Wednesday, 18 October 2017
Is it Possible?
It is possible to slump unknown glass. This glass might be art glass, window glass, bottles, or any other glass whose characteristics are unknown by you. There are some suggestions about the characteristics of some glasses in this post that can be used as a starting point.
Preparation of the Glass
Prepare the edges to their final finish before slumping. This because the slumping temperature will not be enough to alter the finish of the edge significantly. This preparation can be done with diamond hand pads, or wet and dry sandpapers. Start with a relatively coarse grit. You may wish to do the initial shaping on your grinder. This will be between 80 and 100 grit. Continuing with a 200 grit and working your way through 400 and then 600 grit will give you an edge that will become shiny during the slumping.
Clean thoroughly. This is especially important when using glass that is not formulated for fusing. Devitrification is more likely on these glasses. Water with a drop of dishwashing liquid can be enough unless your water has high mineral content. Then distilled water or a purpose made glass cleaner such as Bohle or Spartan should be substituted. Finish with a polish to dry with clean paper towels. More here.
Firing the Slump
Fire up slowly. You should advance at about 100°C to 150°C per hour. Set your top temperature around 630°C for a simple slump, for soda lime stained glass. For bottle or window glass you will need a temperature closer to 720°C although the also are soda lime glasses.
It is best to start with simple curves, as there are fewer difficulties in determining what the glass is doing. It will help you to learn the characteristics of the glass before you tackle the difficult stuff, such as compound curves or texture moulds.
It is necessary to observe the progress of the slump as you do not yet know the slumping temperature. You want to know when the glass begins to deform so that you do not over fire. Start watching the glass at about 10 minute intervals from about 580°C for stained glass and 680°C for window and bottle glass. There is not much light in the kiln at these temperatures, so an external light is useful. You can also observe the reflections of the elements on the glass. When the image of the elements begins to curve, you know the glass is beginning to bend.
Altering the Schedule
Soak for at least 30 mins at the temperature when the glass begins to visibly drop. This may or may not be long enough. Continue checking at 5-10 minute intervals to know when the slump is complete. If the glass is completely slumped before the soak time is finished, advance to the next segment. If not fully slumped, you need to extend the soak time. This means that you need to know how to alter your schedule in your controller while firing. Consult your controller manual to learn how to do these things.
Stop the soak when complete and advance to the anneal. Continue the slumping soak if not complete after the 30 mins. In some cases, you may need to also increase the temperature by 5-10°C.
The annealing point will be about 40°C below the point that the glass visibly starts the slump. If you want a more accurate determination of the annealing point, this post gives information on how to conduct a test to give you both the slump temperature and the annealing point. It also helps to determine the lower part of the tack fusing range (the lamination state), since it is not far above the slumping point that you will observe.
The annealing soak for a single layer, 3mm glass need not be long – 15 to 30 minutes. The annealing cool can be as fast as 120°C down to 370°C. For thicker glass and slumped bottle glass you will need a longer soak – 30 to 60 minutes – and a slower cool. The annealing cool in this case could be about 60°C/hour to 370°C. You can turn the kiln off at 370°C, if you wish, or keep the temperature controlled to about 50°C. The rate for the final cooling can be approximately double the first cooling rate. For a single layer of stained glass this could be 240°C, and for thicker glass about 120°C
Sunday, 15 October 2017
It is relatively common for questions about white deposits on the solder beads of copper foiled pieces to be raised. In reflecting on the cause of the white deposit on solder beads, I recalled that some people use baking soda to neutralise the flux. I put this together with some work on lead corrosion.
I have been doing a bit of research on lead came corrosion in another context. One of the forms of lead corrosion is white lead corrosion, or lead carbonate. It has the chemical compound PbCO3. It is a curious compound, as it is soluble in both acid and alkali. This much you will have seen from a previous posting about lead corrosion.
