Wednesday, 13 January 2021

Annealing Bullseye and Oceanside Together

Credit: Bullseye FAQ_kilnforming_annealing


The question sometimes arises as to whether Bullseye and Oceanside can be annealed in the same firing, since the two glasses cannot be combined in the same piece.  They also have different published annealing soak temperatures (also known as the annealing point).  The explanation requires some knowledge of annealing.

Annealing can be done at other than the annealing point. This is because annealing can be done over a range rather than being a single magic figure. Bullseye did not change their glass when they altered the recommended anneal temperature.  This means that the annealing point is still at 516°C. Their research has shown that good annealing results are obtained by doing the temperature equalisation soak at the lower end of the range.  Temperature equalisation throughout the piece is what happens during the annealing soak. Therefore, it is a descriptive term for what happens at the annealing temperature.

Bullseye's previous annealing temperature was 516°C and Spectrum's was/is 510°C. These are very close, and in the past, many chose to anneal at either - or in most cases, both - of these temperatures. Bullseye's research has shown doing the temperature equalisation at the lower end of the annealing range provides good results and ones that are more reliable than the higher temperature.  This research is applicable to all soda lime glasses, not just Bullseye. Therefore, the same principles can be applied to Oceanside fusing compatible glass, or any other fusing compatible glass. This further indicates that you can anneal both Bullseye and Oceanside fusing compatible glasses at the same temperature. 

Further support to this view of the possibility of annealing the two glasses at the same time and temperature is given by Wissmach.  Wissmach W90 and W96 now are both given the annealing temperature soak as 482°C.  Previously they both had been at 510°C.

If you feel the need to compensate for the annealing point differences, you can increase the 482°C for Bullseye by 6°C to 488°C for both. Although I don't think it is necessary, 488°C will be fine for Bullseye and safe for Oceanside.


Wednesday, 6 January 2021

Consignment Venues

Credit: getlstd-property-photo

Finding suitable shops and outlets

Shops and galleries want stock that meet or exceed their customer expectations of quality, style, function and price.  In other words, they are looking for work that will fit with the other products already on show.  These shops generally will be those that already sell hand crafted work. You need to show how your work fits with or adds to the retail premises.  As you are selling handmade items, you also will be looking for shops with higher price levels to be able to sell to the shop at a reasonable profit.

You need to do your research.
What do you know about the gallery/shop?
  •  What is its perceived standing?  Is it a “go to” shop? Does it get discussed in media? Is it talked about in craft circles? What does its online presence look like?
  • Location.  Where is it? Is it in a prestigious area? Is it unobtrusive?
  • Will there be, or is there already, a good footfall?  Who are its customers? Who does the shop target as their clientele?
  • What is the fit between the shop and your pieces? Will your pieces fit in with the existing items? Will they stand out well, or seem odd?
  • Will the shop advise on the prices they expect to get?  Can the shop get you higher prices?
  • Does the shop have promotional events that you could participate in?

Visit the store/shop as though a customer first to assess the venue.  If the shop is too far away to visit in person, look online to get a sense of the business.  This will show how the shop fits with your products – style, kind, price levels.  Also take note of the presentation of the store internally and externally.

Even after visiting in person, an internet search will be useful, especially to find out about their submission policies and forms. Look at what internet profile they have. And do they have good online reviews? Also enquire around from people you know about the venue, and contact any local crafts organisation for more information.


Local vs regional/national

Should you be looking at local shops or be more ambitious and look at a wider area.  If you are willing to travel some distances for craft fairs, pop-up shops, etc., you may find expanding your search area to regional and multiple outlets a worthwhile activity.  You could take an extra day to investigate shops in the area or meet with the owners.

Some considerations in favour of starting local:
  • Low cost shipping. If your work is large or difficult to post, you can hand deliver.
  • Local helps to start small and get experience for larger volumes, more stores.
  • Local builds an audience for your work near your studio.
  • Getting featured in local press is easier that regional national.
  • Local allows for a more intimate relationship to be developed.
  • Starting local allows you to learn how to build up the volume of your work.

