Quoting for Fused Glass Commissions

When quoting on a fused glass commission, what are all the factors to consider?

Commission for Glasgow University

Quote the same way as for leaded or copper foil.  But if you don’t work in those forms, that statement will not be much help.

The elements to consider are:

·        Design time and value (making sure you retain the copyright of the design).

·        Amount of time to assemble. You need to think clearly about how long it really takes.  You need to be charging a reasonable amount for your time. Think about skilled trades people’s charges and that you have additional artistic skills.

·        Amount of glass to be purchased (rather than used) to make the piece, even if much is from stock – you must replace it after all.

·        Number and cost of kiln firings.  Be clear about how many firings might be required, if something does not work out first time.  Be clear about how much each firing costs including depreciation on the kiln.

·        Incidental supplies.  All the little things that are necessary to supply your practice, such as art materials, kiln supplies, etc.

·        Overheads. This is the cost to run your practice.  If the studio is part of the home premises, add a proportion of the running expenses of the house to the cost.  The cost of business - advertising, promotion, printing, etc., all need to be included.

·        Profit. You do need to make a profit to stay in business. Decide what that is and add that percentage to the cost.

·        Allowance for contingencies (20% of the price already determined is usual).

·        Delivery/installation costs (normally in addition to the cost of design and making).

It is advisable to find out what the client’s budget for the commission is before starting any designing.  If it is too small for their specification, decline the commission.  Otherwise, you can design to the budget.  A large budget allows expansive or highly detailed works.  A small budget restricts the size or detail possible.

Some people charge more for a commission. Some, like me charge less, as I am getting most of the money up front, rather than maybe sometime in the future.  Cash is important.

Some artists take 1/3 to make the design, 1/3 on approval of design, and final 1/3 on completion. This is widely used in the interior design field. You may want to consider requiring a non-refundable deposit of one third to make a start and the remaining two thirds on completion as an alternative. 

A contract of some sort is essential.  It needs to cover the expectations of both parties.  Cost, of course.  When is it to be completed? Requests for colours, shapes, location, style, etc.  If the client wants approval at various stages, you need to either state what these stages are, or more sensibly, decline the commission. 

The contract does not have to be legalistic.  It can be a letter stating the terms of the commission that is sent by you to the client and acknowledged by them.

Determining the price for a commission requires consideration of the costs of time and materials, and the values of what you do.  A contract of some sort is required. It can be a simple letter with a statement of the agreed conditions.

Annealing Range for Unknown Glass

It is possible to anneal unknown glass with some degree of certainty by using what is known as the slump point test.  This will not be as accurate as a factory determined test, so you do have to extend the range over which you do the annealing.  

The annealing of glass with unknown characteristics is possible in two ways - shotgun and calculated.  The examples here are for 6mm thick glass.  The soak and cooling times need to be extended for thicker glass.  

Both the shotgun and calculated approaches exemplified here assume glass of 6mm thickness.  For thicker glass the soak time needs to be extended and the anneal cool rate slowed more than indicated above.  Using the Bullseye chart for annealing thick slabs will give you an indication of the relationship of thickness to speed.

1)  One is the traditional shotgun approach – pick an arbitrary, but slightly high temperature, and soak for a minimal amount of time there. Then go very slowly through the next 55°C.  This may be as slow as 25°C per hour, followed by a doubling of that rate for the next 55°C. Then double again to 300°C or less.

2)  By using the slump point test and the calculations, you will be sure of the annealing point/temperature equalisation point within 10°C.  The approach here would be to soak for half an hour at the calculated temperature, followed by a slow drop of 50°C per hour to 55°C below annealing soak and then at 100°C/hr to 110°C below your chosen temperature equalisation point. The final cooling could be at 200°C to room temperature.

2a) An additional tweak to the slump point test calculations is to use the Bullseye concept behind their recommendations for thick slabs.  Using their concept, you reduce the calculated annealing point by 30°C from the calculated annealing point to do the temperature equalisation soak at the lower end of the annealing range.  Having calculated the annealing point, you reduce that temperature by 30°C and soak for  a longer time of 60 minutes and at a slower rate as noted in the chart.

In using the chart for unknown glass you substitute the calculated temperatures, but continue to use the rates and times indicated.  An example:

• You have calculated that the annealing point is approximately 535°C.
• Subtract 30°C from that to get a equalisation temperature of 505°C.
• Assume the piece is uniformly 12mm thick or 6mm tack fused (when you want to use rates for  twice the actual thickness to account for the difficulties in tack fusing). 
• For a 12mm thick piece your soak time at 505°C will be two hours.
• The cooling rate for the first 55°C is given as 55°C per hour according to the chart. Therefore the first cooling segment will be 55°C from 505°C to 450°C.  The second will be 99°C per hour from 450°C to 395°C.  The third rate will be 330°C per hour from 395°C to room temperature.

You can see that the times and rates are taken as given by the chart (as determined by the thickness of your piece), but the temperature set points are determined by the calculations for the glass you have tested.

