Wednesday, 26 November 2014

Bubbles in Thin Pieces


Bubbles are often blown through frit castings and other thin pieces. This most results from insufficient volume of glass in the mould or on the shelf. Also the design can induce bubbles where there are thinner parts surrounded by thicker parts. As the glass softens, the surface tension of glass - from around 730 - causes it to pull up to equalise at about 6-7mm thick. This causes thinning in certain areas to allow thickening in other areas. This then leads to the risk of blowing bubbles through the glass where the glass has become thinner.

If thinner work is required, you can fire an over-sized piece to about 750C for a short time and then cut it back to the final size. If you want a flat thin sheet, you can also place the glass between two kiln shelves. You need to separate the shelves with a 3mm spacer to keep the upper shelf from coming completely down on the shelf, giving an extremely thin fragile piece of glass.


Wednesday, 19 November 2014

Diagnosing Fractures


What does the nature of the fracture tell about the reason for the break?
  • incompatibility
  • annealing
  • adhesion
  • splits
  • lamination

Incompatibility
Fractures that follow the outline of a glass are normally indicators of incompatibility. The fracture starts at the incompatible glass and then - usually – goes directly to the nearest edge. Occasionally, the stress is not so great, so it only breaks around the offending glass without proceeding to the edge.

Annealing
A sinuous break – often with a hook at the edge – across the whole of the piece is generally an indication of one caused by an annealing stress. Inadequate annealing builds up stress within the glass that breaks through the whole piece in a lazy “S” pattern, rather than a straight line or following outlines of glass pieces.

Adhesion
Another kind of fracture occurs that is most often seen in ceramics. It is a kind of crazing that leaves the glass in granules. I call these adhesion fractures. This is indicative of the glass having stuck to the surface it is resting upon. This can be ceramic, steel or any other rigid refractory material. This comes from inadequate amounts of separator, often at high temperatures.

Split
Sometimes during slumps the piece can develop a tear or split in the lower surface without the upper breaking. This kind of split comes from heating the top of the glass more rapidly than the heat can penetrate the whole thickness. The weight of the relatively plastic upper surface overcomes the resistance of the lower surface by splitting it on the bottom face.

Lamination
Occasionally, a break will have both of the characteristics of incompatibility and annealing stress. The break is relatively straight and goes through differing colours rather than skirting them. This seems to happen most often on tack fused pieces and so is likely to be inadequate annealing. The annealing requirements of tack fused glass are much greater than flat fused glass, as the pieces are to some extent still reacting separately. If the whole piece is not given enough time for each piece to settle with the others they will contain unrelieved annealing stresses, which may have be too great to be held within the whole.

Wednesday, 12 November 2014

Observation


It is the monitoring and observation of the effects firings as they progress that allows confidence in setting firing temperatures and schedules. Although we all have busy lives, planning the firings so you can watch at the forming temperatures enables you to develop your firing practice much more rapidly than firing and waiting to see what comes out the next day. It means that in a single firing you can pretty accurately determine the temperature you need for firing that type of piece, rather than an number of separate firings.

You set your schedule - for the best guess that you can make - at the required temperature, rate of advance, and soak to achieve what you need. At about 50ºC to 20ºC (depending on your certainty) before the set point, you begin peeking to see what the glass is doing. When the glass has achieved the desired result, you advance to the next segment. You of course, have already refreshed your memory on how to do that from your kiln manual.

There is a method of opening and closing kiln to be safe and avoid disturbing the contents. Any observation ports should be opened first. The lid/door should be opened slowly and only enough to see what you had already planned to look at, to determine whether it is ok or a decision is needed for some other action. This opening should be only a few seconds. The air temperature will change dramatically, but the glass temperature will lag behind significantly, so a few seconds with the door only cracked open will not damage the glass at most temperatures. The exception to this is the annealing range – generally around 520C to 400C. The kiln should not be opened at these temperatures so that there is no disturbance possible to the steady and even annealing of the glass.

At temperatures above the annealing, you need to have protective clothing. At the minimum you need natural fibres such as cotton or wool, and eye protection. It is important to check with your hand the amount of heat coming from an observation port before moving your face toward it to look into the kiln. When the kiln is being opened even for brief periods, you should protect you eyes from the infra red given off by the kiln's interior. You should have something to protect your arms and chest too.

