Wednesday, 26 July 2017

Cutting Hour Glass Shapes


Hour glass shapes, wasp waists, or those that are thinner along the length than the ends, should be avoided as much as possible.  They are difficult to break out from the score.  More importantly, they are an inherently weak shape. The longer the piece is with the narrow part along its length, the more likely it is to break; in cutting or in the long term, in the panel.  However, these shapes are sometimes unavoidable.





The principle to use in scoring and breaking out the glass is to remove less glass than that you are retaining at each stage of the process.

This has consequences: 
  • ·         breaking the first score is the easiest
  • ·         only a rough outline of the final piece should be scored and broken from the sheet
  • ·         Relieving scores and breaks will be necessary.  The number will depend on the relative thickness of the thin and thick parts.



You can make the first score and break of one side of the shape from the main piece of glass – usually with little difficulty or need for relieving scores. (1)

You then should score and break off the piece to be retained from the larger sheet.  Be sure to give a margin for the final piece. (2)

Now score the other part of the hour glass shape.  Do not tap the score. Begin gently to run of the score from each end.  Don’t worry if the runs do not meet up.  Do not tap to make them meet up. (3)

If running the score from both ends is not enough to make the run complete, you will need to use relieving scores.  These scores can be like onion rings – generally concentric curves running in the same sort of shape as the curve to be broken out.  




Or you can use the fish scale approach – overlapping crescents.  These are most useful for deeper inside curves.

Either way, each score needs to be planned.  Each relieving score should be smaller than the width of the piece to be retained.  In general, this means the outer relieving scores can be wider apart.  As you approach the final shape, the distance between the scores will need to be less and less. (4,5,6)


More information on scoring and breaking out concave curves can be found here:  http://glasstips.blogspot.co.uk/2008/08/cutting-concave-curves.html


Wednesday, 19 July 2017

Lead Free Solders

Lead free solders have been created in response to concerns about lead, especially in the electronics industry. The following tables present a selection of available solder compositions.  The characteristics of these lead free solders can be compared to the common lead bearing solders in the last table.

Abbreviations for the metals of the compositions:
Ag=Silver; Bi=Bismuth; Cu=Copper; Ge=Germanium; In=Indium;
Sb=Antimony; Sn=Tin; Zn=Zinc



Melting Temperatures of Lead-Free Solders

Alloy  %                     Melting Temperature    Comments
Range (ÂșC)
Sn 65, Ag 25                         233           High strength; patented by Motorola (“Alloy J”)
Sn 99.3, Cu 0.7                     227           Eutectic
Sn 96.5, Ag 3.5                     221           Eutectic. Excellent strength and wetting
Sn 98, Ag 2                          221 – 226
Sn 77.2, Ag 2.8, In 20           175 – 186
Sn 95, Sb5                           232 – 240 Good high-temperature shear strength
Sn 42, Bi 58                         138           Well established; expensive
Sn 91, Zn 9                          199   Eutectic. Corrodes easily; high dross
Sn 95.5, Ag 0.5, Cu 4            217 – 350 Lead-free plumbing solder
Sn 97.25, Ag 2, Cu 0.75        217 – 219
Sn 91.8 Ag 3.2, Cu 0.5          217 – 218
Sn 95.5, Ag 3.8, Cu .07         217 – 220
Sn 95.5, Ag 4, Cu 0.5            217 – 225
Sn 95, Ag 4, Cu 1                 217 – 220
Sn 94.6, Ag 4.7, Cu 1.7         217 – 244
Sn 89, Zn 8, Bi 3                   192 – 197
Sn 97, Ag 0.2, Cu 2, Sb 0.8    287 – 218  High melting range; “Aquabond”
Sn 96.2, Ag 2.5, Cu 0.8, Sb 0.5      217 – 225
Sn 90.5, Ag 2, Bi 7.5             190 – 216
Sn-91.8, Ag 3.4, Bi 4.8          201 – 205
Sn 93.5, Ag 3.5, Bi 3             208 – 217
Sn 94.25, Ag 2, Bi 3, Cu 0.75   205 – 217
Sn90.7, Ag3.5, Bi 5, Cu 0.7     198 – 213
Sn 93.4, Ag 2, Bi 4, Cu 0.5, Ge 0.1         202 – 217
Sn 42.9, Bi 57, Ag 0.1           138 – 140
Sn 48, In 52                         118           Eutectic. Lowest melting point. Expensive

