Claybright
Cape Pottery Supplies Clayman Kiln Contracts
Frequently Asked Questions
View Cart Enquiry Cart: 0
 

CLICK ON QUESTION TO SEE RESPONSE

Search FAQ

Contract All | Expand All


How do I change the elements on my kiln?
  1. Safety is your first concern. Either unplug the kiln or isolate on wall and on mains distribution board. If a locking device is provided, lock the switch off. In a domestic environment, advise the family of what you are doing, in particular to the switches, which have been switched off.
  2. Remove the covers exposing the connections to the elements.
  3. Make a careful note of how the elements are connected, and then disconnect all the wires to the tails of the elements, marking them appropriately for the positions to which they were attached. Old elements tend to become very brittle and will probably break once you attempt to remove them from the grooves. Special care must be taken to protect the groves from mechanical damage.
  4. If the elements were pinned previously then it would be recommend that you pin the new elements in similar positions.
  5. You will need a bolt cutter to trim the tails of the elements, which are normally supplied over-length. Cut them to the same length of the old ones.
  6. Re-attach all your wires making sure that you fasten the wires to the element tails very firmly.
  7. If you are equipped to check the current drawn or if you have a friend who is able to do this for you, it is a good idea to compare the specified current versus the elements that have just been replaced.
  8. Replace all covers.
  9. Then it is necessary to oxidize the elements – this is achieved by firing the kiln empty to 1100ºC and soaking for 3 hours with the bungs open. This process ensures that the elements generate an oxide layer, which protects them from the atmosphere.

NB. A good idea would be to compare your new elements to the elements in the kiln prior to removing the old ones, ensuring that they are visually similar.


My 3D Claybright® colours fall off after they have dried, what is wrong?

Be sure to anchor your 3D by pushing down gently and squeezing at the same time – like cake icing! If they tend to lift when dry, simply wet your finger with water and press gently on the 3D.


I have painted my bisque item with underglaze, but am not happy with my design, what can I do?
Just take a wet sponge and wipe the underglaze off, if you have wet the bisque item too much, wait for it to dry and start over again! The colours are easy to remove with a wet sponge until after the hardening on firing.

What is the highest temperature that slip cast ware can be fired to and why?

The firing temperature of casting slip varies depending on the clay. Our earthenware – then 1000ºC - 1120ºC is best, but we have taken it up to 1200ºC, where it goes slightly grey and it tends to warp a lot. Stoneware - 1000ºC for bisque and 1200ºC for glaze firing. This casting slip is best at 1200ºC, but it can be taken higher. As high as 1260ºC. At this temperature it can warp because it softens in the firing, it also depends on the shape of the object. An open shape is more likely to warp than closed shapes. Porcelain slip has many of the same properties of white stoneware and stoneware casting slips and therefore will react similarly at 1200ºC and 1260ºC. It is only whiter. The charcoal casting slip is best fired to 1140ºC if no glaze is applied for the “blackness”. But, if it is glazed, then is best fired to 1120ºC only. This low temperature is caused by the amount of flux in the body and if it is taken higher it will bloat and ultimately melt into a puddle on your shelf!


How do I calculate the cost of firing my kiln?

Formula is :

((kW rating of the kiln) x (cost of electricity per kW) x (hours of firing time)) x 60%.

1. KW rating of kiln should be on specification plate at the back of your kiln.

2. Cost of electricity per kW – this should be on your electricity account or electricity recharge slip (e.g. 0,20 – 0.36c per kW).

3. Time taken to reach temperature in hours.

4. Then 60 % x all of the above.

In Short :

R3.30 per kw rating of a kiln i.e a 10kw (+/- 8 ft cubic foot kiln)  would cost

R33.00 this is based on R0.55c per kWh and a typical firing of 10 hours.


What information must be sent for an element order, if there is no sample available and not previously ordered?

Send by fax or e-mail a drawing detailing the following information. 1. if a specification plate is visible notate the current drawn, kW rating and Voltage of the kiln, and if the model number is available/ serial number. 2. length of element and number of grooves it occupies. 3. indicate whether tails are twisted or single and the length. 4. measure outside diameter of the coils. 5. thickness of wire – use a vernier for accuracy, as the nominal difference between wires can be 0,1mm. 6. your name and contact details. We can typically have elements made within 48 hrs of confirmed order.


What is the difference between the electric electronic and momentum wheels listed in your catalogue?

CONE DRIVEN ELECTRIC WHEEL

1.      WHISPER QUIET, POWERFUL 0,37kW MOTOR.

2.      TRIED AND TESTED SMOOTH CONE DRIVE

3.      COMFORTABLE SEAT

4.      EASILY CLEANED FIBREGLASS BASIN WITH DRAIN AND PLUG

5.      ALUMINIUM WHEEL HEAD

6.      AUTOMATIC "SWITCH OFF" ON PEDAL RELEASE

7.      SIMPLE AND INEXPENSIVE TO SERVICE.

ELECTRONIC BELT DRIVEN WHEEL

1.      WHISPER QUIET, POWERFUL 0,37 kW, THREE PHASE MOTOR DRIVEN THROUGH A SINGLE PHASE ELECTRONIC CONVERTER.

2.      HIGH TORQUE THROUGH THE ENTIRE SPEED RANGE.

3.      FLEXIBLE RIGHT HAND, LEFT HAND OPTIONS.

4.      FULLY MOBILE FOOT PEDAL

5.      CAPABILITY OF CONSTANT SPEED AND IDEAL FOR DECORATING AND TRIMMING.

6.      VERY FEW WEAR COMPONENTS RESULTING IN SIMPLE AND INEXPENSIVE SERVICING.

7.      COMFORTABLE SEAT

8.     EASILY CLEANED FIBREGLASS BASIN WITH DRAIN AND PLUG ALUMINIUM WHEEL HEAD


What sort of stencils can you use in ceramics?