Always when raising and lowering the lid – or opening and closing the door – do it slowly to avoid creating puffs or billows of air moving through the kiln which might disturb the pieces at low temperatures or move debris over the hot glass at the higher end of kiln forming.

If the glass has not achieved what you want by the end of your soak, just extend the hold until the effect is achieved. You will have reviewed how to do that from your kiln manual before starting the firing. When the glass has achieved the effect you desire, advance to the next segment of the schedule as the kiln manual directs.

You then record the schedule including temperatures, rates, times, effects, etc. You should include a description of the project and its dimensions and nature e.g. full fused, tack fused etc. You will also want to include what this was fired on, what kind of mould – include its description. This will give you the reference for that nature of project for the future without needing to guess.

Wednesday, 5 November 2014

Recognising Devitrification


The appearance of devitrification varies from mild streaks as a dirty appearance on the surface, to at worst a granular surface that breaks away in small pieces. The glass will often have raised sharp corners in cases of severe devitrification.



Avoiding devitrification relates to cleaningfiring rapidly through the devitrification rangeavoiding devitrification-prone glasses, and grinding edges as little as possible.



Repairing devitrification requires the removal of the devitrified surface. This can be done by sandblasting, sanding the surface by hand, using acid pastes to remove the surface. Then the piece needs to be fired again to a fire polish.



To ensure a polished surface a devitrification solution may need to be applied. It can be a commercial product or a borax solution.  Any devitrification solution should be applied evenly.

Wednesday, 29 October 2014

Spray nozzles


Quite a bit of the material we spray is solids in a colloidal suspension. This means that the nozzle can clog easily. Frequent agitation is needed to keep the material in suspension and not building up on the bottom which can clog the screen at the bottom of the tube.




You should not have long delays between spraying in one session, as the solids can begin to solidify within the spray head and so clog it.

When you have finished spraying, take the spray head off and clean it completely and thoroughly to ensure there are no solids left to harden. Then put it back together and it should be clear for the next use as well as sealing the container.




Wednesday, 22 October 2014

Stainless Steel Preparation


Preparing stainless steel rods and moulds for kiln work is done slightly differently from ceramic moulds.

Just to ensure that the steel is of the right grade, I fire it in the kiln to about 720C. This ensures that if the steel is not adequate for the high temperature work, you will find out that it spalls before the glass is put on top. It also has the advantage of removing any dirt and oils on the surface of the metal.




The separator that you need to put on the steel can be done cold if you use MR97 or other boron nitride coating. Its main advantage is that it can be put on cold and also that it has a very smooth surface. This should be put on thinly, or it will come off onto the glass.



You can also put standard kiln wash on the metal. The metal needs to be dry and clean. It could be sandblasted if desired for a bit of extra “tooth”, but is not normally necessary. Heat the metal to about 120C – 150C in the kiln. Remove it from the kiln with tongs or similar thing to grasp the hot metal. Spray or paint the kiln wash solution onto the hot metal. Return it to the kiln as necessary until you have a coating all over the metal. It does not have to be even all over, but noes need to have all of the metal covered.



If the kiln wash boils off the metal, it is too hot. So turn the kiln down a bit.
If the kiln wash runs off without sticking at all, the metal is not hot enough and needs to be returned to the kiln to heat up.
It is best to avoid applying the kiln wash to the metal in the kiln, as water and the hot elements do not mix well.

Wednesday, 15 October 2014

Making Tests


Every time you get a kiln that is new to you – whether new or second hand – you need to do some tests. Recording these tests is essential to your future work.

The first test is to get to know your kiln – where are the hot and cool spots, what effect does the kiln produce at a given temperature and soak. A very good guide to knowing the temperature differentials in your kiln is given by Bullseye Tech Note 1 – Knowing Your Kiln 

The second test is to make a series of small scale tiles to know what your kiln does at different temperatures and rates of advance. What combination of rate and temperature gives the roundness, degree of tack, flat fuse that you want.
How does the kiln perform in slumping and what are the effects of thickness and number of layers on the rate of slump.

These are elements that you may feel are a delay in being able to experience the enjoyment of fusing. However, they are essential to the long term enjoyment and success of your fusing activities.


Wednesday, 8 October 2014

Writing a Schedule


Making your own Schedule

I've been asked about making a schedule rather than using a pre-programmed one. My response is this, but please join in with amplifications and questions.

