CoE as the Determinant of Temperature Characteristics

Many people are under the impression that CoE can tell you a wide number of things about fusing glass. 

What does CoE really mean?

The first thing to note is the meaning of CoE.  Its proper name is the coefficient of linear expansion.  It tells you nothing certain about the expansion in volume, which can be as or more important than the horizontal expansion. 

It is an average determined between 20°C and 300°C.  This is fine for materials that have a crystalline structure. Glass does not.  Glass behaves quite differently at higher temperatures. 

It may have an average expansion of 96 from 20°C-300°C – although there is no information on the variation within that range – but may have an expansion of 500 just above the annealing point. 

The critical temperatures for glass are between the annealing and strain points.  One curious aspect to the expansion of glass is that the rate of expansion decreases around the annealing point.  The amount of this change is variable from one glass composition to another.

The CoE of a manufacturer’s glass is an average of the range which is produced.  Spectrum has stated that their CoE of their fusing compatible glass is a 10 point range.  Bullseye has indicated that their CoE range is up to 5 points. These kind of ranges can be expected in every manufacturer’s compatible glass.

CoE does not tell us anything about viscosity, which has a bigger influence on compatibility than CoE alone. 

Comparison of CoE and Temperature

Among the things people assume CoE determines is the critical temperatures of the strain, annealing and softening points of various glasses.

Unfortunately, CoE does not necessarily tell you fusing or annealing temperatures. 

“CoE 83”

Most float glass is assumed to be around CoE 83.  The characteristics depend on which company is making the glass and where it is being made.

Pilkington float made in the UK has an annealing point of 540°C and a softening point (normally the slump point) of 720°C.

Typical USA float anneals at 548°C and has a softening point of 615°C.

Typical Australian float has a CoE of 84 and anneals in the range 505°C -525°C.

“CoE 90”

Uroboros FX90 has an annealing point of 525°C compared to Bullseye at 482°C, and Wissmach 90 anneal of 510°C. 

Wissmach 90 has a full fuse temperature of 777°C compared to Bullseye's 804 - 816°C.   

There is a float glass with a CoE of 90 that anneals at 540°C and fuses at 835°C.

Bullseye has a slump temperature of 630°C-677°C and Wissmach’s 90 slumps between 649°C and 677°C, slightly higher.

“CoE 93”

Kokomo with an average CoE of 93 has an annealing range of 507°C to 477°C. Kokomo slumps around 565°C

“CoE 94”

Artista with a CoE of 94 has an annealing point of 535°C and a full

fuse of 835°C, almost the same as float with a Coe of 83. 

“CoE96”

Wissmach 96 anneals at 482°C with a full fuse of 777°C and a slump temperature of 688°C.

Spectrum96 and its successor Oceanside Compatible anneals at 510°C and full fuses at 796°C.

Conclusion

In short, CoE does not tell you the temperature characteristics of the glass. These are determined by several factors of which viscosity is the most important. More information can be gained from this post or from your own testing and observation as noted in this post.

Selling online

Credit: 48HoursLogo.com

Once you have achieved a lot of visits to your website, you need to convert the visits to sales. 

The first thing you need to do is consider your products.  Craft products are more difficult to sell online than mass manufactured items that are completely standard and so have known quality.

Is your product suitable for online selling?

·        The general case is that lower priced gift items are easier to sell than expensive ones. 

·        Is delivery expensive relative to the cost of the item? 

·        Do the items have to be sized, e.g., rings?

·        Are the items easy to post safely without breakage?

·        How much packaging will be necessary?

If the answers are that what you sell is expensive to buy or deliver, must made to a size, or are difficult to post, you may have difficulties in generating sales.  If you have items that are likely to sell less well online, consider the other ways you can sell them – trade events, galleries, shops, wholesale.  Also think about making items that are easier to sell online, but still fall within your style.  This approach will help support your more difficult to deliver or more expensive items that won’t sell well online.

Then

Online selling techniques are not so different from in-person selling, except that you have to rely on text and images to do the selling.  This puts more emphasis on words and images and getting your personality into those two things.

The basics are:

Get the viewer’s attention

Stimulate their interest

Develop their desire for the object

Convert these elements into the purchase.

There are many things that can create these three pre-requisites for a purchase.  