Assessment

Arranging a visit to the short list of shops is the next step.  After those meetings there is a further assessment of the venue to be done.  These are the same questions you looked at in your preliminary research about each shop:
  • What is its perceived standing?  How do they perceive themselves?
  • What is their media presence?  What is the customer perception?
  • Location in shopping terms.
  • What is the customer base and how does your work fit with that group of prospective purchasers?
  • How will your works fit with the shop and its presentation?
  • What advice is available on the prices they can get for your items?
  • What promotional activities are presented?
  • What are the consignment commission rates?
Since these are the questions you will be basing your decisions upon, they are the ones you should be asking during the appointment, if not already discovered from your prior investigations.  Assess how you feel about the responses you received after the conclusion of the meeting.  Do not make on the spot decisions.

Selection of a gallery or shop in which to place your work is a complex interaction of commission levels; the value you place on your time in preparing for and attending craft fairs or putting your work online; the perceived prestige of the shop/gallery; the potential relationship between you and the outlet; and the relationship of the consignment, wholesale and retail prices.




Tuesday, 5 January 2021

Expansion at Edges of Tack Fused Stacks

How much will my glass expand if I put glass pieces on top of 6mm base?  I ran some tests for both 6mm and 3mm bases. These showed that the distance from the edge is important.  The amount of glass in the stack has a big influence on expansion.  So does the tack profile and the thickness of the base.

The most expansion for any thickness and at any tack profile is when the stack is placed at the edge.  The further away from the edge, the less the expansion. There is no noticeable expansion of size when the tack stacks are placed 20mm from the edge.  In most cases there is only a little expansion at 10mm from the edge.  Although not tested, it seems that 15mm is a safe distance from the edge to avoid changing the edge.

The amount of glass in the stack being tacked to the base has an effect on the amount of expansion.  This is to be expected based on the concepts behind volume control.  Two tack layers can vary from two to three times that for a single tack layer depending on the profile of the tack.

The tack profile has an effect on the amount of expansion.  At contour there is a greater expansion than at rounded or sharp tack fuse.  This is to be expected, as there is less heat work at sharper tack profiles than at contour.

The thickness of the base has an influence on the amount of expansion too.  Thicker stacks promote greater deformation of the edge at all tack levels.  Thicker stacks need to be placed further from the edge to avoid changing the perimeter.  Thicker stacks create greater change in the edge on single layers than double layers.



The setup and results are given here.



Setup for 2 layer base and 1 and 2 layer stacks at various distances from the edge.


Contour fuse test, 6mm base
1 layer placed at edge, at 10mm from edge, at 20mm from edge, and at 30mm from edge.  2 layer stacks placed in the same way.  
 
Fired results, outlined for clarity

1 layer placed at edge – expansion of 2.5mm
1 layer placed 10mm from edge – expansion of 0mm
1 layer placed 20mm from edge – expansion of 0mm
1 layer placed 30mm from edge – expansion of 0mm

2 layers place at edge – expansion of 9mm
2 layers placed 10mm from edge – expansion of 2mm
2 layers placed 20mm from edge – expansion of 0mm
2 layers placed 30mm from edge – expansion of 0mm
 

Rounded tack test, 6mm base
1 layer placed at edge, at 10mm from edge, and at 20mm from edge.
2 layer stacks placed in the same way.
 
1 layer placed at edge – expansion of 3mm
1 layer 10mm from edge – expansion of 0mm
1 layer 20mm from edge – expansion of 0mm

2 layers place at edge – expansion of 7mm
2 layers placed 10mm from edge – expansion of 1mm
2 layers placed 20mm from edge – expansion of 0mm
 
Fired result of 6mm base with 1 and 2 tack layers, rounded tack.


 
Rounded tack test, 3mm base
1 layer placed at edge, 1 at 10mm from edge, 1 at 20mm from edge, 1 at 30mm from edge.  2 layer stacks placed as above.  
 