In principle, a firing schedule for glass follows these stages:
1 – a gradual, steady heat up to a temperature above the annealing point to avoid thermal shock
2 – a soak or slow rise around the slump temperature to allow any air to escape
3 – a more rapid rise to top temperature to avoid devitrification
4 – a rapid fall in temperature to an annealing soak, saving time and avoiding devitrification. The soak at annealing temperature is to equalise the temperature throughout the glass
5 – a steady slow fall in temperature to well below the lower strain point to complete the annealing
6 – a controlled cool to near room temperature to avoid thermal shock.


The details of schedules can appear complex, but the purposes of these six stages are reasonably simple.
Segment 1 is to heat the glass evenly without causing it to break from too fast an increase in temperature. At minimum this steady increase in temperature must continue to about 40ºC above the annealing point. (This will be about 540ºC)
Segment 2. This segment can include a “bubble squeeze” to enable air to get from between sheets of glass before the edges seal, or it can be a separate segment in your schedule. The slow rise in temperature will occur from about 600ºC to 680ºC. The bubble squeeze soak occurs at around 660ºC to 680ºC. In both cases there is normally a soak of half an hour at least at the end of the range.
Segment 3 is to go through the devitrification range (say 700ºC to 760ºC) as quickly as reasonable, but usually no faster than 330C per hour.
Segment 4 is to get back through the devitrification range to the annealing soak, which will be as long as required to equalise the temperature within the glass. This soak time increases exponentially with the thickness.
Segment 5 is the annealing cool, which should be a slow steady fall in temperature to ensure the glass all cools at the same rate (to around 370C).
Segment 6 continues the cool, although faster than previously, and often is achieved by turning the kiln off and leaving it closed until room temperature.

A schedule for a 6mm piece up to 2/3 the size of your kiln could be even simpler:
Segment 1 - 220 dph to 670C for 30 minutes
Segment 2 - 330 dph to 800 (flat fuse) for 10 minutes
Segment 3 - afap to 516 for 30 minutes
Segment 4 - 80 dph to 370, no soak
Segment 5 - off

You may find a schedule that will work, but you still need to know why it works, or at least what each segment is doing. So, for example, you need to think about what a 15 minute soak at 225C will do. What is the glass doing at that temperature? What do you want to achieve in that temperature range? Is there another way to achieve your objective? These are the kinds of questions you need to think about so you can construct your independent schedule when you move outside the parameters of the pre-programmed schedules.

To make a schedule for yourself can be worrying. But you can see from this example that it does not need to be complex. The principles are simple, although the details can be confusing. It is essential to know something about how heat affects the glass and this Bullseye Tech Note is one of the best descriptions. 
Knowing what the heat up events are is useful too. 

Wednesday, 1 October 2014

Selecting a Kiln


Kiln Forming – Selecting a Kiln
You have been doing some fusing and slumping and now want to get into kiln forming in a serious way. So you need to get a kiln.

The basic kiln choices are ceramic vs. glass. The brand, model and size are up to you. But there is some helpful information on the advantages of each kind of kiln in this post.

Another consideration is the shape of the kiln. Generally the greater the area of the shelf in relation to its size, the better it will suit fusing and kiln forming. Oval kilns seem to waste some space, although they do not have cool corners like the rectangular ones do. Relatively deep and round kilns are best for casting and high temperature work.

Even before you buy the kiln you need to think about where you will be installing the kiln and that will have an effect on the model and size. Some considerations are here.

Think about the kind of work you want to do. This will change with time, but you cannot anticipate that now. Will you be doing jewellery scale, detailed work, lots of forming work, high temperature or even casting work. Each of these have different requirements.

Small kilns are best for jewellery and detailed work – they can be fired quickly and will reach the top and annealing temperatures with a minimum of delay.

If you tend to work larger then you should consider a kiln of about 40 cm square to start with, although smaller kilns will work if they have enough height.

Slumping and kiln forming put a premium on height. If you are going to be doing a lot kiln forming you should consider a kiln with at least 25cm from base to elements. Ex-ceramics kilns can be good for this.

If you are going to be doing a lot of high temperature work, such as casting, pot melts, pattern bars, etc. you might want to consider a brick lined kiln such as a ceramics one as they retain heat by design longer than those designed for glass.

There are a lot of models, so it is up to you to find the combination of style, shape, size and price that suit your present needs.

