Temperature conversions from Celsius to Fahrenheit for some common temperatures in kiln forming:
Temperatures
100C = 212F
200C = 396F
300C = 577F
400C = 759F
427C = 808F
482C = 908F
500C = 941F
600C = 968F
650C = 1123F
677C = 1263F
760C = 1414F
780C = 1450F
800C = 1487F
850C = 1577F
900C = 1668F
950C = 1759F
The formula for temperature conversion is:
ºC divided by .555 plus 32 (for the freezing point of water)
Conversion of rates of advance is different (the freezing point of water does not need to be taken into account):
25C = 45F
50C = 91F
75C = 136F
100C = 182F
150C = 273F
200C = 364F
250C = 455F
300C = 545F
350C = 636F
The formula for rate of advance conversions is:
ºC divided by .555 only
Wednesday 24 December 2014
Wednesday 17 December 2014
Volume Calculations
When
creating a casting, pot melt or other object from glass cullet or
billet, you need to be sure you have a large enough volume of glass
to fill the area. You can do it by measuring the volume or by
calculating the weight. This note is about calculating the weight.
Filling
a damed area with enough pieces of glass provides an illustration of
volume control. To help make sure you have enough glass to fill the
space, measure in centimetres to determine the area. For a rectangle,
measure length by width in centimetres. For a circle multiply the
radius by itself (radius squared) times 3.14 (pi) to get the area.
To
determine the minimum volume required, multiply the area by 0.6 cm.
This is the approximate thickness that glass takes up at full fuse.
As the amount of time and heat that we normally give to the process
is insufficient to allow the glass to fully flow, the glass will tend
to be thicker in the middle when using pieces of glass rather than
sheets. So you may wish to multiply the area by 0.7 cm (to make sure
you have enough).
To
get the weight of glass required for the space, multiply the
calculated volume by 2.5 (specific gravity) to get the weight in
grams. Divide by 1000 to get kilograms. If you must use pounds,
multiply the kilos by 2.2, the number of pounds in a kilo.
Wednesday 10 December 2014
Tack Fuse Temperature
The
tack fuse range is around 730C – 780C. This will give a graduation
in profile from the very sharp, almost barely laminated, to one very
rounded almost flat. Choosing the right heat for the right profile
is one of balancing several elements: temperature, time, speed.
Low temperature, high tack fuse |
If
there were no other considerations, you could go slowly up in
temperature and peek in at infrequent intervals until the right
profile had been achieved. However this tack fusing is happening in
the devitrification range, so slow rises in temperature are not
advisable.
Medium temperature, mid tack fuse |
So
an alternative strategy would be to go quickly through the
devitrification range (700C to 760C) and soak for a bit longer above
that range. However, often the desired profile may has disappeared
by the time you get to 770C.
High temperature, rounded tack fuse |
It
would seem that you can attempt to balance the temperature, time and
speed equation by firing quickly (such as 330C/hr) to your desired
temperature and soak there for 10 minutes only.
To
ensure you get the profile that you want you should begin to observe
from at least 10C below your chosen temperature. If you do not get
the profile you want, you can extend the soak until the desired
effect is achieved. On a subsequent firing, you can set the top
temperature a bit higher, but with the 10 minute soak and again
observe. This can be repeated until the desired combination is
achieved.
Each
of these attempts needs to be completely recorded so that the results
can be used in later firings if slightly different profiles are
needed.
Wednesday 3 December 2014
Steel Pipe for Slumping
Steel
pipe as opposed to stainless steel can be used for slumping. It will
spall, so there will be a need to clean up the flakes of rust after
firing. But since there is so much spalling, putting kiln wash or
boron nitride is a waste of effort. Each firing will flake off any
separator painted onto the metal. Cover the pipe with fibre paper
instead - 0.5 mm at least.
You
need to advance in temperature slowly as the pipe drains the heat
from the glass where it rests. My practice is to advance the
temperature at 100C/hr to 100C with a 20 minute soak, followed by 50%
increases in rate to 250, and to 500 with 20 minute soaks before
proceeding to the next segment. This probably is more cautious than
necessary on all but the first segment.
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 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.
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.
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.
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:
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.
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.
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.
Labels:
Annealing,
Compatibility,
kiln forming,
Thermal Shock,
Verrier
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.
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.
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