When determining what temperature you should use to anneal a glass about which you are uncertain of its characteristics, you can use one of two basic approaches.  Pick an arbitrary temperature and soak for some time there and then proceed slowly in 55°C segments to about 370°C.  A second more certain method is to use the slump point test to determine the annealing point and then apply the Bullseye chart for thick slabs for the soak times and cooling rates.

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

Firing Glass Near the Shelf Edge

“How close to the edge of my shelf can I place a large piece?”

It depends in one sense how thick the piece is.  A 6mm piece that maintains the same footprint after firing as before, does expand beyond that footprint by about half a centimetre during the firing, so it would be safe to have a full centimetre space to the edge.  Thicker pieces will need more space – 9mm will need about two centimetres to accommodate the expansion at the top temperature. 

But

The real answer to this question is: When you know the heat characteristics across your shelf, you will know how close you can go to the edge for a relatively large piece. 

This Bullseye Tech Note number 1 tells you how to test the variations of temperature across your kiln. - http://www.bullseyeglass.com/methods-ideas/technotes-1-knowing-your-kiln.html

The objective in cooling glass is to have less than a 5C difference in temperature over the whole of the glass piece – top to bottom, and side to side.

If you have greater differences in temperature in the middle than that at the edges of your kiln shelf, you need to avoid placing large pieces in the danger area. Small pieces will not suffer by being close to the edges as their temperature differentials will be small.

I have found that the temperature differential in one of my kilns is great enough at the edges that I cannot have the edge of a relatively large piece of glass nearer than 50mm (2") from the edge.

Sand Beds for Kilns

Sand beds can be used for shaping directly into the sand or as a bed for a large kiln to avoid having to place a number of shelves together and patch cracks. It also saves on the purchase of a large shelf, at the expense of some labour before each firing.

Mix (by weight)

The mix is largely sand with powders to coat the sand particles to reduce the amount of sticking to glass. The mix ranges from 75% sand 25% alumina hydrate, up to 67% sand, 33% alumina. The proportions can be adjusted by experience.

Some kiln formers include plaster or china clay (kaolin) in various proportions so the whole mix can range from 70% sand, 25% alumina, 5% plaster/kaolin up to 60% sand, 20% each alumina and plaster/kaolin. Some have found that the plaster started to scum up the glass after a number of firings, and this can happen with china clay too.

Use

The use of sand as the bed of the kiln requires screeding of the sand before each firing. It is possible to smooth the sand with a plaster’s float. This presses the sand down and allows a fine film of powder to coat the surface. A more certain way of avoiding the sand sticking to or texturing the glass is to lay fibre paper over the sand.

You should be aware that a sand bed requires a little different cooling than glass on a suspended shelf does. Effectively you are cooling the glass on the bed of the kiln. Thus, you should use annealing and cooling schedules for one or two thicknesses more than is actually being fired to be really safe.

Sand forming

If you are using the sand for mild shaping, sprinkle alumina hydrate over formed sand to reduce sand particle take up and textures onto the glass.

Renew sand regularly if you are using powders on the sand. An annual renewal using half the old sand mix and the rest new will be sufficient in most cases. The effect of too much powder is to promote large bubbles as air cannot move through the sand with the same ease as when there is less powder. An easy way to tell if you have too much powder is that the mix flows ahead as you screed it.

Revised 22/6/19

Iridescence

What is it?       How permanent is it?

“Many special effects can be applied to glass to affect its colour and overall appearance. Iridescent glass … is made by adding metallic compounds to the glass or by spraying the surface with stannous chloride or lead chloride and reheating it in a reducing atmosphere.” 

Older glass can appear iridised because of the light reflection through the layers of weathering.

“Dichroic glass is an iridescent effect in which the glass appears to be different colours, depending on the angle from which it is viewed. This effect is caused by applying very thin layers of colloidal metals (e.g., gold or silver) to the glass.”

Source: https://www.thoughtco.com/

A rainbow iridescent appearance caused by an oil film on water is seen by light being reflected from both the top oil surface and the underlying water surface.  The light reflected from these two surfaces or boundaries have slightly different wave times and so interfere with each other to create this colourful pattern.

This is also observed in soap bubbles.  Here the light is reflecting from both the inner and outer surfaces of the film.

This iridescent appearance is termed thin-film interference.  It is an occurrence in nature where there is a thin film through which light can penetrate and so reflect off the surfaces of the film.  These surfaces are termed boundaries where the light can reflect. 

The thickness of the film can enhance or reduce the iridised effect. 

At a certain thickness the light waves reflected can cancel each other out.  This is described as a destructive interference pattern as it reduces the reflection.  The phenomenon can be used to provide non-reflective surfaces.

At other thicknesses there is an iridised effect.  This is caused by the reinforcement of the recombination of the two light waves reflecting in phase or nearly so.

Control of the thickness can give the silver or the gold iridised appearance, as in the Bullseye iridised glasses, in addition to the rainbow and other effects.