1 layer placed at edge – expansion of 2.5mm
1 layer 10mm from edge – expansion of 1mm
1 layer 20mm from edge – expansion of 0mm
1 layer 30mm from edge – expansion of 0mm
 
2 layers placed at edge – expansion of 3mm
2 layers 10mm from edge – expansion of 1mm
2 layers 20mm from edge – expansion of 0mm
2 layers 30mm from edge – expansion of 0mm
 
Fired result of 3mm base with 1 and 2 tack layers.

Note: the single 200mm sheet contracted to 195mm in uncovered areas.  Measurements were based on the amount of expansion from the fired dimensions. Even with the greatest expansion the piece was still 2.5mm smaller after firing than at the start.
 

Sharp tack test, 6mm base
1 layer placed at edge, 1 at 10mm from edge, 1 at 20mm from edge, 1 at 30mm from edge.  2 layer stacks placed as above.  
 
1 layer placed at edge – expansion of 1mm
1 layer 10mm from edge – expansion of 0mm
1 layer 20mm from edge – expansion of 0mm
1 layer 30mm from edge – expansion of 0mm
 
2 layers placed at edge – expansion of 2mm
2 layers 10mm from edge – expansion of 0mm
2 layers 20mm from edge – expansion of 0mm
2 layers 30mm from edge – expansion of 0mm
 

Wednesday, 30 December 2020

Float annealing


As a result of various memory failures, I've done a bit of searching on the annealing of float glass.  There are now various compositions of float glass and with different coatings for various applications.

This leads to a variety of annealing points for Pilkington float glasses. The search led to various hard to find documents, which indicate a range of annealing temperatures between 548°C and 559°C. This is not a huge range, so anywhere between 548°C and 560°C can be taken as the annealing point. Pilkington indicate that optifloat has an annealing point of 548°C

The strain point seems to be mostly between 525°C and 530°C for all the varieties.  This indicates the temperature equalisation soak should not be less than 535°C.

The conclusion seems to be that annealing should have a temperature equalisation soak between 550°C and 535°C. It will not matter much where you choose, but remember that the closer to the strain point you do the temperature equalisation, the longer the soak should be.  The length of soak at 535°C can be determined by use of the Bullseye chart for Annealing Thick Slabs. This gives the times and rates for the anneal cooling of glass by thickness.  The temperatures need to be changed, but otherwise the information can be applied.

The softening point seems to be 725°C for all the glasses. This is a good low temperature for slumping.


Wednesday, 23 December 2020

Consignment

Why sell on consignment?


Biscuit Factory, Newcastle


Benefits
Consignment arrangements can add income additional to your other strategies of online, direct sales, craft fairs, pop-up shops, etc. 

It can develop new customers, and develop growth in both commercial and artistic terms. 

It exposes your work to new and different customers.

It can provide opportunities to partner with another small business (the shop) and benefit from mutual promotion.  

Craft fairs are not a long term means to sell your work.  Fairs are concentrated at certain times of the year.  You cannot attend all of them anyway.

Consignment spreads the income over the seasons.

Down sides
Your stock is tied up in the shop.
Your craft fair and online prices need to be similar to the retail prices at the shop(s) to which you consign work.


Further Action
Consignment can be beneficial to your sales, but it does require preparation and effort. 

You need to investigate shops and prepare for a meeting with the owners. You need to have a written contract even with friends and it should include all the elements and assumptions for the arrangements.

If you decide to pursue consignment arrangements, there are several things you need to consider and prepare.
  • Finding suitable shops and stores and assessing them.
  • Preparation for meeting the owner.
  • Knowing your terms
  • Placing and promotion of your work.
  • Maintaining the relationship.
  • Wholesaling


These aspects of consignment are the subject of postings to come.


Selection of a gallery or shop in which to place your work is a complex interaction of commission levels; the value you place on your time in preparing for and attending craft fairs or putting your work online; the perceived prestige of the shop/gallery; the potential relationship between you and the outlet; and the relationship of the consignment, wholesale and retail prices.