In that it is possible for excess whiting left on lead cames to give rise to this form of white corrosion. Baking soda has a chemical formula of NaHCO3. Solder contains a significant amount of lead – usually 37-40%. The chemical reaction of lead and baking soda gives lead carbonate - PbCO3 and NaH -sodium hydride. The sodium hydride is soluble in water, leaving the white deposit of lead carbonate as a corrosion product on the surface.
Putting these things together leads me to recommend that baking soda and other carbonates should not be used in cleaning solder beads. Some other non-carbonate neutralising or rinsing agent should be used instead.
Wednesday, 11 October 2017
Many people are concerned about the cost of kiln forming, but use fibre paper rather than kiln wash or powders, although it is many times more expensive. This may be a matter of convenience. This leads me to an exercise in comparing relative costs and benefits of various separators.
Separators are essential to keep the glass from sticking to the shelf or mould that supports the glass. There are several forms of separators –
· liquids and
The papers include the very smooth Thinfire and Papyros papers and the rougher papers ranging from 0.5mm to 6mm. All these contain a binder of some kind.
· Papyros, Thinfire
· Refractory fibre papers - .5 to 6mm
These are mainly suitable for flat surfaces.
· Kiln wash – there is a variety. Most have kaolin - china clay - as a binder. A few do not. These you can just brush off the shelf or mould after firing.
· Colloidal Boron Nitride – a popular formulation is called Zyp.
These are suitable for both flat and curved surface applications.
Powder separators include:
· Alumina hydrate
These have applications directly onto the shelf or mould and onto refractory separators. If used between glass sheets as in bending, very little is required. This is similar when applied to existing refractory papers. As a shelf bed, much more is required.
This analysis of separators shows the first choice is about the application, as some are not useful in a given situation. But in all cases, there are choices in what separator to use.
I used a popular UK website to obtain comparative prices for the various materials.
Papyros paper is listed at £18.46. This is enough for 5 shelves at 52 cm sq. The per shelf cost, assuming two uses per sheet, equals £1.85.
Thinfire is listed at £10.16. This is enough for 5 shelves at 52c m square. The per shelf cost, assuming one use, equals £2.03.
400 g Bullseye kiln wash is listed at £3.96. This enough for about 80 shelves at 52cm sq. The per shelf cost equals £0.05.
400g of Primo primer is give as £6.06. This also is enough for about 80 shelves at 52cm sq. The per shelf cost equals £0.075 (i.e., 7 and a half pence).
Boron Nitride enough for about 25 shelves at 52cm square is listed at £63.93. The per shelf cost equals £2.56.
25kg calcium carbonate is listed at £14.61. This is a one-use material. Applied at half a centimetre thickness, it is enough for 700 shelves at 52cm square. The per shelf cost is £0.02.
300gms talc is listed at £2.99. this is enough for 8 shelves. As this is a multi-use material, assume 10 uses. This gives a per shelf cost of £0.035.
Alumina hydrate is listed at £9.99 for 500gms. Again, this is a multi-use material, so assume 10 uses. This gives a per shelf cost of £0.04.
Ratios of cost between the least and most expensive (given the assumptions) is as follows:
· Chalk =1
· Talc = 1.75
· Alumina Hydrate = 2
· Bullseye shelf primer = 2.5
· Primo shelf primer = 3.75
· Papyros fibre paper = 92.5
· Bullseye fibre paper = 101.5
· Boron Nitride spray = 128
This illustrates that convenience most often wins over expense, as the boron nitride, Papyros and Thinfire seem to be the most popular separators.
Monday, 9 October 2017
There are three important versions of lead corrosion: Red, Brown and White. In addition, there are other factors that can weaken the lead came.
Red lead is a corrosion product that appears as a bright red surface, is dangerous, and requires water, air and often wood, to form. Sometimes water in the manufacturing process can develop red lead. The chemical composition of red lead (Lead (II, IV) or triplumbic tetroxide is Pb3O4 or 2(PbO.PbO2). It is a bright red or orange crystalline or amorphous colour.
