Getting the Right Firing Temperature

“what temperature should I use to get a tack fuse that is just less than a contour fuse?”

This is the kind of question that appears on the internet often.  Unfortunately, no one can answer the question accurately, because it depends on some interrelated variables.

Kiln characteristics

Top or side elements, size of kiln, relative size of piece, all have an effect. Also no two kilns even of the same model have exactly the same characteristics.

Ramp Rate 

How quickly or slowly you fire has a big effect on the temperature and soak needed to achieve the desired result. This is the effect of heat work.

Temperature

There are no absolute temperatures for a given effect, given the above two variables.

Soaks

The length of time and the number of soaks will affect the temperature required to achieve your effect.

OK. So, what can I do?

Observation

The only certain way to get the effect you want is to observe.

Set a schedule, guessing the top temperature and length of soak.  Know your controller well enough that you can extend the soak or end the segment by advancing to the next.  Your manual will tell you how to do this.

Peek at intervals from 10-15C below the selected target temperature. Peek at 5min intervals until the effect is achieved.  Advance to the next (cooling) segment.  Record the temperature and length of soak at which the effect was achieved.  On subsequent firings you can experiment with reducing the temperature by 5C – 10C with a 10-minute soak.  Observe and record the temperature and effect as before.

The reason for going for a 10-minute soak rather than longer is to avoid holding at the target temperature for a long time, as that can help induce devitrification.  The reason for a soak at all is to achieve the minimum of marking on the reverse or picking up kiln wash or kiln paper on the back.

If effect is not achieved by the end of the soak, extend it by using the appropriate key or combination of keys.  Keep observing at five-minute intervals until the effect is achieved.  Advance to the next segment and record both the temperature and time.  The objective is to get the heat work done with a 10-minute soak, so you will need to increase the temperature on the next firing.  The amount of increase will depend on the length of soak required to get the desired surface on the previous firing.  The longer the soak, the more temperature you need to add.  You will need to repeat the observations and recording until you find a temperature that will achieve the effect with a 10-minute soak.

Use the lessons from the observations to lower temperature, extend soak, raise temperature, reduce ramp speed, or reduce soak as required.  It will also help you judge on other pieces the approximate temperature and time required for the new layups or new moulds.

Bas Relief Moulds

Bas relief moulds that have an image carved into the surface are popular at the moment. They are most often called texture moulds.  The image is “carved” into the back of the glass, creating uneven thicknesses of glass that refract the light to show the image through the smooth plane of the front.

One of the problems with these kinds of moulds is that lots of bubbles are created, often very large ones.  This results from the many places where the air cannot escape from under the glass during the forming process.

Solutions

There are some strategies that can help avoid these bubbles.

Use the 6mm rule

Fuse the glass into a six-millimetre thickness first.  Two layers of glass give more weight to help the glass conform to the texture of the mould.  It also resists bubble formation more than a single layer.

Use the Low and Slow approach

It more important to have low and long bubble squeezes.  The most successful strategy will have a slow rise in temperature to put as much heat work into the glass as you can before the bubble squeeze.  The bubble squeeze is the most important part of firing these texture moulds.  It will start at about 600°C rising at only about 25°C/hr to around 680°C – that is, taking three to four hours. 

Use slow rates of advance

A third element is to rise slowly toward the forming temperature.  Possibly nothing faster than 75°C.  This enables you to keep the forming temperature much lower than a fast rise will.  The usual temperature recommended is about 780°C.

By using a slow rate of advance you can probably reduce the forming temperature by about 20°C.  You will need to peek at intervals to be sure the glass has taken up the required texture. Again, it is about putting as much heat into the glass at as low a temperature as possible.

Use Long soaks

An alternative to the slow rate of advance is to use a long soak at as low temperature as seems suitable.  You will need to peek at intervals to determine when the texture is achieved.  When the appropriate texture is imparted to the glass, you need to advance to the next segment.  This means that you need to know how to get your controller to skip the following segment.  Or, if the texture is not achieved before the end of the scheduled soak, how to extend the soak time.  If you are using 760°C as you target temperature with a rise of 150°C, you may wish to soak for about an hour or more.  Remember that this is in the devitrification range.