Wednesday, 16 December 2020

Mould repairs with ciment fondue



Ciment Fondue
Ciment fondue was a French discovery and so the French name has become common in Europe. The name ciment fondu is used for the formal name Calcium aluminate cement which is also called high alumina cement and aluminous cement.  It is composed mainly of Aluminium oxide (alumina) and calcium oxide (quicklime) with varying amounts of ferric oxide. The Aluminium Oxide varies from 40% to 80% for various applications. The calcium oxide content varies from 40% to 20% and the ferric oxide varies from 16% to none for refractory applications.  For kilnformers, the general purpose composition of 40% aluminium oxide, 40% calcium oxide and 16% ferric oxide is sufficient (the rest is made up of minor amounts of incidental minerals and metals).

It is costly in relation to Portland cement and is used mainly where quick curing strength is required and at low temperatures; in refractory concretes where strength at high temperatures is needed; and in sewer piping and other applications to provide protection against biological attack of the concrete.

It is also used in sculptural applications, both as the casting material, and as a strengthening element in a non-metallic structure.

It is mixed with water to form a paste.  The proportions are not required to be exact, as the ciment fondu separates out of the water due to its weight and very low water absorption.  Slightly different methods are needed to repair breaks, and to fill divots in the surface.

Breaks
To repair breaks or cracks in ceramic moulds the ciment fondu needs to be used on its own.  Mix the dry particles with water until a stiff slurry is formed.  Thoroughly wet the edges of the broken pieces or the cracked area.  Then apply the ciment fondu slurry to both edges.  Press the pieces together and bind them if they would otherwise separate.  This can be with elastic bands or tape or any material that will withstand moisture.

The internal surface must have all the ciment fondue cleaned from it.  It cures so hard that it is not practical to sand it smooth without damaging the ceramic surface.  This clean up can be with a lot of water and paper towels. Any tools you use need to be immediately cleaned with water.  Do not dispose of this clean up water down your drains. It will harden and narrow your drains, potentially blocking them so firmly that whole sections of the drain will need to be replaced.

When fixed together put the mould in plastic or other waterproof material for at least 24 hours to give a wet cure.  The ciment fondu is not completely cured until it is given a heat cure.  This should be above the expected operating temperature.  Although I have never fired any of my ceramic moulds above 680°C, I fire my repairs to 800°C.  The firing is smelly, so ventilate the kiln and room well.  Try to do the heat curing when the smell will not disturb you or your neighbours.


Divots
This mould had glass stuck to it and was damaged in removing the glass.

If there are scratches or divots in the mould surface, you need to add some material that will absorb water into the ciment fondue mix.  Cured ciment fondue rejects water and so does not get as well coated as the rest of the mould when kiln wash is applied.  



To prevent this rejection of water, I add finely ground vermiculite to the mix.  I use 3 parts or less vermiculite to 1 part ciment fondue (measured by volume).  This provides a firm surface that absorbs some water. Although the absorption of moisture is not as good as the ceramic, it is sufficient to get the kiln wash coverage required.



Once the mix is prepared, you need to thoroughly wet the area to be fixed. This prevents the ceramic absorbing the water from the ciment fondue too quickly. Apply the ciment fondue mix with whatever tools seem appropriate.  


You must smooth the applied mixture before it dries, as it is so hard when cured that it is not possible to sand it smooth without damaging the ceramic surrounding the repair.  Smoothing can be done with significant amounts of water and a smoothing tool such as a ceramicist’s kidney or a palette knife. 


Once smoothed to achieve the surface required, pour off the excess water.  Enclose the mould in a plastic bag for 24 hours for a wet cure. Once out of the bag and dry you can further smooth with very fine sandpaper.

Then fire to 700°C to 800°C to complete the cure.  When cool it is ready to kiln wash.  If you warm the mould to around 100°C, the kiln wash will adhere to the repaired areas a little better than the cold mould.  Once the first kiln washing of the mould is complete, further applications of kiln wash will be easier. Of course, if you use boron nitride to coat the mould, there will be no difficulty with the repaired areas.

