady and even rises in temperature (and correspondingly on the reduction in temperature).  Rapid rates risk breaking the glass on the temperature rise, no matter how many or how long the holds are.  

This means a slower rate of advance will accomplish the heating of the glass in the same amount of time, and in a safer manner, than rapid rises with short soaks/dwells/holds.  The slower rate of temperature increase allows the glass to absorb and distribute the heat more evenly.  This slow heating is most obviously required in tack fusing where there are different thicknesses of glass.  

This means that it is possible for thin areas of glass to heat up much more quickly than glass covered by different thicknesses of glass.  It also applies to strongly contrasting colours such as black and white, because they absorb the heat differently - black more quickly than white.

There are, of course, circumstances where soaks at intervals are required – usually because of mould characteristics, in slumping, and in pate de verre.

Sometimes people add a soak at the annealing temperature on the way up in their schedules.  This is unnecessary.  If the glass has survived up to this point without breaking, it is highly unlikely it will break with a further increase in the rate of advance unless it is very fast.  The temperature after all, is above the strain point meaning the glass is no longer in the brittle phase.

Many people add a soak at around 540°C (ca. 1000°F) into their schedule on the increase in temperature, before their rapid rate of advance to the top temperature.  The choice of this temperature relates to the lower strain point.  This also is unnecessary, except possibly for very thick pieces. By this time the glass has reached its plastic stage and if it hasn’t broken by then, it won’t with a rapid rise in temperature either.

Further information is available in the ebook Low Temperature Kiln Forming.

Soaks at various temperatures during the advance to the upper strain points of glass are not necessary.  What is necessary is a knowledge of when the glass becomes plastic in its behaviour, and an understanding of how soaks can overcome characteristics of moulds, or how to achieve specific results and appearances of the glass.

Effects of Dams on Scheduling

 I recently made a statement about the effects of various dam materials on the scheduling.  This was based on my understanding of the density of three common refractory materials used in kilnforming – ceramic shelves, vermiculite board and fibre board.  I decided to test these statements.  I found I was wrong.

I set up a test of the heat gain and loss of the three materials.  This was done without any glass involved to eliminate the influence of the glass on the behaviour of the dams.  The dam materials were laid on the kiln shelf with thermocouples between.  These were connected to a data logger to record the temperatures.

 

The schedule used was a slightly modified one for 6mm:

300°C/hr to 800°C for 10 minutes

Full to 482°C for 60 minutes

83°C to 427, no soak

150°C to 370°C, no soak

400°C to 100°C, end

 

The data retrieved from the data recording is shown by the following graphs.

 

Highlights:

·        The dam materials all perform similarly. 

·        This graph shows the dams have significant differences from the air temperature – up to 190°C – during the first ramp of 300°C/hr. (in this case). 

·        There is the curious fall in the dams’ temperatures during the anneal soak.  This was replicated in additional tests.  I do not currently know the reasons for this.

·        The dams remain cooler than the air temperature until midway during the second cool when (in this kiln) the natural cooling rate takes over.

·        From the second cool to the finish, the dams remain hotter than the air temperature.

 

Some more information is given by looking at the temperature differentials (ΔT) between the materials and the air.  This graph is to assist in investigating how significantly different the materials are. 

This graph is initially confusing as positive numbers indicate the temperature is cooler than the material being compared and hotter with negative numbers.

 

As an assistance to relating the ΔT to the air temperature some relevant data points are given.  The data points relate to the numbers running along the bottom of the graph.

Data Point   Event

    1                Start of anneal soak.

    30              Start of 1^st cool (482°C)

    45              Start of 2^nd cool (427°C)

    65              Start of final cool (370°C)

    89              1^st 55°C of final cool (315°C)

    306             100°C

 

At the data points:

·        At the start of anneal soak the ΔT between the dams is 16°C with the ceramic shelf temperature being 18°C hotter than the air.

·        At the end of the anneal soak of an hour, the air temperature is 20°C higher, although the ΔT between the dams has reduced to 12°C.