Alternative - Frit

A completely different approach is to use fine frit and powder to give a patè de verre appearance by sintering the frit.  This eliminates the bubble problem entirely.

You will need a lot of frit if you are trying to make a sheet of 6mm from the frit.  You could just take the sheets of glass cut to the size of the mould and smash them up to get the required amount of glass.  Or you can use your cullet, by weighing and smashing up enough glass. 

The calculations for weight are best done in the metric system (in cm) as there are easy conversions between volume and weight.  Assume your mould is 20cm square.  The area is 400cm².  The volume is that times 0.6cm or 240cm³.  The specific gravity of glass is approximately 2.5, so you multiply the volume by that and get 600gms of glass required to get a 6mm thick sheet. 

You could full fuse this into a clear sheet, although this would take a much higher temperature and longer soak that would be good for the mould. Better is to sinter the glass.

To sinter the glass, you need slow rises in temperature and long soaks.  A rise of about 75°C to the softening point of the glass (around 600°C) followed by a very slow rise (ca. 25°C per hour) to about 660°C is needed to allow the small grains of glass to settle together.   At the upper end of the bubble squeeze you need a three- to four-hour soak to sinter the glass. The thicker the layer of glass frit, the longer soak needed to ensure all the particles are heated.  The densest glass will be formed by a 50/50 combination of powder and fine frit.

Much better is to have a much thinner sheet formed from the frit.  This will be about two to three millimetres thick.  The weight of powder and or frit can be determined by the formula above, substituting 0.2 or 0.3 for the thickness.  This frit mixture needs to be evenly spread over the mould, with as much on the high points of the mould as the low ones.

If the mould has a lot of variation in height, you can sinter the frit mixture as a flat sheet first.  Then place it over the texture mould and give it a slow rate of advance to the to the top end of the bubble squeeze and soak for an hour or more, as required.  This will ensure you get the same thickness across the whole piece even though there differences in height.

The resulting piece will be very light and translucent.  It will have a fine granular feel to the touch.  It will have the same shape on both sides of the piece, with the upper surface having a slightly more shiny appearance than the bottom.

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

Stand Alone Online E-Stores

Perhaps none of the existing online marketplaces fit enough of you needs to join them.  You can set up your own and make it your only online store, or you can do it in addition to other ecommerce sites that have some of the features you want.

Advantages

The advantages of your own online store relate to control and adaptability to your design needs.

You retain control of the design, layout, branding, etc., of your site.  This helps maintain your identity or brand and aligns it with your product range.

You have control of when the store is live and when it is updated. You, of course make the rules for what can be listed and how it is displayed.

You don’t have to acquire a lot of knowledge about setting up websites and online stores. Website builders offer templates and store services. You can also use professional website builders to get complete control.

You can link to Etsy or other market places from your own store.  You can funnel the traffic from these sites to own site.

Your own site will enable you to build closer relationships with your buyers. You can communicate directly rather than through intermediaries.

Disadvantages

Nothing comes free of course.  There are some disadvantages to establishing and running your own site.

An especially important element of a store is visits – akin to footfall in real life stores.  You must get people to visit.  You get the visits by making the links with people using a variety of communications.  You need to combine social media with the creation of newsletters, direct mail, blogging, etc.  These relationship building efforts are vital to get people to your website and store.

There are costs relating to hosting fees and one-off fees for the building of the website.  The online stores also charge fees in different ways, so a careful comparison of the best-looking services is important.

There will be additional administration in comparison to an online marketplace.

Questions for E-commerce Site Building

       

What are the facilities for integration of Etsy offerings into your own store? Will separate loading be required?

Is drag and drop site building supported? Is there user support or a user group to support you?

Is an integrated shopping cart available?  What are the order fulfilment assistance options?

Will the site support expanded functions as your business grows?  How adaptable it the site to changes in business?

Is the e-commerce site a market place?  How will exposure of the site to potential buyers be managed?

What is the cost for the features you want?

