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.

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.  Record the measurements. 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 the fired. Note 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.

Credit: Paul Tarlow

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

If you do not need a full fuse, you can reduce this needling effect. Reduce the temperature and extend 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. There will be some thinning at the edges 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 will become thinner than 6-7mm.  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 a relationship between temperature and viscosity, and therefore thickness. As the temperature rises, so the viscosity reduces. This relationship allows the glass to become thinner.  At normal kilnforming temperatures, the 6mm rule applies, at higher temperatures it does not.

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

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.

Peeking

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 

gives some ideas of the effects.

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.

Equipment

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

Example

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.

Example

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

Preparation

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.

Example

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.

Example

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.

Example

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. 

Example

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.

Example

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

Profit

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.

Example.

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.

Home Made Devitrification sprays

You can buy a number of devitrification sprays. Some of them are lead bearing and will not be suitable for food and drink containers. Many times people apply them before firing the first time to prevent devitrification. More often these sprays are applied after a piece has become devitrified. However it is applied, these sprays are not cheap.

borax in powder form

It is possible to make your own devitrification solution. It is made from borax which you can buy from your local chemicals supplier, or sometimes as a washing powder – but make sure it has no additives! 

An example of a borax washing powder

 To make a solution, boil a few cups of water. Take the water off the boil and put in 4 – 5 tablespoons of borax. Stir and allow to stand until cool. Pour off the clear liquid and you have a saturated solution of borax. The sediment in the bottom can be added to more hot water to make more of the borax solution.  You will have to break up the remaining crystals of borax to enable suspension in the hot water.

Add a couple of drops of washing up liquid to the solution. This is enough to break the solution's surface tension. It helps to give an even distribution of the solution across the clean glass by reducing the surface tension and therefore, beading of the liquid that otherwise occurs.

You can spray this solution onto the glass, just as the commercial sprays.  Or you can brush it on as you do kiln wash on a shelf.  It requires an even application to ensure there are no streaks left on the finished glass. 

This works because borax is one of the fluxes used in glass making to reduce the melting temperature of glass batch and so serves to soften the surface of the glass enough to overcome mild devitrification.

Permanance 

Description of devitrification
Temperature range

https://glasstips.blogspot.com/2016/02/borax-characteristics.html

https://glasstips.blogspot.com/2009/06/borax-solutions.html

Revised June 2018

Kiln Wash

Kiln forming techniques require separators between the glass and the shelf or mould on which it rests during the heating process. These separators have different generic names – kiln wash and batt wash are two.

There are a number of brands of kiln wash. All of them contain two main ingredients – alumina hydrate (sometimes called slaked alumina) and kaolin (also called china clay). Different producers use these ingredients in various proportions. 

A number of makes also include a colourant that changes when fired above certain temperatures to indicate the wash has been fired.  It also distinguishes between the unfired and the already fired kiln washed shelves.

An important thing to remember is that the kaolin changes its composition once it is fired over 600C/1113F. This change of composition is completed by 900C/1620F.  The change is progressive.  It is so slow that slumping and draping moulds coated with kiln wash will last indefinitely. However this change is great enough by 770C/1419F that the kiln wash sticks to the glass on the next firing. Thus, it is essential to change the kiln wash after every firing that reaches tack fusing temperatures or higher.

It is possible to apply a fresh coat of kiln wash over the old one to save time. However, as soon as the kiln wash flakes you must scrape off all the old kiln wash and apply a new coat to the bare shelf or mould.

Some makers use much less of the binder (china clay) than others which makes them better for the popular casting moulds than those for shelves and slumping moulds as they can be brushed away without abrasion.


In addition, boron nitride is a suitable release from moulds.  It is very stable at reltively high temperatures and so can provide a smooth, "slippery" separator between the glass and its supports, whether shelves, moulds or kiln furniture.  It does seal porous surfaces, meaning that air cannot move through the treated surfaces.  It has to be removed with abrasion and so thought must be given to which surfaces it is applied.

Polarising Filters

Using polarized light filters to show stress works on the principle that stressed glass rotates the polarisation direction of the light as it comes through the glass. As polarized light filters placed at right angles do not allow any light through, only unstressed glass will continue to appear dark. 

If there is stress the light is rotated slightly and becomes visible through the filters.  

You can buy stress testing kits that incorporate a light source. You can also make your own. You need polarizing lighting gels. These come in sheets and are available from theatrical lighting sources. You will need to frame these in stiff card to keep them flat.

You use them over a light source. Place one filter down above the light source. Place the piece to be tested on top. Then orient the top filter so that the minimum amount of light shows through the filters. Any stress will show up as a light source.  The amount of light rotation depends on the stress direction, magnitude and light path length. The greater the intensity of the glow, the greater the stress the glass is exhibiting.   The amount light visible through the filters is wavelength dependent, as the filter transmits light with a particular polarisation direction. If there is large stress, different colours will be visible. 