Wednesday, 24 September 2014

Selection of a Training Course

Often selecting training in glass is a lottery. It most commonly is done through propinquity and incidental knowledge. Most often a course is chosen because information came to hand of a class that is being held nearby. These are not always the best criteria. It may be better to travel for a course that fits your needs better.

There are some things that you can check to help determine whether the course being offered is the one for you.

The first of course, is whether the instruction will meet your interests. Yes, the title has caught your attention, but you need to find out if the syllabus covers your area of interest adequately. 
· Inquire for a syllabus or teaching outline. If there is not one, you may have a question on whether the course is well planned, as well as whether it deals with your interests.
· Ensure the course level is appropriate to your needs. Are there any prerequisites in terms of experience or ability?

Another important element in selection is the person who is leading the course. The leader may of course, may have brought in teacher for this subject, so you need to know things about both. 
What is the background to the course leader? Some of the things you might want to find out are:
· Is the course leader part of a business providing materials,
· Is it in a centre of excellence
· Is it an accredited education provider
· Is the leader a studio owner or artist
· What is the history or experience in providing training courses
Who is the teacher? Some to the things you should ask about in addition to the person’s identity are:
· What is known about her/him? Is there a CV available?
· Where examples of work can be seen
· What experience does the person have in making in general and in making using the technique(s) being offered?
· What length of teaching experience does the person have?

Having satisfied yourself about the instructor(s) you need to begin doing some comparisons with other course offerings. Price is always important, but you need to know what value you are getting, so you need to know what is included in the price. Some of the things that affect price and value are:
· Tools – are they all included, or do you have to provide your own?
· Materials – are the materials included in the price or do you buy as you use?
· Equipment – is the use of all the machinery and facilities included? What is excluded?
· Food – are meals included and which ones? What refreshments are available?
· Length of instruction time – how many days are involved? What are the hours of instruction? Are there any extensions of instruction or working time?
· Numbers – what is the expected teacher to student ratio? This will affect the amount of time you receive from the teacher.
· Accommodation – if relevant, is it included? Is there any assistance in obtaining accommodation?

You should also find out about booking, deposits, cancellation conditions, and when payment is due.

Another element relevant to selection is the premises in which the course is to be held.
· Are they purpose built for the relevant activities?
· Are the premises general educational accommodation? Is it local authority classroom provision? Etc.
· Is the instructor’s studio being used? What space is available?

An obvious important element in selection is convenience.
· Location – is it near or easy to get to?
· Time - are the days convenient? Is the time of the day appropriate?
· Duration – how long are the sessions?

A really important element in selection is the evaluations by past students. These are difficult to get, and if supplied by the instructor, are open to doubt. The best source of evaluation is direct contact with past students. It is important to ask friends and other people in the field about the course being offered.

Documentation about the course is helpful in getting a feel about what is being offered. This might include information about the instructor(s), description of course, dates, times, cost, information on level of instruction, location, travel, facilities, and accommodation.

It is unlikely that you will get the best fit in every category. You will need to make compromises on various elements, so it is important that you think about what you want from the course. If there are one or two elements that you feel are important but not covered, you should contact the provider and ask about whether any accommodation to these requirements are possible.

Gravity


One of the fundamental elements in kiln forming is gravity. When glass is hot it moves according to the effects of gravity and you have to remember that gravity has a big effect on all your firings.

The effects mainly cause:
  • Uneven thickness on shelves that are not level.
  • Uneven slumps into moulds which are not level or the glass is not levelled.
  • Uneven forming due to varying viscosities. Gravity acts on the softest parts of the glass first.
  • Faster or slower forming due to span width. With greater span, gravity pulls the glass into the mould more quickly than with a small span.
  • Gravity acts on things of greater thickness more quickly than those of lighter weight. So a thick piece will form more quickly than the same sized thin piece.
  • Surface tension (affected by viscosity and heat) is affected by gravity also. The glass will attempt to draw up or spread out to about 7 mm if there is enough heat, time, and low viscosity.
  • The effect of gravity causes upper pieces to thin lower ones, as it presses down while the glass is plastic. This has the effect of making the colour of the lower piece less strong.

More information on each of these effects can be found throughout this blog.

Wednesday, 17 September 2014

Screens for Melts




You can buy various stainless steel screens such as barbecue grids for supporting glass melts. The grids need to be of stainless steel. Type 304 is the most common, but there are other grades which work at high temperatures too [link to stainless steels]

You can make your own grid as Cynthia Morgan does. This provides a more flexible arrangement for various effects. 