·        At the end of the 1^st cool the ΔT between the dams has reduced to 9°C and the ΔT with the air is 3°C.

·        At approximately 450°C the air temperature becomes less than the dams. 

·        At 370°C the hottest dams are approximately 17°C hotter than the air.  The ΔT between the dams is 10°C.

 

More generally:

·        The air temperature tends to be between 17°C hotter and 17°C cooler than the ceramic dams during the anneal soak and cool.  The difference gradually decreases to around 8°C at about 120°C.

·        Ceramic and fibre dams loose heat after annealing at similar rates – generally having a ΔT between 4°C and 1°C, with a peak difference of 9°C at the start of the second cool. This means the heat retention characteristics of ceramic strips and fibre board are very close.

·        Between the annealing soak and about 300°C the vermiculite is between 12°C and 9°C hotter than the same thickness of fibre.  Vermiculite both gains and loses heat more slowly than the ceramic or fibre dams do.  This means that vermiculite is the most heat retentive of the three materials.

Conclusions

·        Dams will have little effect during the heat up of open face dammed glass.  The slight difference will be at the interface of the glass and the dams where there will be a slight cooling effect on the glass.  Therefore, a slightly longer top soak or a slightly higher top temperature may be useful.

·        The continued fall in the dams’ temperature during the anneal soak indicates that this soak should be extended to ensure heat is not being drained from the glass by the dams to give unequal temperatures across the glass with the risk of inadequate annealing.  I suggest the soak should be extended to that for glass of 6mm thicker than actual to account for this.

·        The ability of ceramic and fibre dams to absorb and dissipate heat more quickly indicates that they are better materials for dams than vermiculite board.  The slightly better retention of heat at the annealing soak, indicates that ceramic is a good choice when annealing is critical.

Scheduling Effects 

Based on these observations, I have come to some conclusions about the effect of dams on scheduling.

·        There is no significant effect caused by dams during the heat up, so scheduling of the heat up can be as for the thickness of the glass.

·        The lag in temperature rise by the dams indicates a slightly longer soak at the top temperature (with a minor risk of devitrification), or a higher temperature of, say 10°C can be used.

·        The (strange) continued cooling of the dams during the annealing soak indicates that extending the soak time to that for a piece 6mm thicker than actual is advisable.

·        The cool rates can continue to be as for the actual thickness, as the dam temperatures follow the air temperature with little deviation below the end of the first cool. 

·        Ceramic dams perform the best of the three tested materials.

 

Smooth Surfaces on Drop Vessels

It is widely recognised that the usual results of kiln forming are one textured side and a smooth upper side. The common methods of having upper and lower surfaces both smooth is to blow the glass, avoid allowing the glass to touch the mould, and cold working the textured side to smooth.

The question arises about the possibility of getting smooth surfaces on the inside and outside of a drop vessel.  As the glass in a drop only touches the mould at the collar and edge, shouldn’t the glass be smooth on both sides?  The answer to that is in the temperatures and time used.

The temperatures used in a drop are not high enough to be certain of smoothing the outer surface.  But the soak times at drop temperatures are enough to create a fire polish on the upper/inside surface.  This indicates the blank in a drop should be placed with the texture up, facing the heating elements.  The smoother side facing the floor will be stretched and will remain smooth. 

The smoothing effect of firing with rough side up does depend a little on the depth of the drop.  Shallow drops will not have the same heat exposure that deeper drops do, assuming that a moderate heat is being used over three to four hours.

This implies that the design to show on the inside of the drop should be in contact with the separator when fusing the blank.

Firing Records

Bullseye Glass Company

To develop your fused glass practice, you need to record lots of information about your firings.  This tells you what has gone well and not so well.  It hones your expectations about how you should be preparing, scheduling, and analysing your experiences.  It becomes your detailed memory bank of results and gives directions for the future.  This should be done whether fired in your own kiln or someone else’s.