Images

The quality of images is extremely important.  Photographs must be sharp, focused, and with lots of light.  They must be taken to show the quality of craftsmanship.  Multiple pictures of the item help to give a better feeling of the object.  They should be taken from various angles, including the unseen backs of items to show the craftsmanship and help promote the assurance of quality. Lifestyle images bring items to life, but have to be carefully arranged.  This is often done best by a professional photographer.

Get and maintain interest

What you do must be apparent immediately.  Do you have recognisable work or style? Is your business name memorable?  An explanation of what you do and why it is unique is important to maintain interest.  Links from these explanations to relevant individual items or product groups are appropriate to keeping people engaged.

It is important to maintain interest after the initial contact.  Make it easy to find other relevant items. Use links, buttons, suggestions, etc., liberally.

Keep the site alive with case studies. These can be the background to your workday, or events in your business life.   Inside views of the development of new lines shows how you progress from idea to finished work.  You have interesting ways of working, that many people are interested in knowing about. Show your working practices, tell them the story of making.

You need to keep in touch with potential and existing clients.  Direct posts to those you have contact details for, with information on developments keeps you and what you do in customers’ minds.  These must not be direct sales pitches.  You can ask questions of these people to keep them engaged. They may also tell others about you and your work.  General posts to targeted audiences can help spread the word too.  Some paid promotion on social media can help, if targeted to the right people.

Provide information

Explain the potential questions about each item that client may have.  Think about the kinds of questions you ask about non-glass craft products. Use those approaches in stating and answering these questions.

Make the explanations personal and consistent with your site and the glass products you are offering.  In many cases, it is desirable to establish a FAQ section, including terms and conditions.  This can help maintain confidence of the buyer in your ability to make and supply the work.

Purchase

What’s for sale

You need to overcome any difficulties that the client might face in coming to the buying decision.  The website should be immediately clear about what you do and for whom.  Price levels need to be clear, possibly by grouping or sorting. Images need to connect with client desires.  This is where lifestyle images are useful.  Do remember that first impressions are all important.

Develop trust

Development of, or appearance of trustworthiness is essential to buying.  People buy from those they know, like and trust.  Development of this is essential for consistent online sales, as well as anywhere else.  This can be promoted by your presence on a group of platforms that you link between. Good descriptions of products and about yourself are important to maintaining the trust of the client.  Testimonials, if you have them, are useful. 

The website must appear professional.  Knowledge of your location is important to developing confidence in your work. Knowing where else your work is available is also important in validating confidence in your business.  Knowledge of where else your work can be purchased gives creditability to your standing within the craft  buying community. This can include your attendance at craft and trade fairs, as well as any awards or press mentions.

Buying and delivery

Make it easy to purchase.  One-click links can help ease the customer into buying. Use of a known payment provider increases confidence that the purchased item will be delivered and that there is a mechanism to get money back if not.  Make sure you explain about postage and packaging, unless you have included it in the in price.  If P&P is included, make sure that you are clear in the text accompanying the image and description.  If you don’t do that, the price comparisons with those that don’t include P&P are skewed against you.  Include plain English terms and conditions, to engender trust if something were to go wrong.

But

Don’t rely exclusively on online sales. There is enormous competition online, even though it is a means to get your work known to a wide range of people Importantly, it is a way to get year-round sales rather than the summer and autumn craft circuit.  Other sources for consistent sales - without you being present all the time - are galleries, shops and wholesalers.

Think about combining online sales with craft fairs and other selling events.  These face to face events give you the opportunity of getting direct feedback on your work, which will help develop what you do.  Promote your attendance at events on the website and tell about your website at events.  Blog about the events before and after their occurrence on your website and social media.  Tell stories from the events on your social media and in the website, too.

Selling online requires getting attention, stimulating interest and promoting desire to buy.  Some of the things you can do are noted.  But do not put all your effort into online.  You can gain a lot of information by attending face to face selling events.

Overlaying Fibre Paper with Thinfire

Some people use a 1mm or 2 mm fibre paper with Thinfire or Papyros laid on top to get a smoother surface. It also allows assurance that the air can migrate from under the glass through the fibre paper. 

However, if the Thinfire or Papyros is laid down too many times, it fills the spaces between the fibres of the paper, and resists the passage of air.  This results in the large bubbles your are trying to avoid.  Experience will show how many times you can layer Thinfire without blocking the passage of air through the fibre paper.