Instead of imbedding the rods into the brick, you could also place them on top. Place a kiln brick or other kiln furniture on the ends of the rods to secure the metal from moving. Then you can put the glass on top of the rods without them shifting as the glass is placed.



Wednesday, 10 September 2014

Stainless Steel for Kiln Uses


The reason for using stainless steel is that it differs from carbon steel by the amount of chromium present and reduces the spalling. Unprotected carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide, and due to the greater volume of the iron oxide this tends to flake and fall away (spall).


Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion by blocking oxygen diffusion to the steel surface and blocks corrosion from spreading into the metal's internal structure, and due to the similar size of the steel and oxide ions they bond very strongly and remain attached to the surface.


There are a number of grades of stainless steel. Some of the ones that perform better in hot conditions are:

300 Series—austenitic chromium-nickel alloys. Austenitic steels have a cubic crystal structure. Austenite steels make up over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the extremely cold to the melting point of the alloy.

Type 304—the most common grade; the classic 18/8 (18% chromium, 8% nickel) stainless steel. Outside of the US it is commonly known as "A2 stainless steel", in accordance with ISO 3506 (not to be confused with A2 tool steel).

Type 304L—same as the 304 grade but lower carbon content to increase weldability. Is slightly weaker than 304.

Type 304LN—same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.

Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with inconel.

Type 316—the second most common grade (after 304); for food and surgical uses; alloy addition of molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304.

Type 316L—is an extra low carbon grade of 316, generally used in stainless steel watches and marine applications, as well exclusively in the fabrication of reactor pressure vessels for boiling water reactors, due to its high resistance to corrosion. Also referred to as "A4" in accordance with ISO 3506.

Type 316Ti—variant of type 316 that includes titanium for heat resistance. It is used in flexible chimney liners.

Type 321—similar to 304 but lower risk of weld decay due to addition of titanium.

400 Series—ferritic and martensitic chromium alloys

Type 439—ferritic grade, used for catalytic converter exhaust sections. Increased chromium for improved high temperature corrosion/oxidation resistance.

Type 446—For elevated temperature service


500 Series—heat-resisting chromium alloys


Based on Wikipedia 

Wednesday, 3 September 2014

First Firing


Even if this is not your first kiln there are a number of things to do when starting.

The first is to read the manual. Obvious, but in our enthusiasm to get started, reading seems boring. It is essential to understand what the manufacturer wants you to do and to understand how the kiln and controller work.

Then, you can prepare kiln. Test fire the kiln empty to make sure it works and burn out any binders remaining in the kiln materials. You can do this firing at about 400C/hour as there is no glass to damage. Fire to around 800C. Then you can shut off, or programme a dummy anneal. This also ensures you know how to work the controller.

While waiting for the kiln to complete the fast test firing, read the manual again.

When the kiln is cool, apply kiln wash to the bottom (if it is brick) and lower sides, below any side elements. If the bottom or sides are fibre, no kiln wash is required. Kiln wash the shelf and any kiln furniture too.

Now is the time to test for how even the heat is in your kiln. Arrange the kiln furniture around the shelf as described and put glass over. The kiln furniture can be any refactory material, even folded 3 mm fibre paper will be strong enough to hold the small pieces of glass above the shelf. Fire the kiln as described in the Tech Note 1.

Now try out the suggestions in the manual, especially the programming of the controller, even if it has pre-programmed schedules. Look at Bullseye and Spectrum sites to get sample schedules. Enter these as trial schedules.

Then you should be ready to fire the first piece of glass. Place the glass in the kiln, programme it, and record the information about the firing. Now turn the kiln on for the first real fuse.

Wednesday, 27 August 2014

Cleaning Glass Before Painting



If your glass is not really clean, you can get gaps in the paint line. You may also have areas where the paint beads up rather than flows evenly.

You need to clean as best as possible first. Then just before painting you can use a dilute solution of the paint to scrub all over the glass with your finger or other firm material. Wipe any residue off with a paper towel and you will find that the prepared paint will flow evenly onto the glass.