All tools need to be cleaned immediately of the ciment fondue and the cleaning water disposed of on the garden or waste ground.  It should never be put down domestic or public drains.  It does no harm to the soil or plants, but it will certainly harm your plumbing.

Wednesday, 9 December 2020

Clumping Kiln Wash



There are some reports of properly prepared kiln wash (1 part powder to 5 parts water by volume) clumping or going onto the shelf or mould unevenly.

My experience is that this happens on shelves that have been kiln washed and fired several times.  The dry kiln wash that has already been fired absorbs the water quickly leaving unevenly applied kiln wash.  The water is absorbed so quickly that it leaves unevenly distributed kiln wash over the existing, already fired kiln wash.

The immediate response of diluting the kiln wash even further leads to a lot of water being absorbed into the shelf leading to longer air-drying times.  It also risks getting insufficient kiln wash over the existing kiln wash. This risks the kiln wash sticking to the fired glass, which is the opposite of the intention of using fresh separator.




When the new kiln wash solution begins to clump, it is time to stop adding more over the top of the old.  It is time to remove the old, clean the shelf and start with a new smooth kiln washed shelf.  It does not take long and gives the satisfaction on knowing the bottom of your pieces will be flat.


Applying new kiln wash repeatedly over old leads to uneven application and clumping of the new.

Wednesday, 2 December 2020

Consignment Rates


The most common comments about the rates for consignment of pieces to a gallery or gift shop are that they are not fair. They are too high. The gallery is greedy. And so on. How do you judge whether the commission rates are fair?  What are the factors that should be considered?




Time
How much is your time worth?  

Think about the amount of time used to prepare, promote and attend craft fairs, pop up shops, or prepare for and administer online selling. Could you be using that time to make more things, or be with your family?  How much would it improve your quality of life to have to do less selling?



Costs

What are the costs of attending craft fairs?  

    You have to acquire display materials, whether you make or buy them.  You must travel to the event.  You have to be prepared to accept breakage risks from repeated movement of the pieces.  You must pay for the space at the craft fair.



Customer base
Is the shop’s market different than yours at craft fairs or online marketplaces?  
    Shops have a different clientele than craft fairs or online shops.  They spend effort in attracting customers.  They know their clientele and what kind of things will sell to them.  They are aware of the pricing levels needed for their visitors.

Decision
Answering these questions about time, costs and customer base will give you an assessment of whether consignment commission rates being offered are fair. 

[link 20/12/20]

Wednesday, 25 November 2020

Removing Shelves for Slumping



There are those who advocate removing the kiln shelf(s) before slumping.  The advantages claimed include:

Better heat distribution around mould.  The shelf acts as a heat sink. During the firing the shelf absorbs heat and during the cooling the heat is released, so slowing the cool down. 

Additional height. For kilns with little head room, greater height is provided by this practice.

Observations
My observations on this practice lead me to some questions about the necessity, desirability and in some cases the practicality of it.

Elevation of mould above the shelf
This is a widely recommended practice.  I haven’t found the need, but many people do.  One of the points of this is to allow increased air circulation around the mould and under the bottom.  Another is to let air out from under the bottom of the mould to avoid creating air pockets between the mould and the glass.

If the elevation of the mould allows air circulation, what is the necessity to remove the shelf?  There is air circulation around the bottom of the shelf and of the mould. If the mould is placed on the floor of the kiln, the mould will still need to be raised from the bed of the kiln to allow air circulation under the mould. Of course, if the kiln does not have enough space for the height of the mould, it will be necessary to remove the shelf, but not for circulation purposes.

There is also the fact that the floor of the kiln is most often made of refractory bricks even if the walls and top are of refractory fibre.  This also is a heat sink.  I don’t see the advantage of removing the shelf to avoid a heat sink when the base of the kiln works in holding heat in the same way as the shelf.


Difficulty of removing shelves from some kilns
It is difficult to remove shelves from many kilns.  This can be avoidance of damage to the thermocouple; difficulty of getting fingers around the shelf; weight; size; or even depth of the kiln.  It is impractical to remove the shelves from kilns of this nature.  It is still possible to get a good slump in these kilns.