       

The Balance

You must decide whether the advantages of having your own e-commerce site outweighs the disadvantages in terms of traffic, time spent developing relationships, administration and cost

Websites for Selling Craft

This is not a discussion of which site to choose, but a range of things you need to think about when considering which site to use for selling your craft items.  This includes whether to have your own e-commerce site instead of, or in addition to, a market place site.

Evaluating website offerings requires you to think about a multiplicity of things.  Many of these are listed here, although there may be a few additional things you need to think about for your products.

Recognition/Visits/Traffic

You need to think about the amount of recognition the marketplace has.  Is it the place your potential buyers know about?  Are there a lot of visits to the site?

Is it a market place offering where the website promotes the whole site and the shops within it?  Or is it a site where your own efforts to drive traffic are required? This latter element is like having your own site.

What is the competition within the site? Are there many other sellers of your kind of product? How easy will it be to distinguish your things from others?  Are there mass production sellers on the site?

       

How selective is the site in approving sellers?  This also relates to reputation.

Reputation and Products for sale

Is the site restricted to craft made items? How are mass production manufacturers eliminated?

What range of products are allowed? Is it possible to sell services, and digital products as well as physical goods?

Is the site focused on general products or arts and crafts?  What pricing levels are exhibited on the site?  Is the focus on arts-based items, or does it include bargain basement and cheap deals?

What is the level of security of transaction information offered to you and your customers?  It is vital that the site offers good security for transactions to give customers confidence in buying from the site.

Your identity

Whatever site you join, there will be many other sellers or shops.

Do you get your own shop? Or are all similar products grouped? Is there support or templates to set up your shop?

If you have your own shop what degree of control do you have?  How are images formatted?  What amount of text can be included?  What range of formats are allowed?  Does the site brand dominate, or can you have yours as the prime visual?  What number of themes are available to you?

What level of flexibility in store arrangement and titles do you get?  What number of pages do you get at the various plan levels? How much flexibility and customisation is allowed? What number of items per page are allowed? And what descriptions are allowed either in length or number of terms?

What are the restrictions on the number of products you can sell? Are you allowed discount codes?  Is there inventory control with the site? What is the assistance for order fulfilment?  How much and what features? Is there a system set up for returns? How much support is available?

Are searches restricted to your shop or for all shops on the site?  How are the meta tags used by the site? Are hyperlinks within site only or allowed to outside sites too? Are social media buttons available and with what flexibility?  Can you use your own domain name?

Connections with other e-commerce sites

Are connections allowed?  How easy is the linking? Can you link to multiple sites? Are links to social media – Facebook, etc. – allowed? And how are they managed? Can you link the potential customer to mobile phone sales?

Costs

Of course, there are always payments to be made. You need to look at the various options offered, and the charges involved in them.  If you are new to online selling or have low volume sales, it may be that higher selling fees rather than regular payments with lower transaction fees is better for you at the start.

Listing costs are normally linked to number of items you are offering in your shop. There may be refreshing fees – you must pay a fee to keep the product in the shop after a set period.

There will be continuing fees.  These may be in relation to each item – commission - either as you sell or related to the plan level you choose. Are the commission fees in addition to the listing fees?  Are there additional credit card fees?

Plan level costs are ongoing fees that may be monthly or annual. They are often linked to the length of contract between you and the site provider.  They will give different levels of item fees, and levels of features.  What are the costs of the plan levels? What benefits to they give, and do you need them?  What level of functionality do you get in relation to plan level costs?  How are the plan levels related to the volume or value of sales?

What is the ability to expand and grow through graded plans?  How and when can you move from one plan to another?

Administration

The costs of doing business online may be significant. They may also be related to your familiarity with online offerings.

The creation of an entry should be easy and flexible. You should find it easy to move around the listing form, and it should contain a significant amount of flexibility.  You should be able to make bulk changes.  It should be easy to move items and entries around your shop. 

How much control do you wish to have?  With less experience, you may want to have a lot of the listing, editing of pages, especially contact information done for you, or highly guided.   The kind of support is important. Does the site have a maker support community?