This effect can also happen on sand beds.  The separating powders can build up and fill all the spaces between the sand particles. This resists the movement of air through the sand.  It leads to large bubbles just as compacted Thinfire on fibre paper does.

Is this practice of repeated layering of Thinfire and Papyros sensible?  The tipping point between achieving a smooth surface and creating bubbles cannot be known with certainty.  This means there is always a risk of bubble formation.

It is possible to use a vacuum sweeper with variable suction control  to remove the Thinfire or Papyros residue.  Using the lowest power and holding the end of the hose above the paper can lift the Thinfire or Papyros without affecting the underlying fibre paper.  Then a new sheet of Thinfire can be placed over the fibre paper to maintain the smooth surface.

Repeated layering of thin fibre papers can lead to a compaction that no longer allows the passage of air from under the glass during fusing. 

Needle Points

Often fused glass has prickles or needle points around the edges and especially at corners after firing.

This illustration is from Glass Fusing Made Easy

The nature of glass and its interaction with the separators is the cause.  As you heat glass it expands. Once the cooling starts, the glass contracts. Often a particle of the glass sticks to the separator while the rest continues to contract. This dragging of the glass along the separator results in the creation of little sharp points developing as the glass retreats to its final dimensions.

The best solution I have found to reducing the points at corners is to blunt any points or corners before assembly. Only a tiny amount of glass needs to be removed from the corners to reduce the possibility of these points being developed.

Small needle points can also develop along the sides of the glass too.  These are more difficult to avoid.  The most successful method for me is to use a loose separator.  This can be Thinfire, Papyros or a fine dusting of alumina hydrate or powdered kiln wash.  Although less widely available, talc can be used. Talc is known to be carcinogenetic with high exposure, so breathing protection is needed. All these powders provide enough lubrication to allow the runny glass to slide without sticking. 

Of course, you can use boron nitride, which is very slippery, but the cost of it makes it expensive in comparison to the other methods, including using fine diamond pads to remove the needles.

An additional consideration is the temperature you use.  The higher the temperature, the more the expansion.  Expansion rates are almost exponential above the brittle phase of the glass.  Reducing the temperature by 20C and doubling time or more means the glass does not expand so much and the additional time allows the desired profile to be achieved.  

Of course, paying attention to volume control - using 6mm or more thickness - will help to reduce the needle points.  A 3mm sheet both expands and becomes thicker at the edges by drawing more glass from the interior and the edge while attempting to reach 6mm.  This means there is an increase in the needling effect.  Although a 6mm piece retreats on cooling, it does not have the additional thickening effect of a 3mm piece.  Even a 9mm piece retreats on cooling, although the final piece has a larger area than at the start. 

- - - -

There are various preventive measures that can be taken to avoid needle points on fused glass.  These range from altering the edges of the glass, using fibre papers that turn to powder, using refractory powders, or boron nitride. Post firing solutions relate to cold working.

Vitrigraph Pot Liners

Stainless steel vitrigraph pots are durable replacements for ceramic pots that do not last many -  if more than one - firings. But cleaning is not straight forward. Most recommendations seem to concentrate on cleaning by banging the metal to break the glass away from the sides and bottom.  This seems more brutal and noisier than necessary. It will eventually dent the metal and possibly become unusable.

An unlined ceramic pot

An alternative is to line the sides with 1mm or 2mm fibre paper.  Paper of this thickness has enough fibres that the paper will stick together and not contaminate the pull.  It will still protect the metal from glass sticking At the conclusion of the firing and after the cool down, you can remove the fibre paper and have clean sides.

Instead of placing the glass in a bare pot, you can  line the pot with fibre paper 

 

It is possible to put a piece of fibre paper on bottom with a hole in it to match the pot’s hole.  There is a slight risk of drawing fibres into the pull, although I have not experienced it yet.

This method also works with ceramic pots.

Viscosity of Colours

“I have been advised in the past, that blue fires quicker. I was told this by a Master glass maker.”

Viscosity has some relation to colour and intensity.  But you should note black & stiff black are both of the same intensity, and are fusing compatible, but have different viscosities.  This shows that colour is not the only determinant of viscosity, as the stiff black shows the viscosity can be adjusted within the same colour.  The quotation above indicates that the reasons behind any declarative statements need to be investigated.