Source:



Liquidus Temperatures (°C) of Candidate Lead-Free Solder Alloys for Replacing Eutectic Tin-Lead Solder

Alloy Composition%     Liquidus             Melting Range
98Sn-2Ag                                             221-226
96.5Sn-3.5Ag              221                    221
99.3Sn-0.7Cu              227                    227
96.3Sn-3.2Ag-0.5Cu     218                   217-218
95.5Sn-3.8Ag-0.7Cu     210                   217-210
95.5Sn-4.0Ag-0.5Cu                             217-219
95Sn-5Sb                                            232-240
42Sn-58Bi                   138                   138
89Sn-3Bi-8Zn                                      189-199

Where there is a single temperature in the melting range column, the solder is eutectic.

Based on:
V. Solberg, “No-Lead Solder for CSP: The Impact of Higher Temperature SMT Assembly Processing,” Proc. NEPCON West 2000 Conf. (Feb. 28 - Mar. 2, 2000) Anaheim, CA (Source: Indium Corp.) # N.-C. Lee, “Lead-Free Chip-Scale Soldering of Packages,” Chip Scale Review, March-April 2000.
Source:




Solidus and Liquidus Temperatures of Some Leadfree Alloys on Copper

Alloy  %                             Solidus (°C)        Liquidus (°C)
98Sn-1Ag-1Sb                      222                   232 
89Sn-4Ag-7Sb                      230                   230
91.2Sn-2Ag-0.8Cu-6Zn          217                   217
89.2Sn-2Ag-0.8Cu-8Zn          215                   215
89.2Sn-10Bi-0.8Cu               185                    217
85Sn-10Bi-5Sb                     193                   232
52Sn-45Bi-3Sb                     145                   178
42Sn-58Bi                            138                   138

Based on:
M.E. Loomans, S. Vaynman, G.Ghosh and M.E. Fine, “Investigation of Multi-component Lead-free Solders,” J. Elect. Matls. 23(8), 741 (1994)
Source:



Eutectic Composition of Solders

Most solders and especially tin-lead alloys have a melting (or pasty) range between which the metal has moved from a proper solid (solidus) to a completely liquid (liquidus) state.  Wide melting ranges are ideal for plumbers, they are not for electronics, or stained glass.  It is much easier to run a nice bead with a narrow range of melting (pasty) temperatures.

Some alloys of solder have what is known as an eutectic characteristic.  This is where the liquidus and solidus states occur at the same temperature.  A composition of 61.9% tin and 38.1% solder is both eutectic and the melting occurs at a minimum temperature.

For comparison with lead free solder characteristics the following % compositions of Tin (Sn), Lead (Pb) and Silver (Ag) solders are given.

Element % of solders  Melting point        Comment
Sn 62, Pb 36, Ag 2       179                    Eutectic; traces of antimony
Sn 63, Pb 37               183                    Eutectic; traces of antimony
Sn 60, Pb 40               183-191             Traces of antimony
Sn 96.3, Ag 3.7           221                    High melting point. Eutectic
Sn 10, Pb 90               275-302
Sn 3, Pb 97                275-320
Sn 5, Pb 93.5, Ag 1.5   296-301

Source:
http://en.wikipedia.org/wiki/Solder#Lead-free_solder



Conclusions

Most of the alternative solders contain tin as it assists in the formation of bonds with a wide variety of metals.  These solders are also mechanically weaker than tin-lead solders.  Lastly, they are much more expensive than tin-lead solders.  Even within the tin-lead solders there is a variation in price, as tin is much more expensive than lead. If high temperatures were not a problem, you could use a high lead content solder.  However, that also raises the liquidus temperature and increases the pasty range.

