Wednesday, 18 July 2018

Multi Stage Slumping

Deep slumps cannot be done in one slump. Usually, multiple slumps are required to get an even rim with even thickness along the sides.

Special three stage moulds have been developed for deep slumps. The set is expensive even if you have the shallow starting mould already.

When deep slumps are tried in a single stage, uneven sides, hang ups at the edge, needling at rim, and distortion of the image are common in addition to some thinning and significant distortion.

Do it yourself
This leads to investigating whether it is possible or reasonable to try do it yourself methods.

The DIY process involves using two moulds and filling the deep mould with powdered separator.

·        First stage – slump the glass blank into a shallow shape first.  The starting diameter of the blank will need to be about one third larger than that of the finished vessel.  This can be determined by measuring the diameter of the deep mould and adding one third. This means that if your deep mould is 300mm, you will need a 400mm diameter starting disc and an equivalent size of mould. Fire this slump at your standard slumping schedule for large shallow pieces.

·        Second stage – Add powdered kiln wash or whiting to the deep mould.  Fill the mould to half or two thirds of the volume.  Smooth a shallow depression in the powder.  It should rise to meet the curve of the mould shoulder, even if it does not fully match it. This firing is probably the most critical in the DIY process.  The shallow shape will be considerably larger than the diameter of the mould on which you are placing it.  This means that you must fire slowly and you should peek frequently.  As the glass begins to slump, the outer edge will begin to rise at first.  As soon as the outer edge begins to relax, you must advance to the annealing segment.  If you allow the rim to sag, it will not sit very well in the mould at the next stage.

·        Third Stage – This may require more than one firing to achieve the intermediate shape.  In preparation, remove about half of the powder from the previous firing. Shape the remaining powder to a smooth curve. Fire the glass, again watching and advancing to the anneal when the rim begins to flatten.  If the glass has not touched the powder at the bottom, you will need to do another firing.

·        Fourth stage – Remove all the powder from the deep mould. Place the glass and fire.

Keep the kiln wash powder for future use. Its composition will not have changed as you have not fired it to tack fusing temperatures.  Dispose of the whiting, if you used it.  It may be fine for further use, but since it is cheap, it is not worth the risk of it sticking to the glass in subsequent firings.

Remember that long – low and slow – slumps are required at all stages of creating a deep slump.  As a comparison, think about the hours required for a free drop to form and still keep the glass at the shoulder thick.  Deep vessels require long hours of watching just as aperture drops do.

Friday, 13 July 2018

Fire Polishing

Polishing of glass can be done in the flame, in the kiln, by acids or by grinding with successively fine abrasives depending on the nature of the piece and the equipment available.  Fire polishing in the kiln is widely popular as it utilises existing equipment, avoiding purchasing additional cold working equipment. This post indicates some elements about fire polishing in the kiln. 

Fire polishing is the technique most often available to kiln formers. This is the process of heating the glass to less than a full fuse to achieve a smoother texture on the glass. It is often used after sandblasting or hand sanding a piece in order to give a smooth shiny surface to the glass without extensive cold working with successively finer grits to get a polish. It also can be used to give a variety of textures from a sealed but almost unchanged sandblasted surface, through a satin-like finish to a very subtle difference between full polish and slightly textured surfaces in the same piece.

Fire polishing range
The temperature range that this occurs between slumping and tack fusing. The normal range is 650C to 750C depending on the glass, the soak time and the speed of advance.  The purpose of this kind of firing is to get the surface of the glass hot enough to form the desired surface without soaking long at higher temperatures, as this is also the devitrification range (700C - 760C).  Normally there would be a minimal or no soak at the top of the temperature range.

When to fire polish As this temperature range is above the slumping temperature, fire polishing should be done after fusing and before slumping. As this will be the last operation before forming, you also should do any work to shape the edges and deal with any other imperfections, before fire polishing. After doing any grinding or other work on the edges or surface of the piece, thoroughly wash and polish the piece dry.

You can take the fused piece that has been treated to remove the devitrification up at the same rate as for slumping the piece to the tack fuse temperature.  The higher you go, the less soak time is required. Of course, the higher you go, the longer you are in the devitrification zone.  

Some people advocate a quick fire polish.  This is achieved by firing at a relatively slow rate until a low slump temperature is achieved.  Then fire very quickly to the tack fuse temperature with no soak and return to annealing temperature as quickly as possible.