Uneven cooling of the glass
Research shows long soaks lead to a cooler bottom of the glass than top during the anneal – sometimes greater than the +/- 5°C for adequate annealing.  This is a consequence of the fact that the hot air above the glass is not balanced by the same amount of heat below the glass.  So, there may be good arguments for retaining that heat sink of a shelf under the mould to more evenly balance the cooling of the upper and lower surfaces of the glass during the anneal soak and cool.

Height
I don’t have any argument that when extra height is needed, as removing the shelf will provide some.



Some consideration needs to be given on whether to remove the kiln shelf when slumping.  Research implies that increased cooling of the bottom of the glass may go outside the parameters for the even cooling of the glass.

Wednesday, 18 November 2020

Creating Flat Bottoms by Hand


No jokes please!

Often the moulds we use do not have a suitably flat bottom to them, making the resulting item wobble when set on a flat surface.  There are several ways to create a flat spot in the mould, reaching in to re-set the glass while firing, putting the glass in at a complimentary angle for a second firing - but they are not always successful.  

Of course, if you have the money you can use a flat lap or a linisher with a back plate to grind a flat spot on these bowls and other unstable pieces.

But,
You can still make a flat spot on your piece without machine tools.  Use a piece of float glass larger than your piece as your grinding base.  Put a slurry of 100 grit sand on the base and put your piece over.  Holding it level, make circular motions with firm downward pressure.  In only a few minutes you will have produced a large enough flat spot to stabilise your piece.



If you do not like the mess of the slurry, fasten a 100-grit sandpaper onto float glass, add water and do the same as you would with a slurry of grit.


Wednesday, 11 November 2020

Annealing at the Lower End of the Range

Annealing can be done at other than the defined glass transition temperature - also known as the annealing point. Annealing occurs over a range rather than a single magic temperature. Bullseye did not change their glass when they altered the recommended annealing temperature.  Their research has shown that good results are obtained by annealing at the lower end of the range.  

A graph of some aspects of a specific and stiff soda lime glass illustrates this.

Annealing can be between the glass transition (annealing) point and the strain point
credit: Lehigh University

Bullseye's previous annealing temperature was 516C and Spectrum's was/is 510C. These are very close. Bullseye's research is applicable to all soda lime glasses. Therefore, the same principles can be applied to Oceanside fusing compatible glass.  It has already been applied to the Wissmach fusing lines.  This means that you can anneal both glasses at the same temperature.  If you feel the need, you can increase the 482C by 6C to 488 for both, but I don't think it is necessary.

The purpose of the annealing soak is to equalise the temperature within the glass to vary less than 5°C (i.e., +/- 2.5C).  If this is done at the lower end of the annealing range, there is less difficulty of maintaining that small difference throughout the cooling stages. 

Wednesday, 4 November 2020

When to Open a Cooling Kiln

Credit: Glass House Store

Questions about when it is possible to open the kiln during the cool down to avoid thermal shock get the answer, “it depends….”

These dependent variables include:

Temperature Differentials
Thermal shock is related to how quickly a piece can cool without developing stress that cannot be contained within the piece.  So, when the temperature differential is a few tens of degrees between room and kiln air temperature it is less risky than when the difference is hundreds of degrees.

This means that there is a relation between room temperature and when you can open the kiln safely.  If the room is at sub-zero temperatures, you will need to wait for a lower temperature in the kiln, so the temperature differentials are no greater than when the room is warm.  Remember the glass can be much hotter than the air that the thermocouple measures.

Cooling rate of the kiln
The natural cooling rate of the kiln (that is, in the unpowered state) will affect when you open.  If your kiln cools very slowly from 150°C, you may feel confident to open the kiln a little to speed the cooling from that temperature.  If you kiln cools quickly - usually in smaller kilns - then you need to wait longer for a lower temperature to be achieved.