Red lead is virtually insoluble in water or in ethanol. But, it is soluble in hydrochloric acid as is present in the stomach. When ingested, it is dissolved in the stomach’s gastric acid and absorbed, leading to lead poisoning. It also dissolves in undiluted acetic acid, as well as in a dilute mixture of nitric acid and hydrogen peroxide.
When inhaled, lead (II,IV) oxide irritates the lungs. In the case of a high exposure, the victim experiences a metallic taste, chest pain, and abdominal pain.
High concentrations can be absorbed through skin as well, and it is important to follow safety precautions when working with lead-based paint.
This means that anyone dealing with read lead needs protection against skin contact, and breathing protection. Methods need to be implemented to ensure no dust is raised, or that the area is thoroughly cleaned after dealing with red lead. Clothing should be discarded or washed separately from all others.
White lead corrosion, Lead(II) carbonate, is the chemical compound PbCO3. It occurs naturally as the mineral cerussite. It is a curious compound, as it is soluble in both acid and alkali. It is possible, but rare, for excess whiting left on the lead to give rise to this form of corrosion. Generally, it will be neutralised by the weather.
Brown lead corrosion appears as a brown to dull red colour.
Lead(IV) oxide, commonly called lead dioxide or plumbic oxide or anhydrous plumbic acid …, is a chemical compound with the formula PbO2. … It is of an intermediate bond type, displaying both ionic (a lattice structure) and covalent (e.g. its low melting point and insolubility in water) properties. It is an odourless dark-brown crystalline powder which is nearly insoluble in water. …. Lead dioxide is a strong oxidizing agent which is used in the manufacture of matches, pyrotechnics, dyes and other chemicals. It also has several important applications [e.g.,] in the positive plates of lead acid batteries. Source: wikipedia
Air, water and salt are needed to form brown lead. This means coastal areas and those with driving rain are prone to this kind of oxidisation. Lead dioxide also forms on pure lead, in dilute sulfuric acid So, with the acid rain that we are all subject to, it can form in almost any situation, but will be more obvious on areas exposed to the prevailing wind. The corrosion is soluble in strong acetic acid.
Tin corrosion also has a brown, almost copper appearance, very similar to brown lead. The tin corrosion will be confined to the solder joint and surrounding area rather than all along the length of the came.
Corrosion resistant lead
The ideal composition of lead to resist corrosion is 98.5% lead with up to 1% tin. This, combined with fractions of a percent of antimony and traces of silver, bismuth and copper provides a combination of metals and trace elements to resist corrosion of the lead as well as stiffening it. Conservators indicate that, for whatever reason, cast lead incorporating trace elements is the most resistant to corrosion. This is evidenced by the longevity of medieval lead cames.
Conservators also indicate that the higher the lead content of solder, and the better the match it is to the lead came, especially the almost pure lead came, the more resistant it is to lead came fracture at the margins of the solder joints.
Stretching the lead came, rather than simply straightening it, not only weakens the lead, it leaves very small pits in the surface of the lead. These small pits allow the elements of the environment to penetrate the lead’s surface and act as sites for the beginning of corrosion.
Stretching also causes stress points near the solder joint. The stretching creates stress along the length of the lead. When the lead is heated in the soldering process the molecules of lead sort themselves into a stress-free arrangement. As heat does not travel far or fast in lead, there is a stress point formed a short distance from the soldered lead joint where the already stressed and the stress-free lead meet.
Clearly there are a range of factors that relate to the resilience of lead came. 98.5% lead with trace elements including tin and antimony provides the greatest strength and resistance to corrosion. Stretching the came can lead to general weakness and introduce pits into the surface forming sites for corrosion. Stretching can also lead to stress points near the solder joints.
All these indicate that resilient leaded glass windows can be produced by:
the use of lead came with 1.5% of trace elements,
the use of high lead content solders, and
the straightening (rather than stretching) of the came.
Wednesday, 4 October 2017
Diagnosis of breaks during slumping processes is often difficult because the temperature is not high enough to be able to apply the usual rule.