An often-unrecognised level of administration is inventory management. Does the site support that?  If the site does not have inventory control you will have to do it yourself. If you don’t have the stock to satisfy the order, you probably will lose the sale. 

Is the site easy to use?

An important general question is the ease of use for you and for customer. Test the sites out for how easy it is to find and buy an item.  Look at how easy it is for you to use the tools to list your products.

Reviews of e-commerce sites

There are sites that review the offerings of various sites to help you answer some of the questions listed above.  One I have found to be helpful is Ecommerce Guide.

The answers to the relevant questions listed here will assist you toward choosing a website that suits your needs.  It may also lead you toward considering a stand-alone ecommerce site if there are not enough positives in your review of market place sites. It may lead you to consider both.  But the more sites you have the more important it is to be able to link between them and move entries between sites.

A discussion of various things that need consideration on whether to sell on line at all is here.

Orientation of Iridised Glass

Iridised Glass – Which Side Up?

“One rule in using iridised glass is iridised down to fuse, up to slump”

You need to understand the conditions for the “rule" before applying it. Bullseye iridescent and Spectrum System 96 (made by the Uroboros method - not the stained glass) iridescent are not temperature sensitive in the way the other fusing ranges are. Both Bullseye and System 96 can be fired face up.  It is not yet (June 2019) known how Oceanside fusing compatible iridescent will perform when it becomes available.

The Wissmach luminescent and the other fusing glasses seem to be more temperature sensitive and so need to be fused iridised side down.  Avoid using Bullseye Thinfire in contact with the iridescence, as it sometimes reacts with the iridescence and creates pits in the glass.

Use the lowest practical temperature when forming with the luminescent face up. This avoids the partial or complete disappearance of the metal film that creates the iridescence.

More information on what causes the iridescence.

http://glasstips.blogspot.com/2019/06/iridescence.html

Slumping unknown glasses

I had a recent request for help from an old friend who has taken up kiln formed glass. The problem is common enough, that (with her permission) I am adding it to the tips section.

I tried an experiment today to use some of my nice (non-fusing) glass. I cut at 270 mm diameter circle from a 3mm thick sheet and wanted to slump it into my 270 mm bowl mold. I set the mold up carefully and checked it was dead level in all directions and that the glass was absolutely centered on it. I have no idea what the COE is so decided just to use the S96 recommended slumping temperature of 650C. When I checked the kiln no more than 2 minutes after it had reached 665C, the glass had slumped almost to the bottom of the mold but it had slumped very asymmetrically. There was also a small burp on one side which has never been an issue when slumping bowls in this mold before.

The schedule I used was as follows:
200C/hour to 540C, 0 hold
650C/hour to 665C, 10 hold
Then standard S96 anneal programme

Also, the edges were still a bit rough from the cutting, i.e., they hadn‘t fire polished at all. Can you help?

Finding out about the softening characteristics of the glass

Slumping a single layer of glass with unknown characteristics – the CoE is not really relevant – requires that you watch it and other similar ones until you have established a slump temperature for the glass.

There is a way to do it:
Cut a piece of glass 305mm long by 20mm wide. Support it 25mm above the kiln shelf with the posts being 290mm apart. Put kiln furniture on top of the glass where it is supported. Make sure you can see the shelf just under the middle of the suspended glass when you are setting up this test. You can put a piece of wire or other dark element there on the floor of the kiln to help you see when the glass touches down.

Set the kiln to fire at 100C/hour to about 680C. Peek at the suspended glass every 5-10 minutes after 560C to see when the glass begins to move. Then watch more frequently. If your kiln has an alert mode on it, you could set it to ring at each 5C increase in temperature, otherwise use an alarm that has a snooze function to make sure you keep looking. When the glass touches down to the witness sitting on the shelf, record the temperature. This will approximate the slumping temperature in a simple ball curve mould.


Getting smooth edges

You need to have smooth edges before slumping. You can fire polish the piece of glass to get rounded edges, or you can cold work the edges with diamond hand pads, working from the roughest to the finest you have available. If that does not give you the edge you want, you will need to fire polish before you try to slump.