Some factors in viscosity

Opalescent colours tend to be more viscous than their transparent counterparts.

It is the metals that develop the colours that produce much of the difference in viscosity.  The same metal can produce different colours in different furnace conditions, so viscosity cannot be assumed to be directly related to colour. 

Some people in the past have done their own tests on viscosity and colour relationships, but I have no access to them.  More recently Bob Leatherbarrow shows (Firing Schedules for Kilnformed Glass, 2018, chapter 7.2.5, p.88) some slumping tests done with opalescent glass. It shows how much less viscous black is than white, and that white is the most viscous.  The other results show red a little less viscous than white, then some greens, yellows and oranges, other greens, purple, pinks (in that order) and of course, the least viscous is black.

Transparent glasses tend to be less viscous than opalescent glasses.

How does this information relate to kilnforming practices?  It indicates that a piece with the less viscous glasses requires lower temperatures or less heat work to complete the forming of the glass than more viscous glasses.

When you have a combination of more and less viscous glasses in a piece you need to fire more slowly to ensure all the glass is thoroughly heated through and will deform equally.  You will need to observe and be prepared to move the piece on the mould to straighten it up.

Do your own viscosity tests

You can do your own tests for viscosity differences by arranging 10mm wide strips all the same length (about 30cm) of different colours. These should be placed on a kiln washed pair of narrow batts set parallel to each other 25cm apart and about 15cm high.  Fire at about 150°C per hour to about 650°C, setting the soak to 30 minutes.  Observe at intervals from 620°C.  Stop the firing when the least viscous has almost touched the floor of the kiln. When fired all together at the same time you can see the relative viscosity of the colours tested.  You can label these and store them, or tack fuse these labelled curves to a piece of base glass for future reference.

Achieving the Striking Colour

"Is there anything special I have to do to fire striker glass?  Can I mix striker and non-striker in the same kiln or piece?"

Strikers generally need a two-hour soak at slumping temperatures, about 660C.  This heat soak helps ensure full development of the colour. If the soak is not long enough, the colour may not achieve the target colour at all, or be paler than anticipated.

The rate of advance to the heat soak is not critical.  But it does need to be the appropriate rate for the thickness and nature of the assembly of glass being fired.

If you were to have too short a heat soak, you can fire again to help mature underdeveloped colours.  This will, of course, change the profile of the finished piece.

Strikers are compatible within their manufacturer’s own range. So, they can be combined in the same piece as any other of the glass in the fusing compatible range.  That means that they can be fired in the same kiln load as non strikers.

The two-hour soak at slumping temperature will not harm the later stages of firing, but it might lead to use of a slightly lower temperature tack fusing than without the long heat soak.  That is because of the heat work put into the glass at the lower temperature.   Only observation will tell you how much less temperature is required.  It may be possible that only a little less time at the forming temperature is required.  Again, only observation will tell you that.

Strikers require a heat soak to mature the final colour.  These striking glasses are compatible with the rest of the fusing range from a single manufacturer. Glass from different manufacturers must be tested for compatibility before combined into a project.

More Information on Citric acid

Vinegar is not recommended for cleaning of glass, and especially not to soak glass in to remove kiln wash or investment materials.

The reasons for avoiding vinegar are that

·        Dilute vinegar - as culinary vinegar is - attacks glass, giving a mild etch to the surface similar to devitrification.  Concentrated vinegar – oddly - does not attack glass as strongly.

·        It is of variable quality – due to uncontrolled strength, various culinary additives, etc.,

·        It has a strong odour, and

·        It takes a long time to work.

Citric Acid

However, there is an acid which works very well to remove investment materials and kiln wash without affecting the glass.  It is the humble citric acid.

Citric acid is a weak organic acid that has the chemical formula C₆H₈O₇. It occurs naturally in citrus fruits, although that is not the best source for cleaning purposes.

More than two million tons of citric acid are manufactured every year. It is used widely as anacidifer, as a flavouring agent and chelating agent. It is the last that is of most interest to kilnformers.

Chemical characteristics

A citrate is a derivative of citric acid. There are many formulations. Two examples are a salt that is named trisodium citrate (also known as sodium citrate); and an ester called triethyl citrate. We are more interested in the first as it is cheap and widely available.