The nature of the light affects the colours of the iridescence.  If the light is daylight or similar it is a combination of many wavelengths.  The different wavelengths reflecting from the “boundaries” or surfaces provide the multiplicity of colour.  If the film has variations in thickness, there will be variations in the colours created.

A diagram from Wikipedia shows how the reflections work at the microscopic level.

https://en.wikipedia.org/wiki/Thin-film_interference

The permanence of the film causing the iridisation appears to be dependent on the metals used and the way in which they are deposited.

Firing for 3mm Channels

A question has arisen on how to put together a design of pieces for a lamp, but only one layer thick, because 3mm is as thick as the fittings will accept.

The design has no overlaps, so it is a series of butted 3mm thick pieces.  Damming has not been successful in keeping the parts from retreating from one another.  This means that making the design as a single layer will not be successful.

The problem is how to make a two-layer piece that will be able to fit into 3mm fixings.

Design on oversize 3mm base

One way to overcome the fixings’ limitations is to make the bottom layer larger than the top.  The bottom can be any colour you choose.  Make the design on top of that. 

The designed pieces will need to fit snugly beside the fittings. However, the bottom needs to be cut larger than the final size, as it will retreat and become smaller during the firing.  About 20mm larger all around will usually be enough extra for ease of cutting down. If you fire with a larger base piece, you can cut it to size after firing, so it will fit the width of the opening and still fit inside the 3mm fitting space.  This will make your design proud of the fittings.  This may, or may not, be possible for the lamp’s fitting design.

An alternative

Maybe that is not the only way to look at the problem. There is another way.  It is essentially the opposite of the first approach. 

Make the top layer larger than bottom. The design will be on the top still, but with larger than final dimensions to accommodate the reduction in size of the single layer.  The bottom layer will need to be small enough to fit within the space between the fittings of the lamp.

To keep the unsupported parts of the upper layer in one plane, support the larger upper layer with 3mm fibre paper. Coat the fiber paper with boron nitride or cover with powdered kiln wash, Thinfire or Papyros to get a smoother back.  When fired, cut the piece to size.  If you like to score on the smoothest side, you can support the edge with the fibre paper or other 3mm substance.  If you are confident, you can score on the back with no special support.

These are two approaches to making a piece to fit in a 3mm channel.  This will apply to insertions of fused glass leaded glass panels, as the came is designed to accommodate 3 mm glass.

What Cartoon Lines Represent

A frequently asked question by novice glass workers is whether to score at one side of the line or in the middle.  This question revolves around the meaning of the cartoon lines.  What do the lines of a cartoon represent?

Meaning of Cartoon Lines

The lines on a cut line cartoon represent the space required between pieces of glass.  This will vary, depending on the style in which you are working.  In most glass working, a matrix of lead or foil is used.  The space required by these materials needs to be represented in the cut line cartoon. You may have other cartoons for other purposes – painting, came width, foil width, etc., but the lines in the cut line cartoon are there to represent the space required between pieces of glass.

An example of a cartoon for painting

Lead Came

In general, a 1.2mm line is required for standard lead came. This is close to the line made by a new bullet pointed felt tipped marker. If you are working with high heart cames, you will need a 2.8mm wide line. Some chisel point markers, if used on the sharp edge have this approximate width.

The glass is scored at the inside edge of the cartoon line.  This can be done by scoring directly on top of the cartoon, often with a light underneath.  You can make pattern pieces when the glass is too dense for enough light to come through.  If you must, you can draw the score line on the glass. You can score around pattern pieces, but if your scoring wheel goes over the pattern in any place, the scoring pressure will not be delivered to the glass.

Example of came varieties

Copper Foil

In copper foil, a much thinner line is used as the space between pieces of glass needs only be approximately 0.4mm. This is approximately the width of a sharpened pencil or ball point pen line.

The scoring is at the edge of the line as for lead came.  Also, you can score directly over the cartoon, draw on the glass, or make pattern pieces as for lead came projects.

Fusing Cartoons

When preparing a cartoon for fusing, the lines need to be as fine as possible.  The pieces of glass require no space, as they will be butted against each other.  However, unless cutting by computer controlled instruments, the cutting cannot be completely accurate, so the same size of line as for copper foil will do.

As you are going to try to butt the glass pieces together in fusing projects, you score along the middle of the cartoon lines.  As much as possible, cutting over the cartoon will give the best result.  Of course, there are many times when the light is not good enough and pattern pieces will be required. 

Another approach is also possible. Having scored and broken the first piece, you can place it on top of the glass to be cut for the adjoining one.  With a very fine felt tip or fountain pen, trace the edge of the first piece. Score down the middle of that line to create the best fitting second piece.  And so on through the whole project where the glass is not too dense to use a light box.

Conclusion

The line widths in a cartoon are determined by the space required between pieces by the assembly method.  The thicker the matrix material, the thicker the line and vice versa.