The quick fire polish does achieve a minimum of time in the devitrification zone, but it eliminates all subtely in the surface.  A long soak of up to 90 minutes at a moderate slumping temperature will give a satin appearance to an abraded or sandblasted surface.  A shorter soak will seal sandblasted work without eliminating the texture of the sandblasted image.

In all the cases of fire polishing you need to peek at intervals to determine when the desired surface has been achieved.  This requires careful placement in relation to the place from which you will be able to peek at the surface.  For a fully polished piece, you will see the reflections of the elements.  For more subtle textures, you need to think about what you want to see, peek, close the lid or observation port and think about what you saw.  If it is not yet what you want, peek at another interval in the same way, until you observe the surface you want.

Combining fire polish and slumping It is sometimes possible to fire polish and slump at the same time, but this is a risky technique often leading to changes in shape or an uprising of the glass at the bottom of the mould. It is possible to fire polish glass as low as 630 with a long soak – 60 minutes or more. If you are determined to fire polish and slump at the same time, it's essential that you watch the piece very carefully to prevent over-firing.

Fire polishing already slumped items Similarly, re-firing already slumped items to a fire polish rarely succeeds. Distortion of the piece is more likely than achieving a fire polish on an already slumped item.

Again, in these more difficult circumstances, you must observe at intervals to ensure you do not over fire and distort your piece.

The reason that no indicative schedules are given is that different glasses, and different lay ups require different firing conditions.  These are dealt with elsewhere in the blog.

Alternatives Alternatives to fire polishing include acid polishing, which can present a health hazard, and is normally an industrial process. The other common method of polishing is to cold work the piece. This often requires specialized equipment, but can be done by hand if you have the time.

Wednesday, 11 July 2018

Smooth Kiln Wash on Shelves

There are a number of ways of applying separators to the kiln shelf.
These go by a variety of names - kiln wash, shelf primer, batt wash, etc. - all are separators to keep the glass from sticking to the shelf. They are all combinations of alumina hydrate and china clay (or kaolin or EPK) in various amounts.  The china clay provides a high temperature binder for the alumina hydrate which does not stick to glass.

These are some examples of glass separators.  The Primo Primer has very little china clay, and is easy to remove.  It is particularly good for small casting moulds.


The object in applying the separator is to achieve a smooth surface a possible. Remember there will always be some texture because of the particle size of the wash.  For the smoothest surface, use the finest powder you can find.  You can, if you want to spend the time and effort, put the powder into a rock tumbler with ceramic balls to get an even finer powder.  Avoid shelf primer that is intended for ceramics, as it is coarser than that sold as a separator for glass.

It also is important to prepare the mixture some hours before application to ensure all the particles of the powder are wetted.  Immediate use often leads to a gritty surface.

There are several methods for applying the kiln wash to the shelf.  The two I use are spraying and brushing.  Which I use depends on circumstances - spraying requires more set up time.

Spraying the separator onto the shelves can give an even coating without brush marks, runs or ridges.  In this example a mould is being sprayed.  To ensure an even covering on a shelf, it should be horizontal and leveled so the kiln wash is evenly distributed.  Numerous light passes with the sprayer is best, as in air brushing.

Applying the kiln wash with a very soft brush such as a hake brush in a variety of directions will ensure full coverage. The brush should lightly touch the shelf and provide a number of thin layers.  Applying in four directions - horizontal, vertical, and the two diagonals will ensure full even coverage. There may be some residual brush marks.

To reduce the application marks further, you can brush or spray hot water over the still damp kiln wash. This helps to remove brush marks or the stippling that often comes from spraying and brushing.  It is important that the shelf is perfectly level for this operation.

Another way to reduce the texture after the shelf primer dries is to lightly polish the kiln wash with a ball of old nylons or rub a flat piece of paper with the palm of your hand over the shelf.  Be sure to remove the dust that may be left behind from this polishing.

Boron Nitride

Another separator that has become popular in spite of its expense is boron nitride, often referred to by the trade name Zyp.  This is a high temperature lubricant for industrial kiln operations that has been adapted for the generally lower glass forming temperatures.  This is not suitable for kiln shelves, as it completely seals the porous surface of the shelf.  It is difficult to go back to the cheaper kiln wash separator as the water of the kiln wash solution will not be absorbed into the shelf, leaving a patchy coverage of the kiln wash.  Although both separators should be renewed after each firing (above low temperature tack fusing) the boron nitride is much more expensive and cannot provide a smoother surface than the shelf already has.  My recommendation is that boron nitride use should be confined to moulds or other surfaces where the glass may slide or move in the forming process.