Wednesday, 20 August 2014

Fiber Paper Safety


There are often concerns raised about safety relating to the dust from Thinfire, a trade marked product from Bullseye. These concerns also apply to the Spectrum product called Papyros. The main constituents of these are cellulose, aluminium hydroxide, fibre glass and organic binders. It therefore is mainly a nuisance dust and irritant. Greg Rawles, a certified industrial hygenist with a scientific and common sense approach, says that the toxicology of fibre glass is not yet determined. So the best idea is to wear a P95 respirator while dealing with the dust – mainly the cleaning phase of dealing with the shelf. He also recommends keeping the kiln closed during the burn-out phase of the binders, although others recommend venting to avoid anything settling on the glass during the burn-out.

Some people seem to have skin reactions to dust including that from Thinfire and Papyros and tend to wear gloves. If you are wearing rubber or latex gloves for extended periods, it may actually promote a reaction to the gloves rather than a primary reaction to the dusts of the paper. So checking on alergic reactions to the gloves is advisable in addition to the dust. 

If in fact, you are reacting to fibre paper dust, consider using kiln wash instead. If this is not practical or desirable there are several things you should do.
  • Use gloves (for those with sensitive skin),
  • wear long sleeved high necked shirts and pull the gloves over the ends of the sleeves to prevent dust getting to your skin,
  • vacuum the kiln with HEPA filters,
  • dampen the powder with water spray before brushing,
  • take precautions against taking the dust home by changing in the studio and placing clothing in plastic bags to take to the wash.


Wednesday, 13 August 2014

Black Specks in Mesh Melts


The first time you use a mesh for a melt, it doesn't spall until it cools. By that time, the glass has hardened enough that any black specks of metallic oxidisation just land on the top of the melt and can be brushed away.

But, once a mesh has been fired previously, it can spall and drop little bits at any time during the firing process, so some of the bits get embedded in the glass.

The only way I have found to prevent this is to sandblast the mesh between firings to remove any loose flakes of metal. This is time consuming enough that you may wish to use a new piece of mesh for each melt. The alternative is to ensure you are using stainless steel as the grid.

There are several options for grids.

Wednesday, 6 August 2014

Applying Kiln Wash


Kiln wash, or batt wash as used in the ceramics field, is largely made up of alumina hydrate, kaolin (china clay), and often some colouring to indicate an unfired shelf.

These solids are heavy and settle to the bottom of the container quickly. So, you have to agitate the contents with each dip of the brush onto the liquid. To provide adequate - and even – coverage of the shelf, mould or other refractory material, you should paint in four directions. Up, down and the diagonals. You need to apply just enough that you do not see the shelf surface.

Alternatively you can spray the solution onto the surface. This is an easier way to get an even covering, but it sometimes is overly stippled.

A tip I was given for the smoothest kiln shelf is to level the damp prepared shelf and spray a layer of warm water over the wash to form a very shallow puddle. As the water is absorbed into the shelf, the only limitation to the smoothness of the surface is the granular nature of the kiln wash.

Saturday, 2 August 2014

Layering Glass Textures


When using textured glass there is a decision to be made on whether the smooth or textured side is up.

Oddly, the largest, but thin bubbles occur when putting the smooth sides together. it seems that as the glass is not perfectly flat, it holds air within the fused piece.

The fewest bubbles seem to be promoted by placing the rough side down on all pieces. This is easy as cutting is done on the smooth side anyway, and so no reverse cutting is required. It seems that the rough side of the glass provides ways for the air to escape during the bubble squeeze although it does promote micro bubbles within the glass.

If more bubbles are desired, you can place the textured sides together. That seems to allow the majority of the air out, but still leaves the micro bubbles from both sheets.

I have had good results following the Bullseye recommendation to keep the smooth side up on all layers. 

Wednesday, 23 July 2014

Re-firing Poorly Annealed Items

Sometimes you suspect a piece has not been adequately annealed and want to re-fire it to make it sound. The question arises as to how quickly it can be re-rfired.

These pieces are very easy to heat shock, so the initial rate of advance needs to be much slower than for any piece of the same size, possibly less than half the usual rate. This slow rate should be steady without pauses until about 540ºC, which is above the annealing point of most fusing glasses. At this point you can speed up the rate of advance to whatever your normal one is.

Of course, it is best to anneal each piece on each firing to the extent that there is no question  that the piece is properly annealed. Looking at the Bullseye project notes and the annealing of thick slabs can help for evenly thick items. For tack fused and items of uneven thickness, you could review this posting.

Saturday, 19 July 2014

Diagnosis of Breaks in Kiln Formed Glass

Often more can be learned from failures than a number of successes. A common failure in kiln forming is broken glass. The appearance of the break will tell you a lot about the problem so that you know where to look for the solution.