You can do at least two things to find the fire polish temperature. You can do a little experiment by using the cut off pieces of the glass and roughing them up a little before putting in the kiln. Make sure you can see it through the peep hole(s). Set the kiln to fire at about 250C to say 750C. Look in from about 700C to determine when the edges begin to round.

The other is to put a strip of the same glass in with the slump test and set the kiln to go up to 750C rather than just 680C. You can check on progress just as for the separate firing to determine the fire polish temperature. I think about 40C above slump temperature should be enough, but your test will determine that.


Avoiding uneven slumps

Most uneven slumps occur because of too fast a rate of increase in temperature. The piece can hang up on the mould sometimes causing the glass elsewhere to slide down to compensate. The real difficulty in the schedule was the 650C/hour rate up to the top temperature. This was so fast that the glass at the edges would have the opportunity to soften and so hang before the centre was soft enough to begin to bend. 150C / hour would be fast enough from 540C to achieve the slump.  In fact, 150C/hour all the way to the slumping temperature would be fast enough.  The glass reacts well to a steady input of heat rather than rapid rises, even with soaks at intervals on the way up.

Other things can be done too. You mentioned the edges were rough from the cutting. This can cause difficulties of hanging. To avoid that, you should smooth the edges before placing the glass on the mould. A further precaution against uneven slumping is to give a slight bevel to the bottom edge so that it can slip more easily along the mould.

You had already done the leveling of the mould and the centralization of the glass on the mould. These are two other things that can cause uneven slumps.


Avoiding “burps”

The glass slipping a long way down the mould is often accompanied with burps or bubble like up-wellings. These are both indications of too high a temperature being used to slump. I would begin looking at the glass from about 600C in the slumping of any unknown piece of glass. That would apply to any new configuration of the glass or mould too. The fact that the glass slid to the bottom and had a burp means that the temperature was too high and too fast. Once you have established the lowest slumping temperature, by watching to see when it begins, you then can add about 30 minutes soak to that temperature. The length of this soak will have to be determined by observation and experience, though.

A slow heating allows the glass to be at an even temperature throughout its thickness. A rapid rise with a thick piece will sometimes reveal a tear like opening on the underside of the glass that does not come through to the top. This is because the upper surface is sufficiently hot to begin slumping while the bottom is just a little too cool. If there is too great a difference, the glass just breaks all the way through.

Also slow heating allows the slump to be accomplished at a lower temperature, leading to fewer problems and to less texture being taken up from the mould.

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

Sand Beds for Kilns

Sand beds can be used for shaping directly into the sand or as a bed for a large kiln to avoid having to place a number of shelves together and patch cracks. It also saves on the purchase of a large shelf, at the expense of some labour before each firing.

Mix (by weight)The mix is largely sand with powders to coat the sand particles to reduce the amount of sticking to glass. The mix ranges from 75% sand 25% alumina hydrate, up to 67% sand, 33% alumina. The proportions can be adjusted by experience.

Some kiln formers include plaster or china clay (kaolin) in various proportions so the whole mix can range from 70% sand, 25% alumina, 5% plaster/kaolin up to 60% sand, 20% each alumina and plaster/kaolin. Some have found that the plaster started to scum up the glass after a number of firings, and this can happen with china clay too.

UseThe use of sand as the bed of the kiln requires screeding of the sand before each firing. It is possible to smooth the sand with a plaster’s float. This presses the sand down and allows a fine film of powder to coat the surface. A more certain way of avoiding the sand sticking to or texturing the glass is to lay fibre paper over the sand.

You should be aware that a sand bed requires a little different cooling than glass on a suspended shelf does. Effectively you are cooling the glass on the bed of the kiln. Thus, you should use annealing and cooling schedules for one or two thicknesses more than is actually being fired to be really safe.

Sand formingIf you are using the sand for mild shaping, sprinkle alumina hydrate over formed sand to reduce sand particle take up and textures onto the glass.