This example shows extreme stress by the rainbow effect of light rotated in multiple directions

Note that the surface through which the light comes should be rigid, as any deformation of the surface will give a false reading.  The light filters through the slight curve and gives a stress reading, which may not be true at all.  Thus a firm flat surface is required, especially if you have a large light table for your light source.

Also note that the filters are normally on plastic sheets and easily scratched, so the glass should always be lifted and placed, rather than slid, to a new position.

A description of the compatibility test can be seen here.

revised June 2018

Cutting Lead Came

Cutting came is a gentle process rather than an abrupt chopping effort.
There are at least three kinds of implements in common use to cut lead came.

Lead nippers or lead dykes
Lead nippers/dykes are a kind of adapted side cutters, used for cutting wire and by electricians. But these have the bevel only on one side of the jaws, making them almost useless for anything other than cutting lead. This arrangement only crushes the lead on the cut-off side and also leaves a minimum of lead next to the back of the jaws.

In use, the jaws of the dykes are aligned in the same angle as the heart of the lead, cutting across the leaves of the lead. They do not cut from the top and bottom of the came. These are very quick for right angle or very oblique angles on the came. However they are of little use for acute angles.

Lead knives
For more acute angles, blades are more commonly used. These can be either straight edges or curved blades. The straight edge lead knives are essentially putty knives or stiff scrapers sharpened to an acute angle. This kind of knife is normally wiggled from side to side while applying pressure to work through the came.

Other knives are curved to make rocking back and forth easier. There are a variety of knives such as the Pro or Don Carlos. Some look more like a scimitar than a lead knife! These are used to rock along the line where you are cutting the came.

Whatever kind of knife you are using, be sure to be directly above the knife, looking along the blade to ensure vertical cuts.

Saws
Of course, saws are sometimes used. The blade needs to be coarse toothed to enable the soft lead to drop out of the teeth. These saws can be hand held or table saws. Normally, it is quicker to use lead dykes or knives. However, if you are in production mode, a powered table saw may be worthwhile.

Sal Ammoniac

There are sometimes concerns expressed about the use of sal ammoniac to clean the tips of soldering irons.  My conclusion is that there are no elements of the block that will affect the copper plating of the soldering iron bolt.  It is safe to use this as an occasional cleaning method of soldering iron bolts. This is based on the following information.

What it is

The common term, sal ammoniac, refers to the chemical ammonium chloride.  Sal ammoniac is the archaic name for it. The Romans named it from the ammonium chloride deposits that they collected from near the Temple of Jupiter Amun in ancient Libya.  It is found as encrustations around volcanic fumaroles, guano deposits and in burning coal seams. Notable occurrences include Tajikistan; Mount Vesuvius, Italy; and Parícutin, Michoacan, Mexico.

Wikipedia

Ammonium chloride is the product from the reaction of hydrochloric acid and ammonia.  Ammonium chloride is obtained as a by-product in different chemical processes.  It consists of white crystals that are also available in rods or lumps.  The substance changes directly from being solid to gas with no intermediate liquid state. The gas does not consist of ammonium chloride molecules but ammonia and hydrogen chloride. This shows that the salt decomposes easily. When stored, ammonia is continuously emitted and the substance gradually becomes more acidic.

https://www.fishersci.co.uk/shop/products/ammonium-chloride-99-6-analysis-acs-acros-organics-3/p-3586389

Safety

It is widely used in human medicines as an expectorant, diuretic, etc. and in veterinary medicines to reduce gallstones, so it is a relatively benign material in relation to human health. 

There are some hazards though.  It can cause serious eye irritation on prolonged exposure, and is harmful if swallowed.  The precautions are to avoid eating, smoking, and drinking when using it.  Use gloves and eye protection if you are using it for extended periods. If it gets into your eyes, rinse with water for several minutes. https://www.fishersci.co.uk/shop/products/ammonium-chloride-99-6-analysis-acs-acros-organics-3/p-3586389

It is highly soluble in water, and forms a slightly acidic solution. Its main characteristic that you need to protect yourself against is that it vaporizes without melting at 340 °C to form equal volumes of ammonia and hydrogen chloride gas. https://www.britannica.com/science/ammonium-chloride

The amounts of the gas are small when used to clean soldering irons, but as the gas forms hydrochloric acid in contact with moisture, you should use dust masks rated for inorganic acids.  The amounts are small and generally only cause sneezing and coughing upon contact.

The primary hazard is the threat posed to the environment. Immediate steps should be taken to limit its spread to the environment.

https://pubchem.ncbi.nlm.nih.gov/compound/ammonium_chloride#section=Top

Uses

In addition to medicine, it is used to clean soldering irons. It has uses in jewellery-making and the refining of precious metals.  Sal ammoniac has also been used in the past in bakery products to give cookies a very crisp texture.  In some areas, particularly Nordic countries and the Netherlands, it is still widely used in the production of a salty licorice candy known as Salmiak, or Salmiakki.  Formerly it was used as the electrolyte in dry batteries.  It has uses in fertiliser as a source of nitrogen mostly for rice and wheat crops in Asia. It is also an ingredient in fireworks, safety matches, contact explosives, cosmetics and many other applications.

https://thechemco.com/chemical/ammonium-chloride/

Conclusion

Although there are some mild safety precautions that need to be followed, there is nothing in the sal ammoniac block that can harm the copper coating of the soldering iron tip.