Adding Colour to Slumped Pieces

Sometimes an already fused and slumped clear piece needs colour for interest, definition, etc. The problem is how to do it without altering the fused piece.

You can use cold paints. Both acrylic and oven baked paints can be applied, but often they are unsubtle, harsh colours.  These are not permanent.

You can apply enamels.  These can be the ones produced for glass fusing, if fired carefully. The curing temperature of 780°C means that you need to fire slowly to about 600°C and then quickly to 780°C with no soak and AFAP to annealing.  This fast rate of advance is to preserve the shape as much as possible at temperatures above that required for slumping. This will need to be done in the mould, of course.

You can more safely use traditional glass stainer colours, which are also called enamels, although they are slightly different from the traditional ones.  These cure between 520°C and 580°C so can be fired as normal in one steady climb to the top temperature with no soak and quickly down to annealing. To be sure the shape is retained, the glass should be in the mould during the firing.

Use of frits and powders requires the higher temperatures that will distort the piece unless fired in the mould. When firing to tack fuse in a mould, you need to be careful that you do not damage the mould during the higher temperature firing, nor get the separator incorporated into the powder.  If you can place the powder or frit on top of the glass, you will get a better result at a lower temperature as the frit will heat and spread more easily on top than on the bottom. 

In general, it is not recommended to add colour to slumped pieces with frits and powders.  It is hard on the mould, and risks the glass sticking to the mould. Even if successful, the slumping mould will have to have the existing kiln wash removed and new added to avoid the kiln wash sticking to the next piece to be fired.  

It is better to slump the piece to flat, if possible, and then add the frits and powders before fusing.  Then slump again.

These notes show that it is important to assess the flat piece critically before proceeding to the slump.  This can mean setting the piece aside for a few days to review your impression of the fusing result.  This little time elapse can give you a fresh view of what the piece requires, if anything. 

Wednesday, 4 July 2018

Grinding to Fit

In copper foiling, a considerable amount of work goes into getting the pieces to fit with just enough space to accommodate the copper foil and a thin space for the solder fin to join both sides.  This of course, promotes consistently narrow solder lines without the solder melting through to the opposite side.

Grinding to pattern
Many times it is necessary to grind to fit pieces together with this degree of accuracy.  Those who draw onto the glass or stick pattern pieces to the glass, often grind to the template or the drawn lines.  This can lead to inaccuracies in relation to the cartoon.

The object in scoring and breaking the glass is to be as accurate as possible.  This reduces the amount of grinding required.  It saves time. It makes the whole process easier.  Still, we all have to grind relatively often.

Grinding to cartoon
In my view, when grinding to fit, you should be trying the piece out against the cartoon, rather than the template or the drawing on the glass. This will tell you how well the current piece fits in with the rest of the pieces you have already fitted to the cartoon. 

The cartoon drives the assembly of the whole piece.  Thinking you can just make small adjustments as you work along, creates increasing difficulties in making the whole fit together.  If you follow this principle of fitting to the cartoon, you are judging the accuracy of the piece against the cartoon lines, rather than any template or drawing on the piece of glass.  This means that the fit will be correct and the whole will go together with the minimum of difficulty.

Centering Holes for Drilling

When using larger core drills, it is not possible to see the centre point for drilling. So a different arrangement for marking the place to be drilled is required.  This example is to locate the hole for a clock spindle accurately bewteen the points marked by the stringers.

Find the centre point and extend lines at right angles to each other across the centre point.

Then measure the radius on each arm and make a perpendicular mark on each of the radial arms. Depending on how long those lines were, you have something approaching a box.

Approximately center the drill over the center point. In this case it is a portable drill, but the principle is the same for a pillar drill.

Lower the drill bit over the hole. The radius marks allow the drill operator to see the edge of the hole and use any two of the marks to centre the drill bit within the hole.

This procedure ensure the accurate positioning of the hole.  This is especially important when fitting to existing fixing points rather than making new ones.

Wednesday, 27 June 2018

Over Annealing

 I hear the comment "you can't over anneal" all the time. Is it true?

