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. 

Other posts on consignment:

Consignment

Consignment Agreements

Consignment Rates

Consignment Venues

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.

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.  Add a slurry of 100 grit aluminum oxide onto the level grinding base and put your piece over it.  Holding the glass 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.

An excellent video by HIS Glassworks comparing hand and machine coldworking is available here.

Annealing at the Lower End of the Range

Annealing can be done at other than the defined glass transition temperature (Tg) - 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. 

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.

Including Incompatible Glass

The question on whether incompatible glass can be included in a piece gets a range of positive and negative responses.

The real answer, as indicated by the diversity of responses, depends on where you start, and what assumptions are being made.  However, responses such as "Less than 10% of area is ok" are not helpful because they take no account of the conditions.

Degree of compatibility

How incompatible are the two glasses?  The greater the difference, the less can be used. If you have two test pieces of glass that show a little stress upon viewing with a set of polarised filters, 

you can attempt to combine a greater area than if the test pieces show significant stress.

Mass

The relative mass of the two glasses are important.  Thin Bullseye confetti placed sparingly across an Oceanside glass of 6mm thickness and 300mm diameter will usually survive, although there will be some stress visible through polarising filters.  If you are placing a large or thick piece on the disc, you will have much more trouble.

Placing

The placing of the incompatible glass has an effect too.  The nearer the incompatible glass pieces are to the edge, the more likely a fracture is to develop.

Shapes

The fourth consideration is the shape of both the base and the added incompatible glass.  A circular base can contain more stress than a rectangular one.  An angular incompatible inclusion will show greater stress than a circular one.

With included incompatible glass you are asking the main piece of glass to contain the stresses.  The factors affecting the ability of the base glass to contain the stress are:

The degree of difference in stress between the pieces

the mass of glass applied to the base

the shapes of the base and the inclusions

where the incompatible glass is placed.

These all affect how well the main or base glass can contain the stress.  If the piece is at all important to you, do not include incompatible glass at all.  If it is really important, test all the glass you will be using.

Making Thin Sheets

The question of how to make thin sheets arises from time to time.  Unless you are a glass manufacturer, it is unlikely you can make large, thin glass sheets.  But you can approximate making thin sheets by two methods that I know.

Sintering

One of these is sintering.  This is firing the glass to a low temperature and soaking for a long time.  The common form of this is powder wafers. 

By using a screen to deposit an even layer of glass powder you can make very thin, but delicate sheets of glass.  The procedure I would use is a screen of about 45 – 60 threads per inch.  This is coarse enough to allow the powder through, but not so fine as to “reject” large amounts of the coarser particles. 

You can screen the powder directly onto a kiln washed shelf, or onto Thinfire or Papyros.  You will not be able to move the unfired powder on a sheet of paper or fibre paper without changing the thickness and shape of the screened powder.  It must be laid down onto the separator directly on the shelf.  You can of course, move the shelf to the kiln if you can get in without tipping it.

Method

Support the screen about 3mm above the surface to allow the powder to fall through.

Make a ridge of powder at one end of the screen.  Using a smooth straight edge wide enough to cover the whole of the screen, lightly spread the powder from the starting end to the other. Then repeat drawing the powder to the starting end.  Make about five repeats of this – that is 10 passes, to get enough powder laid down to form about 0.5 to 1mm sheet.  You will need to experiment with the number of passes to get what you want.

Do not try to press the powder through the screen.  That will only wear the screen out quickly and may tear it.  Each pass should be a light spreading of the powder.  It is heavy enough to fall through the screen without additional force.

You could, of course, just sift the powder over the area you want to cover and judge by eye how even the layer is.  It is possible that your observation is good enough, but it is more likely that you will have thick and thin areas.  Often even at sintering temperatures, the thin is pulled toward the thicker, leaving small or large holes.   By screening the powder, you know you will have an even layer

Firing

The kind of schedule to use to sinter the glass particles together without changing their structure is the following:

220°C to 482°C , soak for 60 mins
55°C to 593°C, 10 minutes
28°C to 665°C for 5 mins
as fast as possible to 482°C for 30 mins
28°C to 427°C, no soak
55°C to 370°C, no soak
110°C to 50°C, no soak

This will work for most fusing glasses.

This slow firing allows enough heat to penetrate the glass grains that they will stick together without changing shape or developing holes.  I admit the anneal cool is very cautious.  You can experiment with quicker cools if you want to speed the process.

  

Pressing

This is a technique of thinning already existing sheets of glass.  Typically, you will have a 6mm or thicker piece of glass that you want to be 3mm or less.  Paul Tarlow has described this kiln pressed glass very well in his books and on the fusedglass.org site.