The citrate ion forms complexes with metallic cations.  It forms complexes even with alkali metal cations. This makes citric acid an excellent chelating agent, especially of interest in removing kiln wash and refractory materials from glass.

Chelation 

This is a type of bonding of ions and molecules to metal ions. The agents are usually organic compounds. Chelation is useful in applications such as providing nutritional supplements, in chelation therapy to remove toxic metals from the body, in MRI scanning, in chemical water treatment to assist in the removal of metals, and in fertilisers, among other things. 

Citric Acid as a Cleaning and chelating agent

Citric acid is an excellent chelating agent, binding metals by making them soluble. Among many cleaning uses are:

    to remove and discourage the build-up of lime scale, from boilers and evaporators. 

    to treat water by chelating the metals in hard water, cleaners produce foam and work better without need for water softening. Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. 

    A solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing. 

    Citric acid can be used in shampoo to wash out wax and colouring from the hair. 

    In industry, it is used to dissolve rust from steel and to form a coating on stainless steels to resist corrosion.

Its use in kiln forming is to make use of the chelation properties when dealing with kiln wash and investment material residues.  Aluminium hydrate is the main ingredient of all kiln washes.  When it becomes bound to glass, it is impervious to almost all chemicals.  The chelating property of citric acid enables the bond between the glass and the kiln wash to be broken by incorporating the molecules within its own, making a colloidal solution.  This process is approximately 6 times faster than any vinegar solution and without the odour and etching risks.

A sample of the affected glass followed by 4 hours in citric acid and 24 hours in vinegar.
Credit: Christopher Jeffree

A 5% solution made up with 50gm of granular citric acid in 1 litre of water is all the strength that is required. The affected glass can be soaked in this solution for the time required to complete the chelation without the risk of etching, and without needing ventilation to remove smells.  Unless you are using a lot of cleaner, it is better to make up much smaller amounts as mould can grow on this organic solution.

A 5% solution made up of 50gms citric acid in 500ml of water and 500ml of isopropyl alcohol makes an inexpensive and effective glass cleaner. However, if left for a length of time, it becomes sticky.  Apply the solution, scrub the glass and immediately wipe off the solution.  Then polish the glass dry. The alcohol in the solution makes keeping large quantites possible. 

This post was compiled with the assistance of Wikipedia, Christopher Jeffree and my own experience.

Although this post remains valid, there is another chemical for long soaks to remove mould material or kilnwash.

Citric Acid Cleanser

Christopher Jeffree has kindly outlined the reasons for the effectiveness of citric acid as a cleaner for removing refractory mould residue and acting on kiln wash stuck to glass.  This is his work (with a few personal notes removed).

"Citric acid works well for removing the plaster scale that builds up in vessels used to mix plaster, and it helps to remove traces of investment plaster and kiln wash from glass.  Its metal-chelating properties probably help with dissolution of calcium deposits, but I am less clear why it is so good at removing kiln wash.  The constituents of kiln wash are kaolin and alumina hydrate, neither of which I would expect to be soluble in dilute acids.  Equally, the refractory materials in investment formulae I would expect to be insoluble.  However, kaolin forms layered structures in which flakes, molecular layers, of alumina hydrate and silica interact through hydrogen bonding. It is possible (I am guessing here) that citric acid can disrupt those hydrogen bonds, thereby disaggregating the clay.  All we can say is that empirically, it works.

"I prefer to use citric acid partly because it has a defined composition, but also because it is safe and pleasant to handle – no odour, and comes in the form of easily-dissolved dry crystals like granulated sugar.  Vinegar stinks, and glacial acetic acid is  an aggressive flammable, corrosive liquid with a chokingly acrid smell.

"Calcium sulfate has low solubility, but is not completely insoluble in water - gypsum (calcium sulfate dihydrate) has a solubility of about 2.5g per litre (0.25%)  from 30-100 C. Its solubility is retrograde, meaning that it decreases, rather than increasing, with temperature.  Natural gypsum is an evaporite, a type of rock that often forms by evaporation of lake water in a geological basin with little or no outflow. It can also be produced hydrothermally in hot springs, when water containing sulfuric acid passes through limestone.  