The choice in lead free solders is between the high liquidus temperatures of the less expensive compositions and the high price of the eutectic, or nearly so, ones.  The lowest eutectic composition is the Tin-Bismuth solder, but it is also among the most expensive to buy.  You should also note that the inclusion of copper in the composition prolongs the life of the solder bit, as low lead content of the solder leads to the incorporation of small amounts of copper from the tip into the solder joint.



Saturday, 15 July 2017

Needling

Needling is a description of the fine points emerging from the edges of glass.

This occurs in two conditions mainly.

The one that is most commonly seen is in the fusing of single layers of glass. The surface tension of the glass pulls the glass in from its original size, trying to achieve the 6-7mm that is a thickness equilibrium at full fusing temperatures. If the surface the glass is resting on has any rough areas, and most surfaces do, some of the glass will stick and the rest retract. This leaves short, thin and extremely sharp “needles” extending from the edges. 

Two common surfaces allow these sharp edges. Fibre paper of 0.5mm and greater is rough enough to allow the hot glass to stick to tiny depressions in the paper.  Kiln wash is often not smooth enough to prevent this kind of sticking either.  You can smooth powdered kiln wash or aluminia hydrate over these surfaces to reduce the grabbing of the surface by the hot glass. However, the powder is often drawn back with the contracting glass. Thinfire or Papyros paper is fine enough to avoid the needling most of the time without any addition of powders.

The other main condition is in casting, mainly box casting or damming. In this case, the stack of glass sheets or cullet is higher before firing than its final thickness. This means the glass flows out to the dams and sinks down to its final thickness during the firing process. As the glass touches the fibre paper or other separator it behaves just as the single layer of glass does. Some sticks to the surface while the rest is dragged away by the surface tension and reducing thickness of the stack of glass.

Prevention of Needling
Lining dams
Separators for dams

Thursday, 13 July 2017

Quartz Inversions and Conversions

You need to know about this in both casting and when using ceramic pots in the kiln.

Quartz
Crystalline solids are rather temperamental and quartz is no different. Quartz is a crystalline form of silica in that it has a three dimensional regular pattern of molecular units. These form naturally in nature because lengthy cooling times allow arrangement. Quartz is made of a network of triangular pyramid (tetrahedron) shaped molecules of silicon combined with four oxygens.

Unfortunately, the quartz delights in changing the orientation of the tetrahedron shaped molecules with respect to each other, thus loosening or tightening the whole mass (and thus changing its total size). It exhibits twenty or more “phases”. A change to another phase is called a “silica conversion”. The most significant phases are quartz, tridymite, crystobalite, and glass.

Inversions
Changes which occur between these are reversible, that is, the change which occurs during heat-up is inverted during cool down. These changes are thus called “quartz inversions”. These inversions, unfortunately, often have associated, rather sudden, volume changes. That means that quartz conversions are something to consider when optimizing the fired properties; quartz inversions are something to consider when firing to prevent cracking losses. There are two important inversions you need to know about because of their sudden occurrence during temperature increase and decrease.




Quartz
The first is simply called ‘quartz inversion’ and it occurs quite quickly in the 570°C range (1060°F). In this case, the crystal lattice straightens itself out slightly, thus expanding 1% or so. This is therefore an important temperature in casting as it is an expansion on the heat up and a contraction, “grabbing” the glass on the way down. This is the reason for various modifiers when silica or flint is used as the strengthener.




Crystobalite
The second is crystobalite inversion at 226°C. This is a little nastier because it generates a sudden change of 2.5% in volume. This material has many more forms than quartz, so it is complex to say the least. However, while all bodies will have some quartz, you won’t have a problem with crystobalite inversion unless there is crystobalite in your body. Crystobalite forms naturally and slowly during cooling from above cone 3 (1104-1149°C). It forms much better if pure crystobalite is added to the body to seed the crystals or in the presence of catalysts (e.g. talc in earthenware bodies). Thus, this element exists in most ceramic moulds and moving slowly around 226°C should be observed when firing containers made of ceramic materials.