My response to this may be controversial, and I do expect there will be some dispute with aspects of what follows.

My view of the statement “you can’t over anneal” is that it results from a lazy approach to thinking about the process.

The short answer is, in my view “yes, you can over anneal”.

  • ·         Lengthy anneal soaks can induce stress in certain circumstances. More later.
  • ·         Excessive annealing soaks waste time and money.
  • ·         Annealing is more than the soak.  It is a combination of equalisation of the heat within the glass (not just temperature) and the gradual cooling of the glass to below the lower strain point to ensure the glass does not incorporate differences of temperature of plus or minus 5°C.

There is both tradition and research to assist in determining the length of the anneal soak.  The tradition seems to embrace 30 minutes anneal soak for each layer of glass. The research has been done by Bullseye and they have developed a table to assist in accurately determining annealing soaks for thick glass. 

Although this is for thicker pieces, it will inform users of the relationship between thickness and annealing soaks.  The table starts at 12mm thick, but you can extrapolate that a 6mm flat piece cooling from both sides will need a one-hour soak, an initial cooling rate of 110°C, a secondary rate of 200°C.  It is safe to turn the kiln off at 370°C, as the kiln is unlikely to be able to cool faster than the 330°C suggested (although I programme to room temperature). The temperatures used need to be altered for glass other than Bullseye, but the rates remain valid. My advice is to use the research, rather than tradition.

Other considerations include the nature of the kiln.  If your kiln has significant temperature differentials across the shelf, long annealing soaks will incorporate those differences during the annealing cool and result in a stressed piece. You do know the temperature distribution within your kiln, don’t you?  This Tech Note #1 from Bullseye will give you the information to test for the temperature distribution. Using this information will enable you to avoid the cool spots when placing your pieces and utilise the areas where the heat is even.

Economy is another reason that it is possible to over anneal.  Soaking at the annealing temperature uses a significant proportion of the electricity consumed in a firing.  This means an overly long temperature equalisation soak will use more electricity than necessary.  It also uses more kiln time than necessary, by delaying the anneal cooling and the following natural cooling rate of the kiln.

It is possible to under anneal
You need to learn about the effects of your project on annealing requirements, because it is possible to under anneal.  The research on annealing is based glass of uniform thickness. The most popular style of kilnforming appears to be tack fusing of one degree or another.  This is unfortunate for the novice, as it is the most difficult of styles to anneal adequately. There are a lot of factors to consider when setting the annealing schedule. 

I feel this is the origin of “can’t over anneal” thinking.  Instead of thinking about the specific annealing difficulties, many seem to just add more time in a generally random manner.  The post on tack fusing considerations (the link above) is designed to help in thinking about the requirements of the lay-up of your piece. The cumulation of factors can easily treble the annealing soak and slow the rates by three times. In some extreme cases, the annealing time can be extended by as much as five times.

What is the anneal?
Another problem is that most often annealing is thought of as merely a soak at the annealing point of the glass.  It is much more than that.  The annealing point is usually the temperature at which the heat within the glass is equalised in preparation for the anneal cool.  This is because the annealing temperature is that at which the glass will most quickly anneal.  Since the anneal is temperature sensitive, the equalisation of the temperatures within the glass will be most successful at getting a good anneal throughout the cool.

For two-layer flat fused items, the annealing point can be used as the heat equalisation temperature.  The soak is to get the glass to be + or - 5°C throughout the piece. 

Sometimes, especially with thicker or more difficult pieces, the annealing is done closer to the lower strain point. The reason for this is to save time in the annealing cool.  If you look at the Bullseye annealing chart, you will see how slowly thick pieces need to be cooled, so starting 35°C below the annealing point can save many hours of cooling.

Once the glass has equalised in temperature, the object is to cool the glass at a rate that ensures the internal temperatures do not vary more than plus or minus 5°C across and through the piece.  The rate can increase by approximately twice after the lower strain point has been reached (approximately 55C below annealing soak).  This second stage rate should take the glass to around 370C, where the rate can again be doubled to room temperature. 

Difficult pieces
Tack fused and other pieces with uneven thicknesses require more care in the annealing to ensure even cooling of the whole without a greater variation in temperature than +/- 5°C.  As said above, tack fusing is one of the most difficult of styles to anneal adequately.  The blog entry for tack fusing considerations indicates some factors that increase the requirements for more careful annealing.

