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.

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 cleaning, firing rapidly through the devitrification range, avoiding 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.

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.

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.

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.

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. 

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.

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.

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.

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