Renew sand regularly if you are using powders on the sand. An annual renewal using half the old sand mix and the rest new will be sufficient in most cases. The effect of too much powder is to promote large bubbles as air cannot move through the sand with the same ease as when there is less powder. An easy way to tell if you have too much powder is that the mix flows ahead as you screed it.

Revised 22/6/19

Manipulating Slumping Glass in the Kiln

In my view the protection you need at slumping - or combing - temperatures are:

Face and hair protection – a face shield with a coating to reduce the infrared glow is ideal, but expensive.

Good heat protecting gloves – do not cut corners here. You need something more than leather or welders’ gloves.

Heat reflecting sleeves – the aluminised ones work very well.

A reversed denim jacket to protect your chest - you don't want hot buttons!

A helper to hold the lid/door open.

The tools

Wet timbers - at least 40mm in one direction, square is probably the best. They need to be at least 600mm long - a metre is better.

Wet wood handles on any metal tools.

In the case of manipulating glass at slumping temperatures, you need two soaked pieces of timber. If you try to manipulate with one stick you will just push the piece around the kiln. Two sticks allow you to move the glass on opposite sides of the mould toward or away from each other without shoving things around. Glass is amazingly stiff at slumping temperatures. You will wonder how it moves at all when you have to use so much pressure to affect the piece!

Note also that the timbers must be soaked. If they are not they will certainly leave marks on the glass.

When the timbers began to smoke, it is time to close down the kiln - whatever manipulation stage you are at - and let the kiln heat up again while your sticks get wet again.

You need a second person to help with this kind of manipulation in the kiln. They are the door holders, fetch and carriers and anything that needs to be done while you are pushing the glass around the mould. Perhaps after a few trials, you would be able to do it without help, but it is always easier and safer to have help.

Strain Points and Annealing Ranges

I received the following question a while back and thought my response might be useful, although very informal.

“Can you dumb down the concepts of 'annealing point' and 'strain point'? I understand anneal point to be a fixed point (depending on the glass) but the strain point is a range...is this correct? I understand the concept of a hold at the anneal point but I'd like to understand how to bring it down through the strain point.”


I really dislike the idea of dumbing down concepts in kiln forming glass. Glass chemistry is incredibly complicated. Glass physics is still little understood. Glass is a very complicated subject. The marketing of glass for kiln forming has led us all to think it is a simple matter of recipes. Well it's not.

Having got that rant out of my system.... Let’s go ahead.

The annealing point is roughly defined as the temperature at which the glass (if it is the same temperature throughout) will relax most quickly. In the practical kiln forming that we do, it is not possible to ensure that the glass is that temperature throughout. So it is better to think of an annealing soak at the annealing point to allow the glass to become a more even temperature throughout its thickness. As thicker glass means the heat has further to travel from the centre to the surfaces, a longer soak is needed for thicker glass.

The annealing occurs during the slow cool past the lower strain point. The annealing occurs best with a slow, but steady drop in temperature. So annealing is occurring over a range, not at a point. We all rely on a combination of the manufacturers' recommendations, various writings we read, and experience to determine that rate, although Bullseye have published a chart which is most helpful, whichever glass you use.

Strain points.

There is an upper and lower strain point, although this is disputed by some. There are mathematical definitions for these as well as observational definitions. I do not understand the mathematics of either. In lay terms, the lower strain point is that temperature below which no further annealing can take place. It is safe to assume this is 50C below the annealing point (I think it actually is 43C, but I'm not certain of this number).

So it is safest to control the cooling to at least 5C below the lower strain point. Bullseye find that cooling from the annealing soak to 370C is best - this is much more conservative than is theoretically required – 146C below the annealing soak point. This does take care of any problem of thermal shocking of the glass during the cooling.

The upper strain point might be more properly described as a softening point. This also has scientific definitions. The way I think of it is as being the temperature above which no annealing can occur. Another is to think of it as a point beyond which the molecules of the glass are in relatively free motion - which increases with temperature. This again can be considered (on the rise) as 50C above annealing. However on the way down it is safer to consider it to be not more than 30C above annealing. This is because the glass temperature lags behind the air temperature (which is what our controllers measure).