As an example, I cite a piece 6mm thick, with two layers of rectangular and pointed pieces that are just barely rounded.  This adds five factors of complications for the fusing - two levels of tack fusing, rectangular pieces, pointed pieces, laminated tack fusing.  This number of complications increases the practical thickness to 21mm – 6mm of flat base, 3mm each layer of tack (6mm), 3mm for rectangles, 3mm for pointed pieces, 3mm for laminated fuse.  Because this is tack fused, the next practical step up in the table needs to be used. That is the one for 25mm, which requires a four-hour temperature equalisation soak, and 15°C per hour initial anneal cool rate.

Glass other than Bullseye
I have so far talked about Bullseye.  It is possible to apply these times and rates to any glass of which you know the annealing point.  The annealing soak can be set above the lower strain point, which is approximately 55°C below the annealing point.  To be safe, a point 35°C below the annealing point is used.

E.g., if you are annealing a 12mm slab of float glass, the annealing point of which (in the UK) is 540°C, you chose a temperature of 505°C to do your two-hour soak, followed by a cool rate of 55°C for the first 55°C and then 99°C for the second stage cool to 370°C.  The final cool of 330°C per hour to room temperature remains the same too.  So, you can see the rates and soak times remain the same regardless of the glass type.  It is only the temperatures that change.

A summary of this can be seen here.

Bullseye chart for Annealing Thick Slabs

Unfamiliar terms can be searched for on the blog:

Wednesday, 20 June 2018

Pot Melt Formers

There are several suppliers of stainless steel and ceramic formers for pot melts.  They are not always necessary.

If you only want a circle, you do not need a former at all.  The shelf must be kiln washed and level.  The glass will pool in a circular manner ranging in thickness – thickest at the centre and 6-7mm at the edge. The variation in thickness depends on the time the glass is kept at the working temperature after the pot has emptied.

If you are wanting a thicker melt, you do need a dam of some sort.  You can purchase what you want, or you can make some from the materials you have at hand.

You can make a rectangle or square melt from existing straight dams.  You need to make sure the dams are kiln washed and lined with 3mm fibre paper.  You do not need to cut the dams to a predetermined length.  Instead, you can arrange them so that one end of the dam starts at the edge of your rectangle.  The next dam is butted at right angles to the first at the length wanted.  The other pieces are fitted similarly, until the last one passes the end of the first, so that they are butted together.  Then line with the fibre paper.  If you feel the dams are too light, you can back them up with bricks to prevent movement.

Using fibre paper, fibre board, or vermiculite board you can make any shape of melt that you can cut out of these materials.  If you don’t have refractory board, you can make your former out of layers of 3mm fibre paper.  It is possible to make a template for cutting of the multiple layers.  Cut your shape from the required number of layers of fibre to be as thick as your pot melt will become, according to your calculations.  Pin these layers together with stainless steel pins to be sure they do not move or float with the glass.  If you like, you can weight the layers of fibre paper with kiln furniture.

If you have refractory board – fibre or vermiculite – you can cut the required shape from them.  If you do not harden the fibre board, you do not need any further separator.  But you can line the shape with a thin fibre paper to ease the release and refine the edge.  Vermiculite always needs a separator, as it sticks to glass.  You should line the vermiculite board to get an easy release from the glass.

Using refractory materials releases you from the restrictions of commercially available forms and allows your imagination to take over.  It may not be cheaper than the bought ones, but will have the greater feeling of achievement.  In addition, you can develop all sorts of forms and depths not thought of by the commercial suppliers.

Sunday, 17 June 2018

Soldering Lead Came

Soldering lead came is different from soldering electronics or copper foil. Less heat is needed, cleanliness is all important, suitable flux is required, the iron is held differently, among other things.

The lead needs to be clean and bright to start with. If it's fairly new lead it should be solder-able without more than a scrubbing of the joints with a brass wire brush. However, if the lead is dull and oxidized, you should scrape the lead in the area to be soldered with a nail, the blade of a lead knife or other sharp edged tool until the bright metal is revealed.

an example of paste flux
Example of a tallow stick.  It has the appearance of a candle, but without the wick.

Example of the application of tallow to a joint

Then the flux can be applied.  Paste flux or tallow works best as neither flows in its cold state.  This means that you can flux the whole panel at one time without the liquid flowing away or drying.  Once the whole panel is fluxed, you do not need to stop during the soldering process.