In looking for the reasons for a break in fusing processes, sharp edges imply the break occurred on the way down in temperature, but rounded edges indicate the break happened on the way up to the top heat.
This not a universally applicable diagnosis.
At low slump temperatures, the edges will be sharp in both the case of a break on heating up, and in the case of breaking on the way down in temperature.
The best test to determine when the break occurs is to observe periodically during the heat up. You will be able to see if the piece breaks before the top temperature. If it is whole at top temperature, the break occurred on the way down.
If you have been unable to observe the progress of the firing, you will need to diagnose when the break occurred. The test here is not whether the edges are rounded or sharp, because at normal slumping temperatures, the break will be sharp in both cases.
If the break occurred before the top temperature, the pieces will shape separately. Therefore, If the pieces no longer fit together, the break was on the rise. If they do, the break was on the way down. Place the pieces very carefully together to see if they form part of a continuous curve. If they do, the break was on the cool down. If they almost match, or do not match at all, then the break was on the rise in temperature.
In general, when the break is on the cool down, there is an overhang of the glass on the mould which causes the break. But the most common break of a slumping piece is caused by a too quick rise in temperature.
For a flat 6mm piece, the slump temperature rise should be less than 2/3 as quick as the rise for the fusing. If you have a tack fused piece to be slumped you should reduce the rate of advance to at least half of that for a smooth, flat piece of 6mm. Thicker glass with tack fused elements will need to be even slower.
Wednesday, 27 September 2017
You can use high temperature wire for screen melts. This is variously described as Kanthal or nichrome wire. It is the same kind of wire used in the heating elements of your kiln.
|wire with bent ends|
To use the wire, you lay or weave the wire and support it on both ends. Weaving the wire provides more support, but is not necessary, as the wire is strong enough to support a lot of glass.
|first line of wires pushed into board|
You need to have the wires as tight as the supporting material will allow. Straightening the wire before beginning to fix them will help, as will thicker wire.
The wires need support at each end, which can be brick, cut up shelves, or strips of tile. If you do this, you can form a dam or vessel in which to put the glass without fear of it spreading over the edge.
I use fibre board for the support and just bend a right angle into each end of the wire to push into the board. These can be arranged in any configuration, although for ease of illustration, I have used a rectangular arrangement of wires.
|A grid of wires ready for kiln wash|
Put the completed screen over a tray or sheet of plastic to collect the excess kiln wash. Mix the kiln wash very thick and pour over the wires. I put the board with wires into the kiln to heat to about 200°C to help the wash stick. I repeat a few times.
Make sure you coat the area surrounding the screen to avoid the glass sticking to the supports.
When the kiln wash has dried, knock off the stalactites of wash on the underside of the wire to prevent any excess kiln wash being incorporated into the final piece.
Place on kiln washed supports, and put the glass on top of the screen.
This is a relatively quick and inexpensive means of providing a custom shaped screen.
One disadvantage of this over stainless steel rods, is that it is difficult to get enough kiln wash to stick to the wires to be able to pull them out easily.
Wednesday, 20 September 2017
Capping with a clear or tinted top layer is necessary in many cases of inclusions, or desirable when looking for depth or distortion in flat fused work.
Capping inherently has bubble creation potential. The development of a bubble squeeze helps prevent the largest of bubbles. It cannot eliminate all the trapped air that then turns into small bubbles around the inclusions or multiple pieces when covered by a sheet of glass.
An alternative is to do away with the sheet glass capping and instead use enough frit to provide the desired depth, or the necessary material to cover the inclusion. In fusing with two large sheets, a fine covering of powder between the layers will help to eliminate bubbles. However, this will not be enough to successfully cover metal or other inclusions, or provide the amount of glass to give an appearance of depth.
The size of frit to use in a given application can be determined from other styles of glass working. It is known from glass casting that the smaller the frit the greater number of small bubbles will appear in the fired piece. This means that you need to use medium sized frit for cast work. Fine frit is likely to produce many very small bubbles across the whole piece in fusing applications. Large frit is likely to produce larger bubbles, as the pieces themselves trap air as they deform. This means that medium frit is a good compromise between large and small bubbles in capping.