So there is no point in soaking more than 30C above annealing in an attempt to equalise the temperature throughout the glass. However, if you really need to equalise temperature at some point above the annealing point, it might be better to slow the cooling from the working/top temperature and do the final equalisation of temperature at the annealing point.

To answer directly, the strain point by definition of language cannot be a range. There are two points which form the possible annealing range, although the lower one is the critical one. The upper one I described earlier as the softening point. The softening point forms the upper part and the strain point forms the lower part of a range in which the annealing can occur. So the concepts are the opposite of what you propose. They are points which are the boundaries of the annealing range.

To complicate things further, not all glass from one manufacturer has the same annealing point. The published annealing point is a compromise that their experiments and experience have shown to be most suitable. Bullseye glass for example has three annealing points, 532C for opals, 505C for cathedrals and 472C for gold bearing glasses. NOTE: these figures may not be exact; they come from memory rather than documents. Since this list of annealing points was published, Bullseye have conducted further experimentation that shows the best annealing soak occurs at 482C which is below transparent and opalescent, but above gold bearing annealing points.

Schott recommends a range for annealing, not a point, to accommodate these variables. Bullseye, Uroboros, and Spectrum have published annealing points that are practical for people kiln forming in smaller kilns that are less well controlled than the factory lehrs.
If you look at the Bullseye site - education section, you will find a lot of useful information. Especially informative are their tech notes. Spectrum - to a lesser extent than Bullseye - gives helpful information. The information from both sites should be absorbed and the principles applied to other glasses.

Finally, kiln forming is deceptively simple. I have spent 29 years discovering how much more there is to learn. This is one of the reasons that glass is such an exciting medium - people keep discovering new things.

Reviewing the above, I realise that I have not answered your question "... how to bring {the temperature} down through the [lower] strain point". My answer is that you should look at the manufacturer's site for each glass that you use. Look at their rates for annealing for different thicknesses of glass (some also take into account the size). Consider them. Then look at some of the other sites for their published annealing rates for various thicknesses. Comparison of their rates will reveal differences. Think about what they are, how they relate, and whether they reveal that they are using the same principles with slight variations.

Also, if you can, get a copy of Graham Stone's book "Firing schedules for glass, the kiln companion". It provides a handy guide to annealing rates. But DO NOT use it as a book of recipes. Read all the commentary about the schedules, as they (combined with the introductory parts) give principles and tips about how to think about the cooling of the glass.  Bob Leatherbarrow has recently published an excellent book on kilnforming schedules, available from his website.

By the way, experience is so often lost, or misremembered, that keeping a log is essential. My first log consisted of loose leaf binder, so I could file all the same kind of firings for various glasses together (this was in the days when there was not much fusing compatible glass, and I couldn't afford Bullseye at UK prices. I was discovering lots about glass firing and using some schedules that I now wonder how I had any success. I did learn a lot from my failures and recorded them too. Now I use a log, usually an out-of-date A4 size diary, sometimes a manuscript book that is big enough to record observations and illustrations. Bullseye have a good record form on their site.

I congratulate you on your desire to understand the processes. Too many only want to put the glass in and turn the kiln on. That is the desire a number of kiln manufacturers pander to when they put pre-programmed schedules on the controllers. So, don't take any of this as criticism of you or your comments. It is meant in a constructive manner - even though I am told frequently that the manner is blunt, even rude.

Best wishes on continued successful kiln forming.

Revised 22/06/19

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

Applying kiln wash

Applying kiln wash to shelves and moulds have the same requirements.

The kiln wash must be applied evenly

You can use a soft bristled brush like a hake and trail on the kiln wash in four directions – top to bottom, side to side, and diagonally left to right and right to left.

The pigmented kiln wash turns white, indicating that the shelf has been fired

You can also spray the kiln wash onto the shelf or mould. This can provide an even coating, but you must be careful to avoid puddling the kiln wash. Pause for a few seconds after each coat to allow the water to be absorbed before applying the next coat. Apply until the shelf or mould surface is not obvious through the kiln wash. There is no need to dry between coats.  The best coverage is achieved by applying all four coats in different directions at the same time.

