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 when 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 and it needs to be 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 (i.e., 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 patches.

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.

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.

Why Sell Online?

"Why not? Everyone else is doing it."

The more interesting question and response is:

“Why do it at all? There are lots of other ways to sell.”

The first thing you need to help 

decide whether online is for you, is a plan for selling.  You need to think about how you sell. Selling needs to part of whole strategy that can involve multiple ways and avenues of selling. The ways you sell your goods needs to be integrated into social media - whatever platforms you decide to use to sell.  One platform will not be enough.  You need to look at where the main effort will have most return.

Planning a selling strategy

Initially:

Even before you have a whole selling strategy worked out, you need to be using social media to promote your work.  A blog about you and your work is a good place to start.  It gives you a presence, a personality, a brand. Write regularly about your craft life.  Share it with your friends on Facebook, Twitter, etc., and ask them to share it with their friends to get it known.

Linking to and from that blog using Facebook, Pinterest, Twitter, and other social media platforms help to spread the knowledge of who you are and what you do. These are the things you need to do whether you decide to join an online marketplace, a stand-alone shop, or stay with physical sales points.

Making an online selling plan:

The first part of the planning is to decide why you are going to sell online, rather than stick with just bricks and mortar sales locations. These physical locations include galleries, museum shops, gift shops, craft fairs and exhibitions, etc. Some questions to ask yourself are:

• What do you gain by interacting with shop owners and direct customers? 
• What might you lose by going online?
• How does online fit your products?
• Will you need to put in more, or less, effort online rather than selling in physical locations – shops, craft fairs, etc.?
• Where is your market located? Local? Dispersed?
• Is your market a specialised or a general one?
• Will your products stand up to delivery company handling?

The answers to these questions will help you decide if online will help sell your products.  It may of course throw up ambiguous answers – on one hand, on the other hand kind of answers.  This would indicate a mixed platform approach is desirable.

       

But, just a minute!

Don’t sign up to an online marketplace just yet.  There are some things you need to prepare before searching out the appropriate site or sites for your work.

Products:

Yes, you have a lot of different things you could sell. Think about where you want to concentrate your efforts.  A few key products will help to give your work an identity.  It will help you define, and so reach your customers.  It will focus your marketing and build your experience and reputation. 

Online enables you to concentrate on niche marketing more successfully than bricks and mortar can.  It is one of the main advantages of online selling. Once you have identified your products and therefore your market, communicate with them.  Think about their interests, needs and desires as you write.

Work on describing a strong identity – often called a brand. This should be accomplished in a short sentence. Know who and what you are. Tell your values, your story to the world. But especially to your potential customers.  Follow this description up and re-enforce it with photos and descriptions in all areas of social media. This will then be a style to carry over to the online shop, if you decide to set one up. And even if you don’t it will improve your selling at physical locations.

Prepare product titles and descriptions:

Clear titles and descriptions are needed for each item. These should be direct using simple language making it possible for the viewer to understand immediately what is in front of them. These titles and descriptions should include specific words - terms, tags, keywords - that the prospective buyers might use to search for your products.  You should include lots of tags for the photos and descriptions, but they must be relevant.

Include all the detailed information – size, colour, materials, price, delivery mechanisms and costs, projected arrival time, etc. – in the description, but toward the end when they have probably already almost made up their mind. 

Excellent photos are required.

The main descriptive element when the buyer gets to your page or listing is the pictures.  They must be descriptive of all the elements of the piece – several images per item is a good idea.  Consider a professional photographer to make these images.  They have an independent eye and can see things from the buyers’ viewpoint.  In addition, they can take better pictures.

Share these descriptions and images on social media.  This all needs be done before you join a marketplace website.  It helps to test your abilities to present your products successfully before you commit to online.  It will help you in your presentations of your work where ever you place it.

Costing and pricing:

Of course, there is the boring bit – deciding on the price.

These include factual elements:

Costs of production - materials and time, overheads, administration of the materials buying and the object’s selling processes.

Applying this cost information to each item will give you your break-even price.

You then need to consider what profit you want to make for your effort. This might be done as a margin on each piece, or as a part of your desired annual income.

The break-even plus the income or profit factor will give you the wholesale price.  The retail price is at least twice the wholesale.

Will the market bear that price?  Compare similar products and test your pricing with potential customers. If your price is too high, return to the design of the item and consider how it could be simplified. Alternatively, consider how it could be enhanced to achieve an even higher price for little more effort.