Ecxmple of a gas powered soldering iron. The flat face of the soldering bolt is held in full contact with the joint.

An electric soldering iron is held over-handed (as you would a bread knife) in order to get the handle low enough to have the tip flat on the lead. This will be a 15 to 20 degree elevation from the horizontal. Allow the weight of the soldering iron to do the work for you. 

Let it rest on the joint after you apply the solder between the lead and the iron. In order to heat both pieces of lead you may have to rock the tip slightly to contact all leads being soldered. Take the solder away from the iron so it doesn't become attached to the joint. As soon as the solder spreads, lift the iron straight up. This process will take only a few seconds, less than 5.

Example of smooth flat solder joints.

Avoid "painting" or dragging the iron across the joint. The object is to have a shiny, smooth, slightly rounded solder joint. Moving the iron and solder around does two things.  It makes for a weak joint as the solder does not have the chance to become stable and so forms a "pasty" joint.  Moving the iron around during the soldering of the joint often provides sharp points where the iron was moved quickly off the joing. There should be no points sticking up from the solder joint. If a solder joint is not satisfactory you can re-flux and re-heat. Don't apply too much solder. It's easier to add more solder than to remove excess.

Wednesday, 13 June 2018

Volume control

Glass has a surface tension (viscosity) that draws the glass toward 6-7 mm thick at kiln forming temperatures. 

To test this out, prepare three stacks of glass squares.  They all should be the same size.  Place them in a stack of one, a stack of two and the last of three squares.  Fire them to a full fuse.  Compare the sizes of the original to see the expanded size of the three-layer stack, the same size of the two-layer stack and the reduced footprint  and dog-boning of the single layer.

Glass in a single layer behaves differently from the thicker set-ups. When the glass is hot it begins thickening at the edges because the viscosity is drawing the glass both from the edge and from the centre.  This means the footprint of the glass is getting smaller. The result is needling, as the glass retreats leaving small threads where the glass was held in small imperfections in the separator’s surface. 

If you do not need a full fuse, you can reduce this needling effect by reducing the temperature and extending the soak.  This means that the glass does not expand on the heat up so much and the greater viscosity reduces the needling effect.

If you need a thick piece of a certain size, you need to dam the glass to overcome the tendency to expand.  With experience, you can get to know how much a three-layer (or more) set up will expand and cut the glass accordingly.  In this way, you can often do without dams, although there will be some thinning at the edges in relation the centre and a rounding of the corners.

An excellent document on volume control is the Bullseye Tech Note 5.  

Note that this 6mm rule applies at normal kilnforming temperatures.

At higher temperatures, the viscosity is less so the glass becomes thinner.  My experience has shown that at around 1200°C the glass will spread to about 0.5mm thickness.  This is just to point out there is an relationship between temperature and viscosity, and therefore thickness. This relationship between temperature and viscosity -  as the temperature rises, so the viscosity reduces - allows the glass to become thinner.  At normal kilnforming temperatures, the 6mm rule applies, at higher temperatures it does not.

Saturday, 9 June 2018

Dressing the Came

This is a different kind of dressing than what you do in the morning. It relates to how easy it is to slide the glass into the channel of the came.

If you have consistent difficulty in sliding the glass into the came, you should consider dressing the came before use. Dressing the came consists of running a fid or other hard material along each of the four flanges of the came. In doing this, you are pressing each flange in turn down against the bench or other smooth surface.

Dressing the cames gives a slight bevel or ramp for the glass to slide over the edge of the came and into the channel of the came. You can dress the whole length at once, or as you cut the pieces off from the main length. Dressing shorter pieces is less likely to bend the came away from the straight.

Wednesday, 6 June 2018


Observation is desirable for learning and essential for several processes.  How you do that is important to the safety and health of your eyes and skin.  This post gives some guidance on the protections required.

Each observation should take a fraction of a second. It is called peeking to distinguish it from looking or watching. There is a method to doing this. Think about what you are looking for before you open the kiln. Pop open the kiln to record with your eyes, close the kiln. Think about what you saw. If necessary, repeat.  But only after you have thought about what you saw.

Do not spend time looking into the kiln.

Think about the necessity for observation before buying your first or next kiln. 

The best kilns are those with generous observation ports, both in number and size.  These allow you to peek into the kiln without disturbing the heat distribution within the kiln.  Two or more ports are best, as you can shine a light into one of them to illuminate the interior of the kiln at lower slumping temperatures.