You can use a sponge to apply to flat surfaces. A light touch is required. You need to apply in various directions as with the brush.    

Metal moulds are a special case. The water carrying the kiln wash will simply bead up or run off the metal. First clean the metal to get rid of oils and dirt – sandblasting will do well. Then you need to heat the metal to about 125C – 150C (ile., above boiling) and brush or spray the kiln wash onto the mould, one coat at a time. Return the mould to the heat source and then apply another coat of kiln wash. Do this until you have an even layer of kiln wash. Be careful not to put so much liquid on the mould that it begins to run. If this happens, you really need to start again.  Also if the metal is too hot, the water will boil off, leaving bare patched.

Apply the kiln wash thinly

You need enough kiln wash to separate the glass from the carrying surface. Any additional kiln wash will not make for a better separation, but may begin to flake off the surface and adhere to the glass more or less tightly.

The usual recommendation for brushing is one part kiln wash to five parts water. I recommend ten parts water to one part kiln wash if you are spraying the kiln wash. If you have a really absorbent surface, such as a vermiculite mould, you can reduce the water to two and a half parts water to one kiln wash. All these measurements are by volume.

The kiln wash finish must be smooth

There are several ways to smooth the surface.

You can rub your hand over the shelf or mould to remove high spots/streaks. You need to remove the dust before using though.

You can smooth the surface using a rolled up nylon stocking. This relatively open weave allows the powder to be captured in the material. It works well on irregular surfaces like a mould. Again you must clear off any remaining dust.

Another way is applicable to flat surfaces. After applying the kiln wash, but before it has dried, make sure the surface is level. Then brush or spray on a layer of hot water. This both puddles and evaporates quickly, leaving a smooth surface on thinly applied kiln wash. If the kiln wash is thick, the drying process will leave cracks as in a dried-out river bed.

Multiple firings of kiln wash

Iridescence

What is it?       How permanent is it?

“Many special effects can be applied to glass to affect its colour and overall appearance. Iridescent glass … is made by adding metallic compounds to the glass or by spraying the surface with stannous chloride or lead chloride and reheating it in a reducing atmosphere.” 

Older glass can appear iridised because of the light reflection through the layers of weathering.

“Dichroic glass is an iridescent effect in which the glass appears to be different colours, depending on the angle from which it is viewed. This effect is caused by applying very thin layers of colloidal metals (e.g., gold or silver) to the glass.”

Source: https://www.thoughtco.com/

A rainbow iridescent appearance caused by an oil film on water is seen by light being reflected from both the top oil surface and the underlying water surface.  The light reflected from these two surfaces or boundaries have slightly different wave times and so interfere with each other to create this colourful pattern.

This is also observed in soap bubbles.  Here the light is reflecting from both the inner and outer surfaces of the film.

This iridescent appearance is termed thin-film interference.  It is an occurrence in nature where there is a thin film through which light can penetrate and so reflect off the surfaces of the film.  These surfaces are termed boundaries where the light can reflect. 

The thickness of the film can enhance or reduce the iridised effect. 

At a certain thickness the light waves reflected can cancel each other out.  This is described as a destructive interference pattern as it reduces the reflection.  The phenomenon can be used to provide non-reflective surfaces.

At other thicknesses there is an iridised effect.  This is caused by the reinforcement of the recombination of the two light waves reflecting in phase or nearly so.

Control of the thickness can give the silver or the gold iridised appearance, as in the Bullseye iridised glasses, in addition to the rainbow and other effects.

The nature of the light affects the colours of the iridescence.  If the light is daylight or similar it is a combination of many wavelengths.  The different wavelengths reflecting from the “boundaries” or surfaces provide the multiplicity of colour.  If the film has variations in thickness, there will be variations in the colours created.

A diagram from Wikipedia shows how the reflections work at the microscopic level.

https://en.wikipedia.org/wiki/Thin-film_interference

The permanence of the film causing the iridisation appears to be dependent on the metals used and the way in which they are deposited.