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 

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.

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.

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.

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.

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.