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Practical Notes on Pyrometry

by Leonard Smith (c)1995


(originally placed on the Ceramics Gopher (now CeramicsWeb) by permission of the author)

 

Many potters have a basic understanding of pyrometers but few
understand the implications of the term 'Heat Work'. Before I explain
this term and its implications to firing management, I would like to
briefly explain how pyrometers work so that some rules for their use
can be stated and their limitations understood.

A pyrometer consists of two parts: a thermocouple and a galvanometer
The thermocouple works on the principle that if two dissimilar metals
are heated where they join then a small electric current is generated.
The galvanometer is really a millivolt meter, because the current so
generated is in thousandths of volts. Most galvanometers are of the
swinging-arm type although digital ones are now appearing on the
market. The swinging arm type is very sensitive to knocks, dust, heat,
etc., and most malfunctions occur because of these factors. This type
is also less accurate, as its scale is limited in size and it is not
easy to differentiate between the 20 C calibrations so that
effectively we can only be sure of a reading of plus or minus 10 C.

The newer digital gauges offer 1 C read-outs, and are less subject to
shock. In between the galvanometer and the thermocouple are the
compensating leads or extension cables which compensate for the fact
that the cold junction of the thermocouple gets warmer as the firing
proceeds. In effect these leads are an extension of the thermocouple
and should be made of the same wire or be matched electrically so that
they don't change the signal from the thermocouple. These leads also
allow the galvanometer to be placed away from the kiln in a cool and
accessible spot.

Thermocouples can be left bare when used only in oxidation, but when
used in reduction kilns it is better to fit them with a sheath which
is made of impervious mullite or aluminous porcelain. For special
situations, such as wood or salt firings, there are double sheaths
available to give extra protection to the thermocouple.

The following are some general rules that make pyrometers more
consistent:

 

  1. The gauge should be placed away from the kiln in an area protected
    from rapid changes in temperature, moisture, and temperatures above
    the ambient.
  2. Only gauges designed for the type of thermocouple being used should
    be used as all the thermocouple types generate different amounts of
    electric voltage.
  3. Place the thermocouple hot junction at least 75 mm into the kiln,
    and in a position of average heat, i.e. not above burners or near the
    flues.
  4. Check that all leads are of the correct type and tightly and
    correctly attached. Compensating leads are designed to be used with
    specific thermocouples and are polarised, i.e. they have a positive
    and negative side. The wrong wire or the incorrect polarity will give
    incorrect readings. Gauges are usually marked with positive and
    >negative terminals and the table below indicates the correct wires and
    connections. If your thermocouple isn't marked, don't worry, if the
    gauge registers a negative result then you need to change the wires
    around at the thermocouple end.
  5. Gauges in the price range of most potters are rarely very accurate.
    Plus or minus 2 per cent of scale is usual and this represents plus or
    minus 28 C on a 1400 C scale. A pyrometer is a useful tool in kiln
    management, an indication of approximate temperature and whether
    temperature is rising or falling and at what rate. By using this
    information the effectiveness of adjustments to flues and burners can
    be gauged, but a pyrometer is useless in indicating whether glazes and
    bodies have reached maturity. Only cones and other pyroscopes have the
    necessary consistency and reliability to indicate those things. 6.
    Compensating leads and gauges need to be well clear of mains wiring
    (i.e. household power and light wiring) as these can set up currents
    and give misleading readings.

INTERNATIONAL CODE FOR THERMOCOUPLE WIRES TABLE
Type..........Outer cover........+ve leg........-ve leg
K.............Brown..............Yellow..........Red
KX............Yellow.............Yellow..........Red
R.............Green..............Black...........Red
N
NOTE:

  1. Type K and KX are both made of chromel/ alumel
    and are both suitable for those thermocouples.
  2. Type R is made of copper on one leg and an alloy
    on the other and is designed for platinum/ rhodium thermocouples.
  3. Type N is made of Nisil/Nicrosil

 

DIGITAL PYROMETERS

As I have previously mentioned, the gauge section of the pyrometer is
a millivolt meter, and these are readily available at most electronic
shops for anyone with a small amount of do-it-yourself skill.