Cracks and breaks can occur at various times in the kiln. These will have occurred by the time you open the kiln:
  • Curved cracks and breaks are usually caused by inadequate annealing. Often the break will have a hook or sharp curve near the edge of the glass. The edges will be sharp.
  • Cracks and breaks occurring where two pieces of glass meet is usually an indication of incompatibility between the two glasses. This means that you need to perform a compatibility test with the two glasses. Sometimes it is caused by a large difference in the thickness of the glass, especially when light and dark glasses are side by side. This is normally an annealing problem.
  • Breaks in the piece (often more than one) with rounded edges indicate a thermal shock break caused by raising the temperature too quickly for the size or thickness of the piece.
  • Breaks that cross the piece in a reasonably straight line, going across and through pieces of glass are an indication of thermal shock.  The line will be rounded or the pieces even formed together again if it was shocked on the rise in temperature.  If the piece was cooled too quickly, the edges will be sharp.
  • Multiple breaks into small pieces - normally sharp - are an indication that the glass has stuck to the shelf or kiln furniture. This is caused by inadequate batt wash on the shelf and kiln furniture. It tends to happen with high temperature firings more than lower temperature firings.
Other cracks and breaks occur after the piece has cooled.
Breakage occurring long after a piece has been completed are an indication that the stress within the glass has overcome the strength of the piece. There are several possible individual and combined factors:
· improper annealing,
· thermal shock,
· incompatible glass,
· wear and tear.

But the most likely problem is inadequate annealing. Unless you have access to your firing records and can determine how the piece was fired and the materials used, you will need to accept it as experience and extend future annealing times.

The best cure for these is prevention.

First is to do a compatibility test to determine if the glasses fit together in the combination you plan for your piece.
Second, if you check the stresses of the flat piece between polarizing filters, you will be able to see if there are stresses within the piece before you do any further kiln forming with this glass or setup. If the stress is from incompatibility - where you see the stress halos around specific pieces of glass - you will need to destroy the piece. If the stress is more generalized, you can put the piece back in the kiln, reheat slowly and soak at the annealing point for a longer time and use a slower annealing cool.

Wednesday, 16 July 2014

Organic Burnout Marks

Occasionally there is a haze at the centre of the back of large pieces of fired glass. This seems to happen when a large piece of glass is placed over fibre paper (of whatever thickness) that has not been pre-fired. 

 This is based on my experience of doing large pieces on thinfire or other fibre paper with a relatively fast rate of advance. What seems to happen is that the edges of the glass soften enough and early enough that not all the binder in the fibre papers can burn out and the combustion gasses escape from under the glass. The resulting haze is the remnants of the combustion product fired to the surface of the glass.

I have found that flipping the piece over and taking the glass to a low temperature fire polish is enough to return the glass to its usual appearance. You can, for extra insurance, apply a devitrification spray, although I have not found it necessary.

You could, of course, work the back of the glass with pumice and cerium oxide to bring back the original shine without firing. But my impression is that the areas with haze are fractionally depressed into the back surface. This means that a lot of glass has to be removed to reach and polish the hazy areas.




Wednesday, 9 July 2014

Effects of Multiple Layers

Stacking layers of glass fully or partially over the base layer has significant effects on the firing of the whole piece.

Glass is a poor conductor of heat, so you need to be careful to allow the heat to penetrate to the base layer to avoid thermal shock. There also is the effect of the (very small) insulating space between each sheet. The effects of multiple, even layers can be seen from this table based on Graham Stone's* work:

3mm layers
1 sheet – Initial Rate of Advance =1000ºC to 475ºC (less than half an hour)
2 to 3 layers – IRA = 240ºC to 475ºC (ca. 2 hours)
4 layers – IRA = 100ºC to 475ºC (4.75 hours)
6 layers – IRA = 25ºC to 125ºC, then 30ºC to 250ºC, then 40ºC to 375, then 50ºC to 475 before 150C to top temperature (ca. 15.5 hours)

This shows the dramatic effect increasing the number of layers has on the firing schedule to make sure the heat gets to the bottom sheet evenly. If you compare the initial rates of advance (IRA) with the same thickness, but fewer sheets you can see the space between layers is important.