Wednesday, 5 July 2017

Simple Investment Mould Materials


There are a lot of differing recipe options for making plaster moulds. A simple general purpose investment mould making material and method follows:

Equal parts of powdered silica (sometimes called silica flour or flint), plaster of Paris and water by weight.  For example:

1 kilo silica
1 kilo Plaster Paris
1 kilo water
(Do not measure by volume)

Mix silica and plaster of Paris dry in separate bucket by hand.  If you can use a closed container that is best.  Otherwise use breathing protection and do the mixing outside.  Silica is very bad for your health.

Measure the water into a separate bucket with enough volume for three times the amount of water. Slowly sprinkle the entire contents of the dry mix into the bucket of water.  Do not dump it in!

Let the mixture sit for 2 minutes (slaking).  Then mix by hand slowly to prevent bubbles. Using your hands allows you to feel any lumps that are present and break them down gently. Depending on temperature and amount of water, you have 15-20 minutes before the mix begins to become solid.

When mixed thoroughly, pour carefully and slowly into a corner of the mould box or container to reduce the occurrence of bubbles within the investment material or against the master.

When the pour is finished, tap the mould container to encourage any bubbles to the surface.

You can take the investment and master from the container once it is cold to the touch. Remove the master from the investment material carefully to avoid damaging the surface of the investment.

For pate de verre, you can use the mould almost immediately.  For casting, it is important to have a dry mould.


Let the whole air dry. Depending on the temp, humidity and density this can last from several days to several weeks. A way to tell how dry the investment is, is by weighing the mould when it has just hardened. When it has lost on third of its weight (the water component), it is ready for kiln drying. This removes the chemically bound water from the investment material. 

This is only an outline of what to do.  Investment moulds are extremely complicated in their chemistry, physics, and use.

Wednesday, 28 June 2017

Stencils vs. Saw

Saw

Frequently when people want to make a complicated shape they resort to a saw to create the shape.  This is used in both stained glass and fused glass work.  Although it may be necessary in stained glass applications, it is not as necessary in fusing.

One of a variety of saws


Stencils

There is an alternative to an expensive saw – stencils and frits.  You can make a stencil from stiff card. Place the stencil in the appropriate place. Then sift powder or sprinkle frit over the stencil.  Lift carefully and the shape is there ready for fusing.

Example of sifting powder over a complicated stencil


To get the depth of colour obtained from sheet glass, you need to apply the powder or frit to at least the thickness of sheet glass. This also means that you need to go to a full fuse with the powder or frit on the top surface.  You can, of course, later cap and fire again.

Example of the cutting of a stencil


More guidance on stencils is available here

Wednesday, 21 June 2017

Mica



What it is

Mica is widely distributed throughout the world and occurs in igneous, metamorphic and sedimentary rocks. Mica is similar to granite in its crystalline composition.  The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.

Mica can be composed of a variety of minerals giving various colours and transparency. Purple, rosy, silver and grey colours come from the mineral called lepidolite.  


Dark green, brown and black come from biotite.  


Yellowish-brown, green and white come from phlogopite.  



Colourless and transparent micas are called muscovite.  


All these have a pearly vitreous lustre.


The melting point of mica depends on its exact composition, but ranges from 700⁰C to 1000⁰C.

Glass has a specific gravity of about 2.5, and mica ranges from 2.8-3.1, so it is slightly heavier than glass.


Tips on uses of mica powder and flakes

The naturally occurring colours are largely impervious to kiln forming temperatures.  Other added colours have various resistances to the heat of fusing. This is determined by the temperatures used to apply the colour to the mica.  Cosmetic mica is coloured at low temperatures and will not survive kiln forming with their colour in tact.



Mica does not combine with glass, but is encased by glass as it sinks into the glass surface.  You can use various fluxes to soften the surface of the glass.  Borax is one of those.  The cleaving of the mica results in only the layer in contact with the glass sticking.  The upper layers brush off.  This applies to both powder and flakes. One solution is to fire with mica on top in the initial firing and then cap for the final one.