In essence, you use a pair of kiln shelves.  Kiln wash both shelves.  Place the glass to be thinned on one shelf.  At the outer edges of the shelf put down spacers of the thickness you want the glass to be after pressing.  This will keep the upper shelf from settling down too much and more importantly unevenly.  Place the other shelf, kiln washed side down, on top of the glass.  Be sure the spacers are in places where they can support the upper shelf.

If you are thinning from 6mm to 3mm, normally you do not need any additional weight on top of the upper shelf.  But the thinner you want the glass to be, the greater the weight needs to be.  It could be another shelf, fire bricks or steel weights.

When scheduling the annealing remember you must take account of the mass of the weight on top of the glass.  You will need a much longer temperature equalisation soak and a much slower annealing cool.  

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

Keeping Bottles from Rolling

A common problem in firing bottles is that they may into one another and stick, making both bottles useless.

One way to overcome this is to let the bottle find its heavy point by setting on smooth and level surface. It will gently roll to one direction before slowly coming back in the other. When it stops this oscillation, the heaviest part of the bottle will be on the bottom.  Mark the bottle in some way so you can move to the kiln in that position. If after this, it rolls in the kiln, then your shelf is not level. 

Additional assurance against rolling is putting a small piece of fibre paper (1 or 2 mm)at each side of the point the bottle touches the shelf.  Thinfire, Papyros, and other shelf papers are not enough to ensure there will be no movement. But the small bumps of fibre paper are enough to stop the bottle from rolling.  This will work when you want a particular part of the bottle up or down, but it won’t stay in place otherwise.  

Other materials you can use to prevent the bottle from rolling are crumbled chalk, a small pile of whiting or kiln wash powder. 

Preventing bottles from rolling in the kiln is about finding the natural heavy spot, or propping the bottle in place with one of a variety of heat resistant materials.

Altering Annealing Temperatures

Sometimes  it is discovered that a kiln is firing hotter than other kilns, and you need to alter your process temperatures from the generally presented ones.  That your kiln is firing hotter than others is when you recognise the tack fusing profile of your tack fused piece is rounder than expected. 

Altering process temperature and soak times

There are two things you can do.

1)  Reduce the time at the temperature.  If the recommended schedule has the process work being done at 780°C for 15 minutes and the glass is too rounded or more like a contour fuse, you can reduce the soak time to 5 minutes, depending on how over-done the pieces are. 

2)  If the reduction in soak at process temperature does not work, then you can begin to reduce the process temperature.  Often only 5°C with a 10-minute soak is enough.  For some kilns it may be as much as 20°C again with a 10-minute soak.

Remember that the speed at which you advance to the process temperature will have an effect.  The slower you go the lower the temperature can be.  The faster you go, generally the higher the temperature needs to be.  There several factors combining to determine which is the right process temperature and soak.  Experimentation and record keeping are required to find just the right combination.

Annealing temperatures in a “hot” kiln

If your kiln fires hot, you do not need to alter the annealing soak temperature.  I have seen the recommendation that when you need to reduce the process temperature you also need to reduce the annealing temperature by the same amount.  This is not so for several reasons.

The first is that reducing the temperature of the annealing soak runs the risk of trying to anneal below the acceptable range.  These are a few paragraphs to explain.

Annealing occurs over a range.  The annealing point is the temperature at which annealing can most quickly occur.  But there is a range during which annealing can occur.  It is generally around 43°C either side of the annealing point.

If you follow the recommendations to anneal in the lower end of the annealing range, it is possible that you will start the annealing soak at too low a temperature by reducing the annealing soak temperature in line with the reduction of the top temperature.

The second is that the temperature measurement is of the air, not the glass.  On cooling, the glass is hotter than the air temperature in the kiln.  The recommendations for the annealing temperature take that into account.  So, reducing the temperature risks straying outside the annealing range.

Example of the annealing of a tack fused piece comparing temperatures of the air to the under tack stack and exposed base during the anneal soak and first cool

You should note that if you are using the Bullseye recommendations to do the anneal soak at 482°C, you already are in the lower end of the annealing range.  The average annealing point of Bullseye remains at 516°C. This new recommendation for the annealing soak is 34°C below the annealing point and any reduction of more than 9°C will put your anneal soak outside the annealing range, meaning that your anneal will be inadequate, no matter how long you soak there.

The third element relates to the annealing range.  The anneal soak can occur anywhere within that range. But the practical measure is to soak at, or below, the annealing point.  If your kiln fires hot, you do not need to alter the annealing soak temperature.  It will not matter if the glass is in fact hotter at the annealing soak than in some other kilns. 

It does not matter, because the soak at the annealing point, or lower in the range, is to equalise the temperature throughout the glass piece. The annealing point is not some magic number or temperature that sees to producing a sound piece of glass.  The soak at annealing point is to equalise the temperature to + or - 5°C within the glass.  This is referred to by the technically minded as Delta T = 5°C, or in symbols as Δ T = 5°C.  Bullseye has published a table that gives practical information on the length of soak required for this temperature equalisation for different thicknesses.