The layer of frit should be at least 2mm thick. This means a lot of frit is required to do the job. To judge the amount, you can measure the area of a rectangle or circle in square centimetres and multiply that by 0.2 to give you the volume (in cubic centimetres) of frit required. Multiplying the volume by 2.5 (the approximate specific gravity of soda lime glass) will give you the weight of frit needed to cover the area.
Alternatively, if the piece is irregular, you can weigh the base and add the appropriate weight of frit on the top. If the base is 2mm, no further work is required to determine the weight. Weigh the 2mm sheet and use the weight of frit to equal the base. If the glass is 3mm, you need two thirds of the weight in frit, and so on for thicker glass.
Using frit to cap is unlikely to eliminate all bubbles, but it will reduce them to a minimum.
Wednesday, 13 September 2017
Not all steep slumps are deep.
|An example of a deep, steep slump|
|An example of a soup bowl with steep sides|
|A square bowl with slightly less steep sides|
|A shallow plate or platter|
Relative to the size, the above platter mould is a steep slump, although not deep.
This can be slumped in two stages to obtain confirmation of the glass to the mould without distortion.
One way to do this is to place powdered kiln wash in the mould so there is a gentle curve to the bottom. Place glass on the mould and do a slow, low temperature slump.
After first slump, empty the kiln wash back into your container (it can still be used as kiln wash). Fire again using the same slow low temperature schedule as for the first.
It may also help to retain the rim on the shallow plate to cut your circle 12mm larger than the diameter of the mould. This will allow a margin for the slight shrinking that even a low and slow temperature slump will cause.
Wednesday, 6 September 2017
What is boron nitride? What makes it a good separator?
Boron nitride is a heat resistant refractory compound of boron and nitrogen with the chemical formula BN. It is also chemically stable at elevated temperatures. It exists in various crystalline forms that are similar to a structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN forms. It is the form most useful in kiln forming as a smooth release separator, especially for steel. It is also used as a high temperature lubricant, and has a wide use in cosmetic products.
There is a cubic form that is similar to diamond (called c-BN), but softer. It has a superior thermal and chemical stability. There is a harder form called wurtzite, but which is rare. Neither of these is of much use in kiln forming.
Hexagonal BN (h-BN) is the most widely used form of boron nitride. It is a good lubricant at both low and high temperatures (up to 900C, even in an oxidizing atmosphere). Another advantage of h-BN over graphite is that its lubrication properties do not require water or gas trapped between the hexagonal sheet layers. So, h-BN lubricants can be used even in vacuum, e.g. in space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cements, and pencil leads. In kiln forming, the high temperature lubricating properties are made use of as separator between metal, ceramic and other supporting materials for the glass.
“Hexagonal BN was first used in cosmetics around 1940 in Japan. However, because of its high price, h-BN was soon abandoned for this application. Its use was revitalized in the late 1990s with the optimization h-BN production processes, and currently h-BN is used by nearly all leading producers of cosmetic products for foundations, make-up, eye shadows, blushers, kohl pencils, lipsticks and other skincare products.”
It has wide application in materials to give them self-lubricating properties. Boron nitride has the properties of stabilisation of materials, reducing expansion and resistance to electrical conduction, making for wide use in plastics and electronics among a wide variety of other products.
Health and Safety
There are some health issues related to its use. It is reported to have a weak association with the formation of fibrous material in the lungs and so result in pneumoconiosis when inhaled in quantity in particulate form. It is best to wear a dust mask when applying and to do it outdoors, as simple ventilation will not prevent dust settlement indoors.
Wednesday, 30 August 2017
Petra Kaiser is reporting that there are people finding cracks in white W96, which she cannot be replicate. However, they are using strange firing schedules.
The most popular one appears as follows, in Celsius, with my comments.