When encasing mica exercise caution. Micas flakes must be applied thinly, as air is easily trapped between layers which leads to large bubbles from between layers of glass.  This is the result of the shearing of layers of the flakes allowing air between layers.  Although powdered mica is less likely to create large bubbles, air bubbles are often created for the same reason.  This is the reason it is most often recommended to fire the mica on top. 

Of course, one use of the mica to make complicated designs is to cover the whole area and fuse.  Then sandblast a design removing the mica from areas of the glass. You can then fire polish, or cap and re-fire to seal the mica.

Mica safety

MSDS for mica only mentions the inhalation of the dust as a risk. Mica is resistant to acid attack and is largely inert.  Inhalation of the dust is a (low level) risk.  Any significant health and safety problems relate to the coloured coatings.



Wednesday, 14 June 2017

Deep slumps


One of Karl Harron's deep slumped bowls


Deep slumps require multiple stages to get even drops without thinning the sides.  There are several makers of staged slumping moulds which allow progressively deeper slumps in a series of firings into deeper moulds.





If you have a steep-sided mould, you will find slumping directly into the shape difficult.  There will be uneven slumps, thinning of sides, hang-ups, etc., among your attempts to achieve the slump in one firing.  It is possible to mimic this series of moulds without buying the whole set. 

To avoid these difficulties, you can build up the inside bottom of the mould by placing powdered kiln wash in the bottom and smoothing it to a gentle curve. You should aim for a gentle shape as in a ball mould. 

After the first firing, remove some of the powder, placing it in a clean container.  Shape the remaining powder into a deeper slump than the first one. 

It takes some time and practice to achieve a smooth even curve.  You can ease the shaping process by cutting the intermediate shapes from stiff card.  This can be rotated to achieve an even curve in the powder.  Remove any excess powder and do a final rotation to give the powder a final smoothing.  Place the glass back on the mould and fire.

It may be that you will need to repeat this several times to get the full slump.  Separate template curves need to be cut for each slump if you are doing more than one intermediate slump. It does depend on the steepness of the mould sides and the depth of the slump as to how many stages are required.  Sometimes the slump can be achieved in only two stages.

After firing the powder, pour it back into your kiln wash container, as it still is good for mixing to apply to shelves, moulds etc.


This method is useful for any mould that is too deep for achieving the slump in one firing, and without buying intermediate moulds.  Remember the final result will be smaller than the size of the deep mould, as the span of the glass becomes less with each deeper slump.

Wednesday, 7 June 2017

Effects of Annealing at the Top End of the Range

It is possible to begin your annealing at any point in the annealing range.

The annealing point is the temperature at which the glass most quickly relieves the stress within.  This occurs at the glass transition point

The  annealing range is between the softening point and the strain point of the glass.  No annealing can be achieved above the softening point, nor below the strain point.  This range, for practical purposes can be taken to be 55°C above and below the published annealing point.  For thick slabs, Bullseye has chosen to start the anneal 34°C below the published annealing point of 516°C.

High Annealing Point

They could have chosen to use a higher point, even up to 571°C, the approximate strain point of the glass.  The effect of this is an extended anneal cool.  The reasons are as follows.  

The anneal soak does not need to be extended, as the purpose is to get all the glass at the same temperature in preparation for the annealing cool. 

The cooling rate must be slower (approximately one third the rate) than an anneal soak at a lower temperature, as the glass must be maintained at the same temperature throughout the long cool.  

Also, the initial rate of cool needs to be maintained down to the strain point, which is 110°C below the softening point.  Of course, after that initial cool, the speed of cooling can be increased.

Low Annealing Point

Starting the anneal cool closer to the strain point requires a longer soak to ensure the glass is all at the same temperature (+/- 5°C) before the anneal cool begins.  Typically, this initial soak would be for an hour before the initial cool begins (for a 6mm to 9mm thick piece).


Effect of the Differences in Approach

The advantages and disadvantages centre around these needs to 

  • soak long enough to get all the glass to the same temperature and secondly, to 
  • cool slowly enough to maintain the even temperature distribution throughout the glass.