If you do not have ports you will need to open the kiln. This is easiest to do with top hat kind of kiln.  The top hat kiln keeps a lot of heat in the upper portion of the lid, making the amount of heat dumped less than on other kinds of opening.  You can peek in at the level of the shelf, so minimising the amount of heat being dumped.

The problems with lids opening so you have to peer down into the body of the kiln, and with doors opening to the front, is that you are dumping a lot of heat directly at yourself.  You also are losing a significant amount of heat from the kiln.  The large air exchange also will disturb any dust in the kiln and that may fall onto your work.

It is possible to make ports in your kiln by drilling a large diameter hole in the side of the kiln and through the insulating material (assuming you do not have side elements).  This post gives some ideas.

In all the cases where it is necessary to open a lid or door, you must close the kiln slowly and gently to minimise disturbance of the air within the kiln.

The effects on the glass of peeking at various temperature ranges varies between the rise and the fall in temperature.  This post 

Pricing Classes

There are lots of things related to preparing to give instruction whether for a few hours or days.  This is only about the easier element – calculating the costs.

Costs – these are both variable and fixed costs.  These are important in calculating costs per student.

Variable costs. These are the expenses you have no matter how many may attend. E.g.:
Tools – how many people do you intend to cater for?  What kind of tools do you need? Does everyone each need the tool? Can some doubling up occur?

Advertising, promotion – The costs in printing and time to distribute leaflets and other promotional efforts.  Costs of advertising online or in print.

Course Preparation and delivery time – Every course requires time to plan the course and organise the materials, take bookings, set up at venue.  The amount of time you spend delivering the course/class needs to be claimed.

Travel expenses and time to get to the venue.  Mileage expenses only cover the transport costs, it does not include your time.

Venue costs – If you have been invited, check the venue is free to you.  In some cases, you will have to pay for the venue and this needs to be added to your variable costs.  In this case, I am assuming the host is providing the venue free.  If it is your own venue, you should add a sum to cover the overheads of your premises at the very least.

Fixed costs. These are the costs for each person on the class/course.

Materials and consumables. The materials the students will use varies directly with the number of people.  Any provision of food, refreshments will depend on the number.  Any handouts or demonstration materials are also related to the number of people.

Price.  The price of the course/class will depend on the costs and the profit you wish to have as well as what the market will stand.

Working out the Costs
I will go through the main areas of costs and give examples to demonstrate.  Not all these areas will apply.  When they don’t the cost is zero.

You can estimate the number of classes over which the tools and any other equipment will last. 

Say, you plan on 6 students and the cost of the tools for that many will cost 440.00 and you expect the tools to last for 10 courses. (you will have to top up on tools due to loss during that period, so you may want to add an additional sum to your calculations.)  Ten classes of 6 gives you 60 people to spread the cost across.  This gives you an individual cost of 7.34 on this basis.
Cost sub total:  7.34/student

Promotion and administration
The promotional effort includes time to prepare, costs to produce and time to distribute.  You can prepare the advertising before the pricing begins, so that amount of time can be known before you start.  The costs of printing or publishing can be determined by getting estimates.  The amount of time you spend in distributing leaflets, and in using the internet to advertise needs to be included in the promotional time. 

I am going to assume your administration of the applications and payments for the course are taken by the venue.  If you do them, you need to add a notional amount of time to cover that aspect of delivering a course.

Say, you have determined the printing and advertising is going to be limited to 50.00.  In addition, you are going to spend 5 hours distributing the printed leaflets and 10 hours of social media time promoting the classes.

As you can see, your time is going to be a large part of this promotional work, so this is the occasion you must decide how much you are going to pay yourself.  There are ways to do this.  Here is one.

Say you have decided that you need to pay yourself 20.00 per hour.  This is the figure that needs to be applied to all the time-based costs you incur in preparing and delivering the class.

The promotional costs for the course are going to be 50.00 plus the 15 hours (distributing leaflets and 10 hours social media) at 20.00 (=300.00) totalling 350.00.  Will this promotion be useful for subsequent classes? Probably not.  This means that you can’t spread the cost over more than one course unless you redesign the materials in some way.  It may be that subsequent classes will require less effort to promote them, but don’t count on it.