6mm layers
1 sheet – IRA = 320ºC to 475ºC (ca. 1.5 hrs)
2 layers – IRA = 240ºC to 475ºC (ca. 2 hrs compared to 4.75 hrs for 4 layers of 3mm)
3 layers – IRA = 200ºC to 475ºC (ca.2.5 hrs compared to 15.5 hrs for 6 layers of 3mm)

These are the fastest safe firing speeds for evenly covered sheets. 

This difference in firing times for stacks of thicker glass, shows how important it is to fire sections of the stack before the final firing of all the layers together.  It also reduces the risk of bubbles developing within the stack. 

If you are thinking of tack fusing with thicker and thinner areas, you need to take account of the differences in thickness in the various areas of the piece when preparing your schedule. You will need to decrease your IRA by quite a bit. So you might want to be thinking of firing some of your pieces to be added to the base layers before tacking them in an additional firing to reduce the risk of thermal shock to the base layer.


* Firing Schedules for Glass; the Kiln Companion, by Graham Stone, ISBN 0646 39733 8

Wednesday, 2 July 2014

Cleaning


A lot of devitrification resembles dirty smears over the glass that will not clean away. This kind of devitrification results from inadequate cleaning.




The glass needs to be made “squeaky clean”. The glass needs to be free of dust, oils and minerals before firing. An initial wash of the glass with a minimum amount of liquid soap will dispose of the dust and oils. However it may leave behind minerals and additives from the soap and water, so a rinse in clean water followed by a polishing with unprinted paper towels or lint free cloths washed without softeners. As the glass dries you may very well hear the squeak of glass that is well polished to dry.



If there are still residues of labels or markers, use of a spirit may be required to remove these marks. Then the glass will have to be cleaned again in the normal way to remove the residues from the spirits.



If you are fortunate to be in an area with very few minerals in the water, you will not have to take as many precautions as those in areas with hard water. If you have hard water, you may need to think about using distiller water for the final rinse if you have streaks of devitrification after the standard cleaning process. The use of spirits is not necessary. The glass still needs to be polished dry with unprinted paper or dedicated towels.

An alternative (that I use most often) is to use a window cleaner without additives, such as supplied by glaziers. This avoids the local water supply, and most often is sufficient to remove dust and oils.

Wednesday, 25 June 2014

Annealing Range


Sometimes, people feel they need to use the lower part of the annealing range for all their glass, as Bullseye has published annealing tables for thick slabs. To determine whether or when to use these tables needs some understanding of the annealing range.

The annealing range of a glass is approximately 40ºC on either side of the annealing point, but for practical kiln forming purposes it is normally taken as 55ºC. The annealing point is around 510ºC for System 96; 516ºC for Bullseye and Uroboros for example. So the range for a fusing glass will be around 565ºC to 465ºC.

So it would be possible to start the annealing at about 560C for any of these glasses. But the slow rate of decline in temperature, following the equalisation soak, would need to be maintained for all 110ºC for the whole range, rather than just the 55º from the anneal soak point. This would double the time of the annealing cool. This high temperature anneal is a much slower process, which – together with the more rapid relief of stress at the annealing point – is why the top of the range is never used for the temperature equalisation point.

The annealing point is the temperature at which, if all the glass is at the same temperature, the most rapid cooling can take place. To achieve that equalisation temperature (+ or – 5ºC throughout), the glass needs to be soaked at the annealing point for varying lenghts of time relating to thickness and other variables. To complete the anneal and keep the glass within that tight range of temperature, the anneal needs to be continued at a steady slow rate of temperature change.

Bullseye still uses 516ºC as the annealing point for things up to 9mm thick, but chooses to use the lower part of the annealing range for thicker items. Choosing to start the annealing process at the lower part of the annealing range speeds the process for thick slabs. Bullseye have not changed the composition of their glass so the usual annealing point is 516ºC for things less than 12mm.

Using the bottom end of the annealing range for thick items, means there are a fewer number of degrees of very slow cooling to the strain point. But this lower soak, or temperature equalisation point, requires a much longer soak to equalise the temperature within the glass before the slow steady decline in temperature to maintain the temperature differentials within the glass to less than 5ºC.

Bullseye have found that using a temperature a bit above the bottom end – 482ºC – with a long soak reduces the total time in the kiln, but continues to give a good anneal. In the case of Bullseye, 476C is the bottom end of the annealing range. However, this low temperature equalisation soak is mostly applicable to thick slabs, and not necessary for things less than 12mm thick.