Thermal Shocking Ceramics

When firing glass in ceramic moulds, and especially ceramic pots for pot melts, you should be aware of the temperatures at which the ceramic material quickly expands and contracts.

There are refractory ceramics which are not as sensitive as the kind of ceramics we are using in most kiln work.  The ceramics we use are not refractory materials and contain, among other things, quartz and crystobalite. These two elements are important, as they have considerable effect on the survival of the pot or mould during the firing.

The effects are called inversions.  This is because the rapid expansion experienced upon the heating is reversed as rapid contraction on the cooling of the ceramic.

The first element to be affected by the heat up is crystobalite.  This element has a sudden expansion of 2.5% at 226°C.  This does not seem to be much, but compare it to the expansion of glass at this temperature - .0085% - almost 300 times that of glass at the same temperature.  And of course, the ceramic contracts by that amount when it reaches 226°C on the cooling.

The second element affecting the heat up is quartz.  There is quite a bit of this in clay.  The critical temperature for this is in the 570°C to 580°C range.  The expansion and contraction is not so great here – only 1% - but it is still more than 100% that of the glass, and in a critical range for the glass on the cooling.   

More information on the quartz and crystobalite inversions are given here.

The importance of these inversions for us are to remind us to be careful at these temperatures of about 225°C and 570°C - 580°C to prolong the life of the ceramic pots and moulds that we use.  

It is probable that 150°C per hour is as quickly as we should increase the temperature when using ceramic moulds or pots.  Some thought should be given to the cooling of the moulds too.  They should not be taken from the kiln while hot nor subjected to draughts of relatively cold air.

Leading Small Circles

Putting came around small circles such as lenses and small bullions often leaves an irregular curve. There is a way to avoid this.

Use oval or round came to reduce the kinking of the leaves of the came. As there is less material at the edges of the leaves of oval came, there is less kinking than on flat came, where the thickness of the leaves is constant.

Begin to form the lead round the circle, about half way. Then take the circle out of the came and cut, at a right angle to the length of the lead, at an angle from top to bottom. The degree of the angle is not important at this stage, only that you can repeat the angle – so it must be fairly shallow and natural for you.

Put the circle back into the came and continue to form the came round it until you meet the angled cut at the beginning. Again at right angles to the length of the came, cut a repeat of the angle.

Then fold this end toward the other end. Push the two angled ends together. If they slip up and down from each other, the came is too long. Open the came and cut a sliver off.

Try again until they meet with very little “slippage”.

Then the piece is ready to put into the panel. Place the join at a lead joint so you don't have an additional solder spot.

This technique can be used for small ovals too.

Tin Bloom

Using float glass sometimes produces partial clouding as though devitrification were present. Although float glass is prone to devitrification, not all the cloudy film on the surface is due to devitrification.

Float glass, which these days, is almost all clear smooth glass, gets its name from the process of floating the glass on molten tin. The tin in compression gives an apparent devitrification effect which is called tin bloom.

it is different from devitrification, to which float glass is particularly subject. Devitrification sprays and solutions will not have an effect on this surface defect called tin bloom. 

When the tin layer is stretched, it does not create a tin bloom on the surface.  Therefore, it is important to have a means to detect which is the tin surface.  Always fire the glass with the tin in the same relative location to each other.  I.e., on several layers of glass have all the tin side down or all up, but not mixed. 

This example of a test by Glass Art by Margot shows the tin bloom on the outer portions of the platter where the tin side was up, causing the tin too be compressed and show.  The flatter portion of the piece did not show this tin bloom as there was not the same extent of compression. You can visit the description of the experiment here.


When forming the glass (slumping, draping, kiln carving) make sure the tin sides will be stretched rather than compressed.  Of course, you can take advantage of the tin bloom by controlling the compression of the tin layers.

Leading Procedure

Cut the leads exactly as the cartoon indicates. In other words, where one line runs into another, that is generally a stopping/starting point for the came.

Always lead to the cartoon line, not the glass. This ensures accurate completion of the panel. If the glass is slightly too small, the cement will take up the gap (assuming the flange of the came covers the glass – if not, you need to cut another piece of glass that fits). If the glass overlaps the cut line, it needs to be reduced.  A description of the process is given here.

This shows the use of a guage to determine where to cut the horizontal lead came.

Cut the ends of the came shorter than the glass. The best way to determine this is to place a piece of came of the dimensions being used for the next edge on the cut line. Use it to determine the length and angle for the cut. The object is to have each piece of came butt squarely against the passing came, to make a strong panel and to make soldering easier.