Categories of information for the record
There is quite a bit of information that needs to be included in such a record.  This is my view of what needs to be included  in your logbook for future reference.

Date
Record the date of the firing as that will give you historical information on similar projects.  It can show you what you have changed over time and the variations you have introduced.

Glass used
This is not only the type of glass (Bullseye, Float, Oceanside, Wissmach, Youghiogheny, etc), but the colours used.  This should include the manufacturer’s code numbers to enable you to replicate the glass used.

Lay up
This can be a description, a drawing or pictures of the set-up of the piece prior to firing.  This is vital to later understanding what you did in this firing.  Record any glues or stabilising elements you use. Any frits or powders used should be recorded. The placement in the kiln is important - centred, one corner or another, level/ height in kiln, etc., can affect the results.  You can make a sketch or take a photo to attach to the record rather than writing separate descriptions. How it comes out is recorded later.

Dimensions
The dimensions (h x w x d) including any variations in height are needed to compare with other projects.  This might be included in the lay-up diagrams or pictures, but it is most useful to have the dimensions and their variations recorded as numbers too.  You might think in terms of layers, but remember to record the thickness of each layer/piece (e.g., 2mm, 3mm, 4mm, 6mm, etc)

Kiln used
This is especially important if the kiln is not yours. Every kiln has variations and it is important to compensate for that in scheduling and placing of the piece in the kiln.

Process
This is essential in gaining an understanding for planning any modifications.  The process can be described by standard terms - e.g.,  sinter, slump, tack, contour, full fuse, casting, melt – or by your own terminology (if it is consistent).

Description
A statement of your project and aims is very useful for the future.  It is a reference point to use in comparing what you wanted with the results of the firing.

Support system
This includes essential information affecting the firing – shelf type (e.g., fibre, mullite, ceramic tile), mould type (e.g., ceramic, fibre, steel), and a description or sketch including any reference codes.

Kiln furniture. The kind and quantity of kiln furniture (dams, stilts, posts, etc) can affect the firing results, so need to be recorded.

Separators
This includes kiln wash (type, whether new or the number of uses), fibre paper type and amount, mould coatings, and anything else you may use to keep the glass from sticking.

Schedule
This is the thing most everyone remembers to record.  You need to record it each time you use it – even if you have used it many times before.  You need to record each step of the program.

So many times, people report that “it [the schedule] has always worked before”, only to discover that some element had been intentionally or accidentally altered from past firings.  I normally write the schedule in a logbook and then enter it into the programmer. I use the written record to check against what I have entered into the controller.  Then I know I have programmed what I intended.  I can also check on earlier, similar firings to see the variations I have used in the past.

Results
Drawings or pictures of the finished item are essential.  A description of the results is also needed as a picture does not tell the whole story.

Comments on results
You should also give a commentary on the results of the firing.  This should include successes as well as disappointments.  Thoughts for future similar firings should be written down.  They will be forgotten soon, if you don’t.

How to keep all this information
As you can see there are many elements that need to be recorded as they each can affect a firing. I see these as a minimum, and you will add elements important to you for this list.

It does not matter much in what form you keep the information.  It can be a ledger, spreadsheet, database or your phone or tablet that you carry with you always.  There are several apps for recording the kiln firings that can be used.  What is important is that you can record the information immediately, or as you prepare the work for the kiln, into the chosen form of recording.  I use a logbook and convert that in my leisure moments to a spreadsheet (usually at new years day).  This allows me to compare information over time and especially the kinds of firings that I rarely do.  It also allows me to search by various processes.

It is important that you back up any electronically held information to the cloud or other device to protect against loss or corruption. 

Forms
It is useful to have a form for compiling this record.  A number of elements of the records can be reduced to tick boxes to ease the recording.  It helps to remind you of the information you need to log for each firing.  Bullseye have an excellent form that you can use or adapt to your needs. There are a few apps that can be used on phones or tablets which are useful for those who record everything on their phone.  Remember to back it all up to the cloud for preservation in case of loss or damage.