There are many models around, which cost less than $80. The advantage
of this gauge is that with a set of tables you can read any type of
thermocouple with it, it is very portable and it still functions as a
multimeter for all those little repair jobs around the home and car .
In conjunction with a Type K (chromel/alumel) or Type N
(nicrosil/nisil) each movement in the millivolt scale represents
approximately 2 5 C So you get an immediate response to any kiln
adjustment with a high degree of accuracy.

Another approach to digital gauges is to buy a millivolt kit from one
of the electronic shops. Dick Smith* sells a digital millivolt panel
meter kit (CAT K3450) for about A$30. This kit needs a fair bit of
skill with a soldering iron so it may be better to buy a pre-assembled
one. Jaycar* have one (D.PM. 50), which only needs a small amount of
soldering, that costs A$40. I have built several of these and, as they
have an adjustable potentiometer, by using another pyrometer as a
reference they can be adjusted with a screwdriver to read directly in
degrees Celsius when used with Type K or N thermocouple. There can be
some point made as to the accuracy of this but, again, accuracy is not
the point of using these gauges, it is kiln management.

(*The above are Australian versions of Radio Shack)

 

PYROSCOPES


Before the knowledge of pyrometers was used in a potter's kiln, firing
was done with the use of pyroscopes. The earliest forms were draw
trials and test rings, followed by Buller's Rings and finally by
Holdcroft's Bars and cones. I personally cannot over-emphasise the
importance of these devices for firing management.

Traditionally, temperature inside a kiln was gauged by colour, and the
potter had to be very experienced to judge this correctly but even
then the state of the bodies and glazes was judged by drawing samples,
usually in the shape of rings from the kiln. If you wish to learn more
about your firing and glazes you should have draw rings in your kiln.
It is usual to put two or so earthenware rings in to determine whether
oxidation has been successful before commencing reduction. Two or
three rings glazed with the most important or sensitive glazes in the
firing, when pulled, will tell you more about the success of the
firing than either a pyrometer or cone. Before you make any dramatic
changes, though, a little warning about matte glazes. Some matte
glazes, those with a crystalline matte surface, could be quite glossy
when pulled from the kiln as the mattness occurs mainly in the
cooling.

Although Holdcroft's Bars and Buller's Rings are later developments
than pyrometric cones, I will discuss them first and briefly as they
are rarely used by artist potters. Buller's Rings are flat discs 60 mm
in diameter with a convenient hole in the centre for ease of
withdrawal from the kiln. The diameter is measured on a special gauge
which relates shrinkage to temperature. Rings are effective for the
range 960 C to 1400 C and can be withdrawn and the heat work
calculated at any time during a firing. New ones are withdrawn for
each reading that is required (Diagram 2). Holdcroft's Bars are based
on the same principle as cones, being made of glaze minerals, and
graded to soften at different temperatures. They are 75 mm long and
are supported horizontally at both ends with their centre unsupported,
so that they sag as heat is applied They offer an alternative to cones
but are not as commonly used. Holdcroft's Bars are available from
Harrison Mayer Ltd (see Diagram 3).


Buller's Rings and measuring gauge drawing Diagram 2. Bullers ring and measuring gauge.


Holdcroft's bars illustration DIAGRAM 3: Holdcroft's Bars (a) before firing (b) after firing

 

PYROMETRIC CONES


When Dr Hermann Seger was testing various materials for refractoriness
in 1885, he originated the cone shape that we are familiar with today.
When heated, the effect first penetrates the narrow upper part and
fusion progresses down the cone. A cone therefore bends slowly. In A
Handbook of Pottery Glazes by David Green (Faber and Faber, 1979)
there is an excellent description of Seger's early experiments and the
eventual outcome of not only cones but of the rational molecular
formulae which we have come to know as Seger Formula.