Example

If you think of an example of a piece of Bullseye glass 12mm thick, it will show the differences in approach.

High temperature soak
A soak of 30 minutes at 571°C (the highest possible start for an annealing soak) is required to even the temperature.  To ensure the temperature differentials in the glass do not deviate from the +/-5°C, the cool needs to be at 18°C per hour down to 461°C.  It is possible then to increase the speed to 36°C down to 370°C.  This gives you a total annealing cool of just over 5 hours.

Low temperature soak
Starting the anneal at 482°C requires an hour soak followed by a decrease in temperature of 55°C per hour to 427°C, and an increased rate of 110°C to 370°C.  This gives an anneal cool time of 3 hours and 30 minutes.


The example shows how, although the annealing result may be the same, there is considerable time saved (for thicker pieces) in using the lower part of the annealing range to begin the annealing.  It also will save some electricity.

However, an anneal of 30 minutes at 516°C with a cool of 80°C per hour to 370°C will still give a perfectly adequate anneal for 6mm thick pieces.

Wednesday, 31 May 2017

Breaks after the Piece is Cool

People sometimes fire a piece only to have it break after it is cool.  They decide to re-fire with additional decoration to conceal the break.  But it breaks again a day after it has cooled.  Their questions centre around thermal shock and annealing. They used the same CoE from different suppliers, so it must be one of these elements that caused the breakage.


Thermal Shock

This is an effect of a too rapid heat change.  This can occur on the way up in temperature or on the way down.  If it occurred on the way up to a fuse, the edges will be rounded.  If it occurred on the way up to a slump the edges may be sharp still, but the pieces will not fit together because the slump occurred before the slump.  It the break occurs on the way down the pieces will be sharp.  The break will be visible when you open the kiln.  More information is here.


If the break occurs after the piece is cool, it is not thermal shock.

If the break occurs some length of time after the piece is cool, it can be an annealing or a compatibility problem.  They are difficult to distinguish apart sometimes.

The annealing break usually crosses through the applied pieces and typically has a hook at each end of the break.  If the piece has significant differences in thicknesses, the break may follow the edge of the thicker pieces for some distance before it crosses it toward an edge. This kind of break makes it difficult to tell from an incompatibility break.

An incompatibility break may occur in the kiln, or it may occur days, months or years later.  Typically, the break or crack will be around the incompatible glass.  The break or crack may follow one edge of the incompatible glass before it jumps to an edge.  The greater the incompatibility, the more likely it is to break apart.  Smaller levels of incompatibility lead to fractures around the incompatible glass pieces, but not complete breaks.

There is more information about the diagnosis of the causes of cracks and breaks here.


Annealing

Another possible cause of delayed breakage is inadequate annealing.  Most guidelines on annealing assume a flat uniform thickness.  The popularity of tack fused elements, means these are inadequate guides on the annealing soak and annealing cool.  Tack fused items generally need double the temperature equalisation soak and half the annealing cool rate. This post gives information on how the annealing needs modification on tack fused items. 


Compatibility

The user indicated all the glass was of the same CoE.  This is not necessarily helpful. 

Coefficient of Linear Expansion (CoE) is measured between 0°C and 300°C. The amount of expansion over this temperature range is measured and averaged. The result is expressed as a fraction of a metre per degree Celsius. CoE90 means that the glass will expand 9 one-thousandths of a millimetre for each degree Celsius.  If this were to hold true for higher temperatures, the movement at 800C would be 7.2mm in length over the starting size.  However, the CoE rises with temperature in glass and is variable in different glasses, so this does not tell us how much the expansion at the annealing point will be.  It is the annealing point expansion rate that is more important.  More information is here.

Compatibility is much more than the rate of expansion of glass at any given temperature.  It involves the balance of the forces caused by viscosity and expansion rates around the annealing point.

Viscosity is probably the most important force in creating compatible glasses. There is information on viscosity here.  To make a range of compatible glass the forces of expansion and viscosity need to be balanced.  Each manufacturer will do this in subtly different ways.  Therefore, not all glass that is claimed by one manufacturer to compatible with another’s will be so. 