If your product is the kind where purchase of multiples is possible or likely, give yourself enough margin so that you can offer discounts for quantity purchases.

Also think about whether yours is a luxury product. If it is, the price needs to be higher than for general work.  E.g., if your jewellery is set in silver, it needs an extra margin for the luxury element of the item.

Cost of delivery – packaging, time, delivery charges. You need to decide if you are you including this in the price, or is it separate?  This will need to be reflected in the price, or the cost of delivery must be stated in the description.

Benefits

This process of preparing for online selling will benefit your selling, even if you decide against online selling.  It will clarify your offering to the potential customers.  It will help to define you as a producer and develop the awareness of your products. It will assist the customer to understand your story and your work, so encouraging them to buy.

A discussion of choices about choosing a website is here.

Preparing for a Craft Fair

“What should I take to the craft fair?” 

This is a frequently asked question.  The question gets a variety of answers from the materials to make the stand out of, the variety of supplies, to taking a variety of things to the show with different price points.

I’m sorry to say that the last piece of advice leads to set ups that look more like a jumble sale rather than a display of finely crafted works.  These stalls have a lot of closely arranged pieces to fill the space.  They have a lot of price points.  They have a wide choice of colours in each range.  What is the potential buyer to make of so much choice?

It is important to have an identity on display.  Are you a maker of votives, table ware, suncatchers, garden objects, panels, recycled and repurposed glass, etc.?  If they all are displayed, it is difficult for the viewer to determine what it is that you do.

This identity can be a unified colour range throughout the items you are selling.  It could be a range of bowls, platters and other table wear with a consistency of colour or design.  It could be your concentration on making objects from recycled glass. Essentially, you are telling the viewer what you are about.  It could be from “I make jewellery” to “I make glass objects in a range of blue green” (not that I think the latter is a real identity, but it is a possibility).  This “identity” can change from venue to venue, of course.

The identity can change to suit the venue, of course.  Selling at a garden festival will lead to a different display to a craft fair for the general public, or one at a school.

The usual way shops and stores distinguish their important items, is to give them display space.  This is vital for craft shows too. Space around items allows each item to have its own significance.  It is possible to give this space by concentrating on just a few lines of work.  You can always have spare stock under the table or out back to fill the gaps caused by purchases.

You may decide to go for the jumble sale approach. One of the reasons might be the people coming will be looking for lower priced objects. (Should you be selling your pieces at bargain prices?). You may be unsure of the nature of the visitors to the show and want to appeal to a wide range of people. You still need to give each piece its space.  You may have a range in multiple sizes and colour ways.  Display only one product in the full range of sizes and only a single item in each of the colour ways.  Your “under the counter” stock will fill in the sizes for the colour range.  In this kind of way, you can make space for each item even when you bring a multiplicity of objects.

So many times, I see little evidence of descriptions relating to the object or range of products.  You need to illustrate the thought you have put into the creations by the descriptions you give to the pieces.  Simply putting the pieces out in a nice arrangement is not enough.  You need to tell people what it is, the concept that led to its development, possible uses, how it might fit into a life style and its benefits – unique art piece, display of the owner’s ability to choose beautiful objects, etc.  And of course, you need to give the price.  You might also want to make it possible for a buyer to have it shipped, so you need information about delivery costs as well.  In giving this kind of description, you are showing you have put thought into the piece and into the buyer’s needs too.

It is very unusual to see anything about the maker.  Unless people are looking for bargain basement items, they want to know about the maker.  They are buying your story almost as much as the object. You have a story relating to your life and your approach to making.  Tell it. Display it prominently.

Where else can your products be found?  People want to be assured that they are buying things in demand. Telling where else your creations are sold helps assure them that their choice is validated by other people’s interest (even though they are buying a unique piece).  Put this information in a few sentences and put it up somewhere on your stand.

Telling people about where you can be found in the real world, on social media, your blog, and any online marketplaces is important.  Often people can turn into repeat buyers through maintaining contact. At the least it can promote continuing relationships, which may turn into commissions.

Research the market. You need to look at the profile of people that will be attending the event.  If it is a gardening event, you might take garden-based objects, or set your pieces in a garden-like atmosphere. If it is a school event, think about the background of the parents.  If it is a stand-alone event, then you need to think about the population of the area, or the people that the event will draw.  The organisers in all these cases should be able to give you information on the people who are likely to come. 