Assume your promotion cannot be spread over more than one course, which is the most likely.  This means the cost for each of the six people is 58.34
Cumulative cost sub total:  65.68/student

You need to prepare a course syllabus or outline at least.  This will guide you in the presentation of the class and will help you determine the equipment and materials your students will need.  Any additional materials and equipment needed for the course will need to be added to your initial equipment calculations.  You need to keep track of the amount of time you spend on designing the course.

You may have spent 10 hours researching and writing your course.  You can decide that little alteration of the course will be required over the life of the equipment.  This then becomes 200.00 divided by 60 students or 3.34 per student.  You may also have course handouts.  These will be fixed costs as they relate to each student.

Cumulative cost sub total:  69.02/student

Some of these promotional and preparation costs will have to be guesses until experience is gained. How long will the advertising last? One class or more? Will you need additional preparation time each time the class is held?  The answers you give will affect the calculations by the number of students or classes the cost is distributed.

Travel and accommodation
Of course, The travel expenses and time, and the class delivery time will remain relatively constant; varied only by the distance and the facilities at the venue.

The venue is 25 miles away. The travel time is 45 minutes each way. The set-up time is 30 minutes.  The course is for 4 hours and clean-up is 30 minutes.  This means that at 0.50 per mile the expense is 25.00; the travel time is 1.5 hours (1.5*20.00) gives an expense of 30.00 to get to and from the venue.  Set-up and clean-up is 20.00. Class delivery time of 4 hours equals 80.00.  All these on the day costs are 155.00.  For six students, this will be a cost of 25.83 each.
Cumulative variable cost sub total:  94.85/student

Of course, your accommodation expenses for a multiple day course will need to be added, although not the time between class sessions.

Fixed costs are the ones directly related to each student.  These will vary according to venue, style, length etc.

Materials/consumables. The materials the students will use varies directly with the number of people.  The costs of this relate to the materials the students will consume during the class/course.
The glass used by each student will cost 20.00
Additional consumables will be 10.00
Firings for each student will be 5.00
fixed cost sub total:  35.00/student

Hospitality - provision of food, refreshments - will depend on the number. 
Refreshments at the beginning and middle of the course 5.00 each
fixed cost sub total:  40.00/student

Any handouts or demonstration materials are also related to the number of people.
Handouts and examples for students – 10.00
fixed cost sub total:  50.00/student

Student accommodation – this is relevant for those who need to stay overnight either because of their travel or the length of the course.  Normally, this is the responsibility of the student.  It may be that you wish to include the cost of this in the course fee, although that usually makes the course appear to be very expensive.  In this case, I will assume accommodation is the responsibility of the student.

Cumulative variable cost sub total:  94.85/student
Cumulative fixed cost sub total:  50.00/student
Total cost for a one-day, six-person class:  144.85/per student

You do need to make a profit on this class, or you can’t continue.  You can’t continue with an “at cost” basis, because this is time away from making where you can generate an income.  A small profit margin of 20% is the minimum. It does give you some recompense for your knowledge and provides a small margin for contingencies.

A margin of 20% on the above example would add 28.97 per student.
Cumulative cost total:  173.82/student

If you feel this does not represent good value for your experience and knowledge, increase the price.  This exercise only provides the base cost level for setting the price.  Also remember that it is easier to reduce prices than it is to increase them.

On the other hand, you may decide that your hourly charges are sufficient profit for the course.  This is not advisable, but is a choice you can make.  You should always be aiming for doing your work on a cost, plus profit basis.  Not simply covering material costs and expenses. Remember your time is also a cost factor.

If you were to feel this is too expensive, there are some ways to reduce expenditure.  The only ones that you cannot reduce are your hourly rate and the profit margin.

Possible reductions include:
Tools and equipment – get the venue to share costs or underwrite costs.
Promotion – reduce your expenditure, or get the venue to take all or most of the promotion costs.
Venue – get them to set-up and clean-up, or better, get the students to do these things.  Get travel expenses from the venue.  Move the venue closer.
Accommodation – get the venue to provide at their cost.
Student numbers – get more students which will reduce the variable cost per student.

But – to repeat – do not reduce your hourly rate, ever.

This is a general introduction to costing, profit, and pricing.  There are a lot of more sophisticated ways of calculating these things, but until a lot of experience is gained, most of the work will be from estimates and guesses.  So, investing in highly detailed methods will not make the pricing more accurate, as all the calculations are based on estimates.