All is not lost.  It does not need to be left to chance.

Testing glass from different sources is required, as you can see from the above comments.  It is possible to test the compatibility of glass from different sources in your own kiln.  The test is based on the principle that glass compatible with a base sheet will be compatible with other glasses that are also compatible with that same base sheet.  There are several methods to do this testing, but this is the one I use, based on Shar Moorman’s methods.  

If you are investing considerable effort and expense in a piece which will use glass from different sources or manufacturers, and which is simply labelled CoE90, or CoE96, you need to use these tests before you start putting the glass together.  The more you deviate from one manufacturer’s glass in a piece, the more testing is vital. 

In the past, people found ways of combining glass that was not necessarily compatible, by different layering, various volume relationships, etc.  But the advent of manufacturers’ developing compatible lines of glass eliminated the need to do all that testing and experimenting.  While the fused glass market was small, there were only a few companies producing fusing glass.  When the market increased, the commercial environment led to others developing glass said to be compatible with one or other of the main producers of fusing compatible glass.

If you are buying by CoE you must test what you buy against what you have.

The discussion above shows that even with the best intentions, different manufacturers will have differences that may be small, but can be large enough to destroy your project.  This means that unless you are willing to do the testing, you should stick with one manufacturer of fusing compatible glass. 

Do not get sucked into the belief that CoE tells you anything important about compatibility.



Wednesday, 24 May 2017

Sticking Kiln Wash

Sometimes people experience kiln wash sticking to the bottom of their glass. 

You need some understanding of what kiln wash is to know why the wash sticks. It is largely due to the chemical changes in the kaolin at fusing temperatures.

Opalescent glass does tend to pick up kiln wash more easily than transparent, and does it more at higher temperatures. It is the case that at higher temperatures and longer soaks, the kiln wash is more likely to stick to any of the glasses than at lower temperatures and with shorter soaks. This re-enforces the mantra of "low and slow" to avoid problems in kiln forming.

To achieve the same effects at lower temperatures as at higher temperatures, your rate of advance needs to be slower from the slump point to the top temperature.  This additional heat work will achieve the desired effect with a lower temperature.


One kiln wash, Primo, does not contain china clay.  If you use this and it is sticking to the bottom of the glass, you may be firing too high. Try a lower temperature with a longer soak to reduce the kiln wash pickup. 

Wednesday, 17 May 2017

Compatibility Tests

These procedures are based on the observation that glasses compatible with the base glass are compatible with each other. This means that you can test opaque colours’ compatibilities with each other by testing each of them on clear strips.

Annealing test

These tests must be combined with an annealing test.  This conists of putting two pieces from the same sheet of glass together - so you know they are compatible - and firing them along with your compatibility test.

Viewing the results of your annealing through the polarised filters shows whether there is stress left in your annealing.  If there is, the compatibility tests are inconlusive as there is no difference in appearance of stress whether from incompatibility or from inadequate annealing.  Once you have the annealing right, you can then interpret the compatibility tests done at the same time.

Strip test




Cut a strip of base glass ca 25mm wide and as long as convenient for you or your kiln.

Cut clear glass squares of 25mm to separate the colours.

Cut 25mm squares of the colours to be tested

Start with a clear square at one end of the clear strip and alternate colours and clear along the strip finishing with a clear square.

Add a stack of two layers of clear to the kiln before firing. This is to test for adequate annealing. If the annealing is inadequate, then the whole test is invalid.




Test the result with polarising filters. Start with the clear annealing test square. If no stress is apparent, go to the test strip. But if stress is apparent in the annealing test, look to your annealing schedule as something needs to change. Usually the requirement is a combination of a longer soak at the annealing temperature and a slower annealing cool.




To test for compatibility, look carefully for little bits of light in the clear glass surrounding the colour. These are indications of stress – the more light or the bigger the halo, the greater the stress. Really extreme stress appears to be similar to a rainbow, although without the full spectrum.