Take things that will appeal to the people who will be browsing and arrange them appropriately to the event as well as getting the display right.  This might involve different table coverings, or backgrounds to reflect the ethos of the event to which the craft fair is attached.  You need to make a connection with the potential buyers.  Show that you are tuned into their lifestyle.

Preparing for a craft selling event is much more than what structure to hang or display your work on.  It is about reflecting the environment of the event. Will the people be browsers that need to be converted to buyers, or are they going to be coming to buy? Those two groups have different display requirements. What your identity will be in relation to these buyers will determine the range of material you bring to the show. 

Summary

A craft fair is very similar to a shop display or your offering on an online craft marketplace. 

• Your display needs to immediately say what you do. This is assisted by the walls, coverings, etc. of the stand, but is most importantly shown by the selection of objects to show.  This is what tells people what you do.  It is your identity for the time being. 
•  
• You need to lovingly describe each object on display. This gives the buyer a feeling for the concept behind the item, an idea of the benefits and value of it to their life.
•  
• You need to tell your story. They are buying an idea of the maker as well as the object.  It is a good idea to tell people where else your work can be found too – even if it is at other craft fairs just now – but especially if you are on any online marketplaces such as Etsy.  Engaging them with your social media is also a good idea to help maintain the currency of your story.
•  
• You need to find out about who will be attending the event.  The social and economic characteristics, the age profile, the status of the event and many other things will be important to what you bring and how you dress it.

Finally, all these elements of an approach to selling your work apply to anything you are thinking of selling on an online craft marketplace.  Getting it right at craft fairs prepares you for successful online selling too.

Hot Spots in the Kiln

You may suspect you have hot spots in your kiln because of bubbles or one side of pieces being more fully fused than another. A good method for determining the temperature distribution across the kiln is given on the Bullseye site.  It does not require any sophisticated equipment – just supports equal distances apart and strips of glass equally wide and long – to be witnesses for the hotter and cooler parts of the kiln.  You fire slowly to a very low slump temperature – ca. 620C - for only 5 minutes.  Go as fast as possible to the annealing point and soak for 15mins. Then you can turn the kiln off, and let it cool as fast as the kiln can.

This test will show where the hotter areas are.  You will see from the test results that there is a gradual change of temperature across the shelf, rather than small hot areas that would be required for localised large bubbles originating from under the glass.  It will tell you where the cooler areas are, so you can avoid placing pieces in that area when you need precise profiles on the finished piece.

There is little to no relation between hotter areas of the kiln and localised bubbles.  Do not think hot spots are the cause of large bubbles.

Bubbles more often relate to:

Bubble squeeze

Alack of an adequate bubble squeeze, as described here.

Avoidance is the best approach.

Do not be lead into the idea that mistakes are automatically art, or that all of them can be rescued.

Rapid firing rates

Firing rates need to be adjusted to the materials you are firing.

As fast as possible firing rates can cause problems.

High temperature rapid firings can also cause problems.

Rapid firings are more likely to harm the glass than the kiln.

Damaged shelves

Distortions or damage to shelves can trap air and so cause bubbles to form between the shelf and the bottom of the glass.

Checking for flat shelves and fixing.

There are some remedies.

Volume control

Varying volumes within the piece can give problems.

There are a variety of related things that can cause large bubbles.

Glues

Glues and adhesives have a variety of effects and dangers, especially if generous amounts are used:

There are a variety of glues each with their own characteristics.

Uneven layers/layup

You must think of ways for the air to escape from the interior of the glass and from under the glass.  Most often we set up things in a way that creates bubbles. There are two main ways that we do this.

Encased items

When we put glass or other materials between an upper and lower sheet of glass we are creating conditions for bubbles to form.  The encased items hold the upper glass above the lower glass by an amount related to the thickness of the inclusion.  Routes for the air to escape must be planned. 

One of the ways to reduce the height of the space taken up by the enclosures, is to fire upside down with the inclusions on the shelf. This allows the glass -which will be the bottom layer - to form around the materials, reducing the air space between the bottom and capping layer.  This is known as flip and fire.

You then clean the face which will be capped very thoroughly.  Place the capped piece on fiber paper – which can have Thinfire placed over it, or coat with kiln wash.  This is to allow the air in the uneven bottom surface to escape from underneath through the fibre paper.