"Calcium citrate is not very soluble either, only in the order of about 0.85g per litre, but the important thing from our point of view is not to get the material into solution but to separate its crystals and make it detach from the glass.

"In other contexts, warm citric acid is used by jewellers and silversmiths as a pickle for dissolving copper oxide (firestain) from silver and gold alloys  after heating / soldering.  It is a safer alternative to the traditional jeweller's pickle of 10% H₂SO₄.

"Citric acid also dissolves rust from iron, without much etching the iron itself, so is good for cleaning rust off tools etc.

"These pictures show a plaster mixing bowl with (presumably) CaSO₄-rich deposit on the surface, cleaned by soaking with 5% citric acid for 4 hours,

and flash from the pate de verre castings with tightly adhering kiln wash, cleaned using 5% citric acid soaked for 4 hours, and vinegar (white wine) soaked for 24 hours.

"I'm not sure about reaction products - I was speculating a lot there, running through hypotheses that I can't support. We don't really have data on the composition of the layers that are stuck to the glass, or a clear idea of why they sometimes stick and sometimes don't (e.g. the differences between transparent and opal glasses in this respect). Maybe this would be a topic to discuss with technical people at Bullseye."

Hope this helps
Best wishes

Chris Jeffree

Subsequent to this work Christopher has done more work and found that Tri-sodium citrate is an even better chemical for cleaning glass of kiln wash and mould material.

Placing of glues

The placing of glues to hold the glass pieces temporarily is important.  Often unsightly black marks appear due to inappropriately placed glues.  Bubbles can form between layers  and even appear to come from underneath the glass for the same reason.  Placing is often more important than the amount of glue used.  Still, the amount used should be the minimum to hold the glass from moving from bench to kiln.

Place glues at edges of the pieces to be secured during movement.  This allows the burn-off of the glue to evaporate without being trapped under the glass.  If you use very runny or diluted glues, the capillary action will draw the required amount of glue under the glass piece to form a secure adhesion.

Glues burn off and leave the glass pieces unsecured long before the glass becomes tacky enough to stick together.  This means that if your stack of glass will not stay in place without glue as you build it, the glass will collapse or move in the kiln.  Glues are only suitable to stabilise the glass pieces while moving to the kiln.

Two recommended glues that burn off cleanly are the Bullseye Glasstac (more fluid) and the Glasstac gel (more viscous)

Bubbles on Drop-out Rims

Sometimes people doing dropouts get bubbles or unevenness on the rims of their pieces.  This means that it is not suitable to leave the rim on the piece.  Most times, this does not matter, as you intend to cut the rim away. But if you do want to have a rim these uneven surfaces are unsightly and not suitable for high quality pieces.

One person has indicated that they used a schedule of 250°C per hour to 520°C for a 30-minute soak and then proceeded at 330°C to the top temperature of 710°C.  This is probably too fast a heat up at the second segment.  Slower rates of advance are advisable.

One of the advantageous methods of scheduling for dropouts is to put the heat into the glass steadily.  I suggest there are two problems with the rates of advance and soaks in the above (partial) schedule.

The soak at 520°C would be more useful if it were at around 600°C.  This would allow the heat to be distributed throughout the glass before it begins to droop significantly.

The rapid advance of 330°C is much faster than needed, or desirable.  This rapid rate of advance allows the glass to move into the aperture, before the rim is plastic enough to stay on the supporting ring.

These rough drawings show how the rim initially rises from the ring, pivoting on the edge of the aperture.  This happens on all moulds (drops or slumps) where there is a rim.

With a rapid rise in temperature the raised rim edge gets more heat than the depressed middle, as it is closer to the elements.  This additional heat allows the edge of the rim to curve downwards forming air pockets as the edge touches back to the supporting ring.

Some people use fibre paper between the ring and the glass to prevent bubbles. This addition allows a passage of air from under the glass and reduces bubble formation.

Others have developed sloped drop out rings that eliminate the rising of the glass from a flat ring.  The glass is suspended above the aperture and only touches the edge of it as the glass softens. These crude drawings show the process.

To be certain of avoiding air bubbles under the rim of dropouts whatever the style of ring, you should use moderate rates of advance, with a possible soak at around 600°C which is before the glass begins any significant movement. The moderate rate of heating should be continued after this soak, rather than increased.