You can use this test to determine if you annealing is satisfactory for larger pieces. In this case you should use at least 100mm squares. Stack them to the height of your planned project and dam them with fibre board or other refractory materials to prevent spread. Fire to full fuse and anneal. When cool check for stresses.


The tile method looks at compressive factors too.

Cut a 100mm clear tile

Cut two strips of glass 25mm wide and 100mm long for each test

Cut two rectangles of 25 by 50mm of the same glass for the two remaining sides

Cut a square of 50mm for the centre. The glass in the middle is normally the test glass. To be very certain of what has happened you can do the reverse lay up at the same time. You put coloured glass around the outside, but in this case the inside needs to be clear or transparent. At least one element needs to be transparent enough to view the stress patterns, if any. So you could have a clear middle and black exterior, and vice versa.

This test is a more time consuming process and you may wish to use it only for larger projects.

Also look at the use of polarising filters

Charges for Repairs


Repairs always cost more than the owner or artist expects on initial inspection.  The cost is very similar to, or more expensive than, the cost of a new panel if the whole has to be taken apart and renewed.


If it is a repair to part of the window or object, you need to be careful that you do not under price.  The cost elements you need to consider are these at minimum:

  • Glass
  • Materials
  • Time
  • Overheads
  • Travel
  • Installation
  • Contingencies
  • Profit


Glass - and the cost of obtaining it.  Can you obtain the same or very similar glass to the original?  If you can’t, is the client willing to have the repair in different glass?  If you get approval, you need to cost it – whether you already have it or not.  If you do not have it in your stocks, you need to add in the cost of getting it whether that is travel or postal order.  You need to include the time either or both methods involve in the costs.



Materials – The materials you will use in addition to the glass need to be considered.  These include solder, Foil or lead, flux, patina, cleaning materials, etc.


Time - labour and admin. You need to assess how much time it will take to do the repairs.  Then multiply that by your labour rate. You do have one, don’t you?  If not, get down to it and create one. Use steps one and two of this description.   You also need to take into consideration the time to recreate a pattern for the broken area if extensive.


Overheads – If your overheads are not included in your hourly rate, this is the time to include them in the pricing.


Travel = Your mileage rate + time to get there and back.  If you don’t have a mileage rate, look at what your local authority allows.  This will be lower than what businesses allow, but are reasonable, and publicly available.  (At the time of writing the allowance in Scotland is approximately £0.50 per mile.)  It takes time to get to the location, so this needs to be included in the cost too. Of course, if they are willing to bring the item, it reduces the cost to the client.


Installation – If you are expected to install the piece, you need to include travel (there and back at least twice) and time.  You also need to include the estimated time to remove and install a substitute (and its cost) as well as installation of the repaired piece.


Contingencies - All repairs have uncertainties.  You do not always know what the progress of repairing will reveal.  You can agree with the client that any work required in addition to the initial agreement will be notified for the client to decide whether to proceed or not.  However, you can take on the risk. This is what the contingency is for. You need to build allowance for these unforeseen developments.  A 10% to 20% of the total costs addition to the price is sensible if you are taking the risk.


These seven elements added together give you the cost of doing the repairs.  That is the bottom line.  But there is one more element to consider:



Profit – You do expect to get a profit from all this work, don’t you?  If not, why do the repair at all?  You are not a charity.  Of course, you can decide to give away your profit.  Before you do, think about what you have to pay for repairs – to your car, your plumbing, etc.  You deserve some profit on everything you have invested in this craft that you love.  The love will die without profit.

The profit level will depend on your objectives, but will range from 20% (very low) to 100% (what shops charge). If you put your work in a gallery or shop on a sale or return basis, you expect to have to pay at least 30% on the sale.  That should be the minimum basis of your profit level on any repair.



This may all sound like it is too much trouble for a simple repair.  Yes, it does take a bit of consideration to start with.  But once you have established the basic labour, travel, overhead and profit levels, the rest is pretty straight forward.  You will have an idea of how long it takes to do the work, to travel, the glass costs, etc., and the profit level. You only need to multiply by the rates you have established to give you the price.  I should warn you - it will be much higher than you initially thought.