Weight

Even when there is no encased material, the weight of the glass pieces on top can create areas where the air can be trapped.  On a single layer the arrangement of pieces can create areas where the glass cannot resist the air pressure that cannot disperse from the pockets caused by the glass on top.  Very clear and generous exits for the air are required.

This can happen with two layers as well, although usually a higher temperature is required.  A means of avoiding large bubbles when there is glass – powders, frits or pieces of glass – placed on top is a two-stage firing of the piece.  First fire the base layers together at full fuse so they become one whole.  Then add the decorative elements on top and fire.  Remember to fire more slowly than for two unfired layers.  The main piece is now 6mm thick and needs a slower rise in temperature.  The additional heat work this entails may mean that a lower top temperature, or a shorter soak will be required than normal.  You will need to peek at intervals to check on the progress of the firing.

There is a multiplicity of ways that bubbles large and small can be created.  Careful layups, bubble squeezes, slower rates of advance and lower top temperatures can minimise, but not always eliminate, bubbles.

Revised28.12.24

Devitrification Temperature Range

Devitrification is the beginning of crystallisation of the surface of the glass. It can look like a dirty film over the whole piece or dirty patches. At its worst, the corners begin to turn up and become “wrinkly”.

This piece shows both mild devitrification and more severe wrinkling on the right side.

The visible occurrence is in the range 720° – 760°C. This means that you need to cool the project quickly as possible from the working (or top) temperature to the annealing point. There is evidence to show that dwelling for a long time in this range on the way up to top temperature can promote devitrification too.

The lower graph line shows the  temperature relationships between annealing (glass transition), devitrification and blowing temperatures.

For more information see this post.

Revised 28.12.24

Kiln Washing Kiln Surfaces

“Having just got my first kiln, I was wondering how often I have to add kiln wash to the bottom of the kiln.”

It has become common practice to kiln wash the bottom of a new kiln.

This may be fine for brick lined kilns.  Kilns with fibre blanket or fibre board do not need to be kiln washed at all.  The fibre is a separator already and does not need additional material which will turn to powder and need to be carefully cleaned to avoid damaging the refractory fibre lining.  This has led me to reconsider the value of kiln washing the bottom of the kiln.

I have followed the practice kiln washing of the bottom of the kiln in the past.  However, I have found that small glass pieces falling to the floor, do not stick to the bare brick or to fibre.  They can be vacuumed or picked from the surface of the kiln without creating any damage.  This means that at fusing temperatures, the brick and fibre does not stick to the glass and kiln wash is not needed.

The main idea seems to be to help protect the kiln surfaces from the molten glass if a relay becomes stuck, raising the kiln temperature to very high levels. Kiln washing the bottom of the kiln does not protect the brick or fibre from a large amount of glass running off the edge of the shelf onto the bottom. Of course, pieces of glass resting on the floor of the kiln may become stuck when higher temperature work is being conducted, such as combing, the various melts, and casting.  The solution is not to kiln wash the floor, but to clean the floor of the kiln before entering the high temperature processes.

There is not really a need to kiln wash the bottom of the kiln at all.  The kiln wash will not protect the kiln brick or fibre in the event of a high temperature accident.  The kiln wash turns to powder which needs to be cleaned from the kiln to avoid contamination, as with other dusts, of the glass being fired.  The main objective is to keep the kiln clean and free of dust rather than adding another source of dust.  A dusty atmosphere in the kiln can promote devitrification, so anything which avoids introducing dust will be beneficial in reducing the incidence of devitrification.

I suppose if you really want to protect the bottom of the kiln from molten glass, you can add a high temperature separator such as a refractory fibre board, or a thin layer of sand.  The sand will resist the molten glass and can be scooped out of the bottom before adding new. 

After some consideration, I no longer think kiln washing the bottom of kilns is worth the potential for dust accumulation, as it doesn’t really protect the kiln floor during high temperature accidents. Low temperature spills of frit, glass powder and shards will lift off the kiln surfaces easily without damage to surfaces.

Kiln Cleanliness

Problems with finished pieces can be caused by an untidy or dusty kiln interior.  Pieces can be affected by devitrification or specks of refractory material in or on the fired project.

Dust is a common problem.  Kiln wash, fibre papers and combustion products all produce particles that are collectively referred to as dust.

Vacuuming the kiln regularly is a good practice to keep the dust down.  It is best if the vacuum has a variable suction control to avoid damage to the refractory materials which make up the kiln.  It is best to use a brush attachment rather than the bare hose.