Dr Seger followed his experiments on natural materials with simply
compounded ones and devised a series of cones which would collapse at
predetermined temperatures, and which could produce an accurate scale
of refractoriness against which natural materials and glazes could be
measured. Pyrometric Cone Equivalent (P.C.E..) is still the industrial
standard of refractoriness of materials.

Pyrometric cones measure heat-work, i.e. not only the temperature but
the length of time that heat has been applied. As they are made from
glaze materials, they closely represent the effects of heat on clay
and glazes and are therefore the only real indicators of firing
progress. Harrison Cone 8 which is given a temperature of 1250 C
would collapse at 1200 C in an extremely slow firing, say 3 days,
whilst in a firing of, say, 3 hours, it may not bend even at 1300 C.

Unfortunately, all the different cone manufacturers don't standardise
their cones and Cone 8 can be designated anywhere between 1250 C and
1300 C. The important thing to do is to gain experience with one
particular type. Generally Orton (made in USA) cones are the most
readily available in Australia now, and although I have found that I
need to use Orton Cone 10 where I used to use Harrison Cone 8 it
doesn't matter once you have established at what cone your glazes and
clays look the best.

When using cones there are some guidelines that you should follow:

  1. Always face the cones to a source of heat, i.e. face them to the
    burners. In a cross-draught kiln if you face them toward the flues,
    they will bend backwards, towards the flame, and cannot be considered
    accurate.
  2. Always follow the manufacturers' advice, as to the angle to set
    them. Standing them straighter will make them melt later, more angle
    will make them melt sooner
  3. If you have a pyrometer you only need 3 cones. The first one to
    indicate if you wish to reduce the kiln atmosphere, say No 6 (990 C).
    This cone will turn to glass at Cone 8 so it needs a small bowl to
    melt into (see Diagram 4). Also the cone for the temperature you wish
    to fire to plus, usually, a guard cone one or two numbers beyond that
    so that you can see if you have overfired.
    If you don't have a pyrometer you can use several more lower
    temperature cones, say, one each for 700 C, 800 C, 900 C, etc., to
    let you know how the firing is progressing.
  4. Rather than cone stands, use a small amount of wadding clay (50
    ball clay, 50 sand by volume) to set the cones and use a match or nail
    to spike it all over once the cones are in place. This will stop the
    cone pad from exploding when the water in it turns to steam.
  5. Ensure that you can see the cones through the spy hole. A lot of
    kilns have very small holes and I would consider enlarging them to
    somewhere near 75 mm in diameter This allows some flexibility in
    setting the cones and allows drawing of test rings.
  6. Keep a record of what cones you used and where you placed them in
    the kiln. I put this at the top of my kiln log.
  7. For accuracy's sake, always place the cones in the same place in
    the kiln. It doesn't hurt to have a few in other places in the kiln so
    that when you empty it you can decide if you need to make any kiln
    adjustments to get evenness.
  8. Use gas welding glasses to look at cones at temperatures above
    1150 C as the radiant heat damages the eyes, and the cones are more
    easily seen with them on.

typical cone pack with 'boat' to catch melting cone DIAGRAM 4: Cone with boat to catch molten cone

Temperature measurement alone, no matter how accurate, is not the way
to consistent and repeatable glaze firings, as glazes and bodies react
to both temperature (heat) and time (work). As can be seen pyrometers
are a useful tool in kiln management, some potters would never be
without one, but they do have limitations in regard to accuracy. At
best they will indicate a rate of climb or fall of temperaturenot
glaze or body maturity. Finally, let me make a plea for the use of
test rings. More can be learnt about firings from draw trials than
staring at a digital readout.

clock positions of cones as they deform
DIAGRAM 5: Clock positions as a method of recording cone deformity

 

Leonard Smith is a potter and teacher who works at The National Art
School East Sydney and lives at Arcadia, Sydney, NSW, Australia. He
is a co-author of Handbook for Australian Potters pubilshed by
Methuen, Sydney. His E-Mail address is:

smithl@ozemail.com.au

 

 

 

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