166°C per hour to 232°C and hold 20
166°C is relatively slow. It is a rate I would use for a fused 6mm piece. An unfired two-layer piece I would fire at 200°C to the bubble squeeze. There is no effect in soaking for 20 minutes at this temperature. If there is a worry (often expressed) that there will be thermal shock unless you let the glass catch up, slow the rate of advance to 134°C. This is of course excessively slow for a two-layer piece.
If, however, you are tack fusing onto two un-fused layers, then 166°C may be appropriate, as you are shading parts of the base from the heat of the kiln. But the soak is not necessary. It does not do anything useful.
166°C per hour to 538°C and hold 20
As the rate for this segment is the same as for the first, I repeat the soak is not necessary. If the glass survived the first 200°C at this rate, it will survive the next 300°C too.
This rate for two layer pieces could be increased to 200°C without damage.
The 20-minute soak at this temperature again does nothing useful. If the glass survived to this point, you can continue the temperature rise to the bubble squeeze at the same rate as in this segment.
278°C per hour to 621°C and hold 30
Although this rate is not excessive, there is no real reason to speed the temperature rise. If you use 200°C from the outset to the bottom of the bubble squeeze, no time will be lost in getting to the bottom of the bubble squeeze.
However, this schedule leaves out the important second part of the bubble squeeze. This is a slow rise to about 50°C above the start of the bubble squeeze process.
Insert an advance of 50°C per hour to 670°C with a 30-minute soak
278°C per hour to 788°C and hold 15
788°C is a temperature given in the Wissmach tutorial on firing schedules. However, Petra Kaiser has found that 771°C with a 10-minute soak is sufficient for a full fuse (or 765°C with a 12-minute soak).
The speed at which you reach the top temperature affects what you need to use as the top temperature. This rate of less than 300°C will not require more than 771 as a top temperature. However a faster rate will require a higher temperature, and with it potential bubble problems, over firing, needling, and inconsistent results.
afap to 527°C and hold 120
This seems to come from the old Spectrum 96 schedules where a temperature equalisation soak was established above the annealing point. Even if it were necessary, two hours is excessive.
The temperature equalisation of the glass should occur at the annealing point. Therefore, this segment is unnecessary. And should be replaced by an AFAP to 510°C
55°C per hour to 510°C and hold 120
If the previous segment is eliminated, the rate in this one should be AFAP to 510°C with a soak of 30 minutes for a full flat fuse of 6mm. There is no need for a longer temperature equalisation soak, as this is enough time for all the glass to be within 5°C of each part.
If you were tack fusing, a soak of an hour would be sufficient for a single layer of tack on a 6mm base.
28°C per hour to 399°C and hold 1
This rate is appropriate for a piece of 19mm. A 6mm piece could use a rate of 80°C per hour. A tack fused piece as described above could have an annealing cool of 60°C per hour.
Depending on the natural cooling rate of your kiln, it is possible to turn the kiln off at this point. If you kiln cools off faster than the cooling rates given above, then you do need to programme a second stage cool.
55°C per hour to 93°C and hold 1
This is excessively slow for a 6mm thick full fused piece – a possible rate would be 200°C per hour.
The one-minute holds in these two down rates are only required where your kiln controller will not accept “0” as the number. If the controller will accept 0, then use that, as 1 minute will not do much of anything, except confuse.
Writing and evaluatingschedules
When you are writing or looking at others’ schedules, review what is happening to the glass at various temperatures. This excellent guide tells you what is happening to fusing glass at various temperature ranges. Float glass has some different characteristics.
Combine that knowledge with what you are trying to achieve in the firing.
This table shows the lack of correlation between CoE and tempereature characteristics of the glasses. See the previous post for the discussion.
|Pilkington UK Float||83||540||720||835|
|Sorted by annealing point, averaged
CoE Anneal Slump Full fuse
|Pilkington UK Float||83||540||720||835|
|Sorted by Slump point, averaged as
CoE Anneal Slump Full fuse
|Pilkington UK Float||83||540||720||835|
|Sorted by full fuse, averaged as necessary|
|Pilkington UK Float||83||540||720||835|