Dust on the brick or fibre board floor of kilns can be vacuumed easily if you remove the shelf.  Usually you need to use the most suction available to pick up heavier particles such as glass frit along with the dust that accumulates on the floor.

Dust also accumulates on the sides and top of the kiln too.  If you have brick sides and tops, you can continue to use the high suction.  You need to be careful around the elements so that you do not bump them.  This is where the brush attachment is most useful, as you can gently brush out any accumulated dust and any loose particles from the brick.

If you have fibre sides or top, the high suction setting on the vacuum will pull fibres from the refractory material.  You need to use a low setting to avoid damaging the insulating materials.  Gently pass the brush attachment along the insulating fibre and along the elements.

This vacuuming of the kiln does not need to be done on every firing, only at regular intervals.  It is also a good time to check the condition of the elements and condition of the interior of the kiln.  Any element tail connections can be checked for tightness.  The condition of the bricks can be checked as you vacuum. 

Of course, if you are going to fire an important piece, it is a good idea to make sure the kiln is clean before you start. But daily cleaning is not required.

It is not only the interior structure of the kiln that needs to be clean.  You should be checking the cleanliness of your kiln furniture too.  Make sure you keep the shelves dust free and regularly kiln wash them.  Check the kiln posts for flaking kiln wash and dust.  Clean off any dust or loose material and re-coat as necessary.  

And while you are doing all this cleaning, you could vacuum the outside of the kiln too.

Firing Bullseye and Oceanside Together

Is it possible to fire Oceanside (formerly Spectrum) and Bullseye at the same time?

Yes, it is possible to fire pieces made of Oceanside and pieces made of Bullseye in the same firing – as long as the glass is not mixed in one piece.

There will be differences in profile as the temperatures for Spectrum are a little less than for Bullseye at all stages.  A rounded tack for Spectrum will be a much sharper edged tack for the Bullseye, etc.  If you can accommodate those differences you can continue to fire.

It is a bit easier on slumping operations as you can use the lower slumping temperature for Spectrum and extend the soak for the Bullseye glass.  Or, choose a mould for the Bullseye that requires less time than the Spectrum, so they complete the slump at the same time.

The annealing points are different, of course.  But not by much – Spectrum is 510°C and Bullseye 516°C (for any but thick pieces).  These are not far away from each other.

There are two main approaches to annealing different glass in the same firing.

One is to use a shotgun approach.  This means that you choose your upper anneal soak – in this case 516°C – and hold the temperature for the required amount of time.  Then proceed more slowly than usual – say 50°C /hour rather than 80C/hour – until about 55°C below the lower anneal point.  Then proceed to the rest of the cooling.

The other approach is to anneal soak at both annealing points before proceeding to the anneal cool.  This approach is probably best with thicker than 6mm pieces than the shotgun method.  It is also required if you use the Bullseye lower annealing point of 482C.  You would anneal at 510°C and again at 482°C and soak at each point for the required time for thickness.  This doubles the annealing time, thus reducing the advantage of one over two firings.

There is a third approach for pieces less than 9mm that will eliminate the double anneal soak.  Choose a single annealing temperature.  The two annealing points for Bullseye and Spectrum are so close (510°C and 516°C) that you could chose a mid-point between them (say 513°C) and soak there before proceeding to the anneal cool.  

It might be even better to choose a temperature midway between 510°C and 482°C (say 499°C) and soak both glasses for a longer period to ensure the temperature is equalised before proceeding to a slow rate of anneal cool.  This will be especially applicable for tack fused pieces, which require more care than full fused pieces.  Remember that you should be soaking at the temperature equalisation hold for at least twice the thickness of the thickest part of the piece.  Then reduce the temperature at the rate recommended for the thickness indicated.  Look at the Bullseye chart for annealing thick slabs for the rates. 

The reason that you can anneal at different temperatures is that annealing occurs over a range of temperature.   The annealing point is the temperature at which annealing can most quickly occur.  There are several of physical changes that are affected by temperature and rates of cooling. 

If you cool too quickly after the anneal soak, you will induce stress and probable breakage.  The cooling after the anneal soak is an essential part of the whole annealing process.  Annealing at a lower temperature requires more certainty that the glass is all equal in temperature.  This means a longer anneal (or temperature equalisation) soak is required.  It is also a good bet to slow the anneal cool to be less than you would use for a single glass.

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