Once the temperature is equalised within these limits, you can begin the anneal cool.  This is an essential part of annealing and is designed to maintain the equality of temperature differentials during the cooling.  The rate of cooling is directly related to the length of the temperature equalisation soak required for the piece which in turn is related to the thickness of the piece.  This forms the fourth reason that starting the anneal soak slightly higher than recommendations, will not affect the annealing process adversely. The first slow cool is essential to achieving a sound piece as it maintains this small differential in temperature during the early part of the cooling into the brittle phase of the glass.

Annealing Temperatures in a Cool Kiln

Exactly the same reasoning process is applied to both hot and cool firing kilns.  You do not need to alter the anneal soak, even though it means you will start the temperature equalisation at a slightly lower temperature than the published schedules.  This is because you have to increase the top temperature to get the effect you want and so would also be annealing in a cooler kiln.  Since you are measuring the air temperature, the glass temperature will be above the air temperature and will still be in the safe annealing range.

Summary

The reasons annealing temperatures do not need to be altered if you kiln fires hot or cool are related to:

·        annealing range

·        air temperature measurements

·        rate of the anneal cool

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

Uneven Slumps

A common problem in kilnforming is that the glass slumps into the mould unevenly. Several of reasons are given in this post about high temperature or fast slumps for uneven results.

There are two other things that can be done to alleviate uneven slumps.

Place the mould in the centre of the kiln to reduce any uneven heating of the glass.  Uneven heating is a common cause of off-centre slumps.  Where you have persistent uneven slumping with a mould it may be better to fire it on its own so the conditions can be best for it.  Sometimes it is more economical to fire a single item rather than a crowded kiln shelf where the firing conditions must be for an average rather than the optimal firing schedule and conditions for one mould.  Less of the resulting slumped glass is disappointing.

There is an alternative. Cut the glass so the fused piece will be slightly smaller than the mould top. This will allow the glass to sit inside the mould rather than on top. Frequently there is evidence of the glass hanging up on the side of a mould.  Sometimes there are spikes where the glass stuck and stretched. (Another reason for Low and Slow)

A third method has been suggested, but I have not tried it.  This is to lightly bevel the underside of the piece to be slumped.  The basis for this suggestion is that a bevelled edge will fit the mould better by having a slope rather than a relatively sharp edge resting on the mould surface.  I do know the other two suggestions work, but not this one, although it sounds logical.

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

Slumping sizes

A question about slumping into a bowl mould.

·        Will the end result always be that the glass drops down into the mould and so the bowl size is smaller than the mould?

·        Does the firing schedule affect the outcome?

You need to think about what is happening to the glass in the slumping process to understand what the outcome of a slump will be.

During slumping, the glass is being bent rather than stretched (very much) into the mould.  In other, higher temperature processes the glass is being both stretched and thinned.

The reason for using low temperatures is to reduce the thinning effect while obtaining the shape of the mould.  If you use higher slumping temperatures, the glass will slide down the mould more than at low temperatures, and will begin to thicken, or create an uprising, near the lower portion of the mould.

Take the simple case of a ball mould. One which has no flat bottom but a simple curve from edge to edge.

If the glass was 300mm diameter when flat, it will still be (approximately) 300mm from edge to edge on the bottom.    If you put a flexible tape measure around the bottom from one side to the other, it will be very near the 300mm diameter around the curve.

This means the new diameter of the rim will be smaller than the flat diameter.

Steep moulds have less change in diameter, but greater change in the height of the piece in relation to the mould.  Again, the glass measured from side to side on the bottom will be about the same as the flat piece.  To achieve this, it will slide down into the mould more than a shallow one.

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

Anneal soaks

An odd concept was presented recently.  This in summary was that if you have long soaks on the way up to top temperature you do not need to have such a long anneal soak as normal.

This is a fundamental misunderstanding of the physics of glass.  As the glass temperature rises above the upper strain point (about 55°C above the annealing point), the molecules become disordered.  No amount of soaking at any temperature on the way up to the top temperature will change that. 

The glass (and the molecules of it) will need to be cooled relatively quickly from the top temperature to avoid crystallisation of the glass.  This is the reason for the fast cool to the annealing soak.  It is also a reason to avoid a soak at approximately 50°C above the annealing point – there is a slight risk that crystallisation could form.  This would appear as scum marks on the surface, rather than in the interior.

Whatever soaks you have performed on the way to top temperature, you will need the full length of soak for the full or tack fuse.  And you will need it for the slump too.

No amount of soaking on the way up to top temperature in kilnforming will have any effect on the requirements for the annealing soak at the cooling part of the schedule.  The soaks in the early part of the schedule, no matter how many or how long, do not change the annealing requirements.

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