Size of the piece
The size of the piece(s) relative to the kiln size has a bearing on when it is safe to open the kiln to speed cooling.  The more space the piece takes up in the kiln the cooler the temperature reading needs to be before you open the kiln.

Placing
The placing of the glass has an affect too.  If the glass is at the front of a front opening or top hat kiln, it will cool more quickly and unevenly than one at the back. A large piece placed more to one edge than another will also require lower temperatures before opening.

Thickness
The thickness of the glass also needs consideration.  The thicker the glass, the hotter it will be in relation to the measured air temperature, and so the longer it needs to be left to cool before opening.

Type of kiln
Your kiln may cool slowly or quickly, but the style of the kiln is important too.  The kiln may be brick lined or fibre lined, or a combination.  The greater the mass of the insulation, the earlier you can open, as the dense brick will radiate heat back toward the glass.

If you have a top hat kiln it is probable that you can open earlier than if you have a top opening or front door opening kiln, as they will dump hot air slower than top and front opening kilns.

The venting method
The way you open the kiln to increase the cooling rate is important.  If you open vents, that provides a gentler flow of cooler air than opening the lid or door.  If you open lids or doors, you need to wait for a lower temperature than for opening vents.

And I am sure there are other considerations.  But these are enough to show that there is not a single answer.  The answer is in relation to the kiln and its contents.

Acceptable Cooling Rates

The speed of cooling that a glass can sustain is indicated by charts giving the rate of cooling for the final rate of decrease to room temperature.  Faster rates might be induced by turning the kiln off at 370°C and opening the door/lid at some slightly lower temperature.

This means that you need to know how fast a cooling rate is acceptable.  The bullseye research suggests that 300°C per hour for the final cooling is as fast as you would want to cool a 12mm thick piece.  This is in a closed environment.  Therefore, you will want to be slower – at least half the speed for a partially opened kiln of say 5cm. 

My predictions for acceptable cooling rates are (with a room temperature of 20°C; a piece evenly thick and 30cm square, but less than half the area of the kiln floor; and a top hat kiln):

6mm -   300°C per hour (although I never use more than 200°C per hour)
12mm - 150°C per hour
19mm - 75°C per hour
25mm – 45°C per hour

Note: Tack fused items with these total heights need to have these rates halved, or use the rate suitable for a piece twice the thickest part.


But!

You cannot open the kiln until the natural cooling rate is at the predicted acceptable rate of cooling or less, to be safe.

The natural cooling rate at various temperatures can be determined by observing temperature falls in relation to time intervals between those observations.  You can make a chart to indicate the cooling rate at different temperatures.  The kiln will naturally cool more slowly at lower temperatures. 


Schedule to room temperature

A protection against too rapid cooling is programming to room temperature.  If your kiln is cooling less rapidly than you predict is acceptable, you are using no electricity – OK, maybe a tiny fraction of a kilowatt to keep the controller operating. But there is no worry of using excess electricity.

The point of programming to room temperature is that if the air temperature in the kiln cools faster than predicted, the controller will turn the kiln on.  You will need to be present for a while after venting the kiln to hear if it turns on and you can lower the lid to a point where the kiln does not turn on, indicating the rate of cooling is less than put into the schedule.

An example:
Assume you predict that 150°C per hour is the appropriate rate of cooling from 370°C. Also assume you open the kiln at 100°C and a minute or so later you hear the kiln start.  Then you know that you have opened the kiln too far causing a more rapid cooling than 150°C per hour and you need to close the opening to less than the current state.  This probably will be a progressive thing.  You will come back, say, half an hour later and open a little more.  Everything seems fine, but 10 minutes later you hear the kiln switch on again.  Oops! You opened too much – you need to close the kiln a little.  This may repeat several times.

The real answer to when you can open your cooling kiln is dependent on many variables.  You will have to decide on how critical these are in relation to the piece(s) you have in the kiln.  Once you have decided on the appropriate rate, you should program that into your schedule for the final segment.  This means when you partially or fully open the kiln the controller will switch the kiln on when the cooling rate is faster than you wanted.