Paper stencils, adhesive backed stencils, stickers, masking tape, contact paper, stencil paper (acetate), paper doilies, lace – all work as stencils.


How can I purchase when I am far away?

Fax or e-mail your order, including your address, we will then contact you with the total price, weight and availability/ dispatch date. You then make payment via credit card or electronic transfer, fax us proof thereof, and then we dispatch either by post, courier or rail depending on size, weight and costs. Transport costs are for the clients accounts.


Do you have credit card facility?

Yes, alternatively payments can be made directly into our bank account.


How do I keep glaze mixed?

Use CMC to keep stains etc in suspension or add +/- 2/3 % of Bentonite to your glaze.


How much water into glaze?

Transparent glaze likes to be thin (about 1 – 1 ½ litres of water per 1 kg of glaze), but colour glazes works better if it is a thicker consistency (+/- 800 ml – 1 litre of water to 1 kg of glaze).


Can I use mix Claybright® underglaze colours together?

The short answer is “yes”. However, underglazes are not like ordinary paints. In the firing process they undergo a chemical reaction & change colour (Compare Dark Blue in its raw and fired states). The safest procedure would be to mix a ratio of two colours together and then do a test firing of that mixture, before you commit yourself to applying it to a precious piece. That said, as primary colours of red, blue and yellow go, the Claybright® Poppy, Electric Blue and Bright Yellow create good mixtures of purple, green and orange.


What is crawling & what causes it?

Crawling occurs when a glaze separates into lumps and beads. Dust and/or grease on bisque can repel glaze, causing crawling. For this reason always wipe bisque with a damp sponge before glazing. Bisqueware that has been standing for a long period should ideally be re-bisque fired. Some glaze ingredients (including zinc, colemanite and opacifiers) are prone to crawling when applied too thickly. Some underglaze colours when applied too thickly could have the same result.


Can I use both Claybright® and Gare underglazes on my pot?

Yes, both are suitable for earthenware temperatures. They can be mixed. When firing to 1200°C stoneware some of the Gare reds and pinks can fade. We have also noticed that the purples tend to fire to a blueish purple, but 90% of the Gare colours will fire successfully to 1200°C. It is always a good idea to test fire colours before using them on your pieces. There are fired samples available in all our showrooms.


Have you Spodumene in stock?

Li2O.Al2O3.4SiO2.

We no longer stock Spodumene, a  good source of Lithium, a flux at stoneware temperatures.

Spodumene is used in white bodies and glazes and lowers the vitrification point. 

You could try substituting 4 g of Silica and 1 g of Alumina for every 1 g of Lithium Carbonate instead.


I fired my Carmen Red/ Cad Red glaze to 1040 - 1060 C/ cone 05-04.  It came out transparent.  Is there something wrong with this batch?

°Cadmium glazes are notorious for being temperature sensitive.  This means that when firing a cad glaze be it yellow, green, orange or red, care should be taken in the firing.

Cad glaze will appear transparent if over-fired or to thinly applied.  If you have access to a brush on cad glaze, more glaze can be applied to the surface where it burnt out and re-fired to the glaze temperature (1060°C).  Just remember that cad glazes should be applied very thickly.  When firing these glazes do not soak them at temperature.  FIrst thing to do is make sure that your kiln is not packed too tightly.  Cad glazes must "breathe".  Then start your firing by going to 100°C and holding it there for an hour or two to warm up the ware in the kiln.  Thereafter go up to 1060°C at a rate of 250°C an hour.  If your firing consists of bigger platters and sculptures keep on firing slowly to 600°C ( a rate of 150°C per hour) and only then go to a rate of 250°C per hour to 1060°C. 

If you are firing manually (with cones), put the kiln on setting 1 for 2 hours then on 2 for 2 hours then on 4 for 1 hour and then on 6 until the cone bends.

Carmen red glaze is not the only red cad glaze in our range, we also have some Gare glazes that are cadmium based like the Xmas Red and Fire Engine Red.  These should not be fired higher than a 1000°C.  In application they state paint first coat wait 2 hours then paint the second coat wait a further 4 hours and then paint 3rd coard wait 8 hours and then pain the 4th coat.  The finished thickness should be between 1,5 and 2 mm thick.  When firing it - same as for Carmen red but it only fires to 1000°C with no soak.


Is my kiln over-firing?

 

 

 

In modern times, cones are still a very important tool in a potter’s armoury.  It is the only definitive measure of heat work done on pottery.

 

With the current regime of power failures, without a cone in your kiln you will not know whether your pottery reached temperature unless it is badly under fired.  More recently we have had glazes from prominent suppliers, which have been problematic.  It is useful to bench mark good firings with cones and to save the cone results.  If you are experiencing unsatisfactory results, cones will quickly tell you whether the kiln is misbehaving or whether the glaze is misbehaving.  If the fired results of cones significantly differ to your benchmark then look to your kiln for the cause.  If the fired result of the cones is the same as your benchmark look to your glazes and or clay body for the cause.  It is useful to place cones next to your thermocouple and in areas where you get your best results, as these are your litmus tests.


Is my glaze faulty?

 

 

 

In modern times, cones are still a very important tool in a potter’s armoury.  It is the only definitive measure of heat work done on pottery.

 

With the current regime of power failures, without a cone in your kiln you will not know whether your pottery reached temperature unless it is badly under fired.  More recently we have had glazes from prominent suppliers, which have been problematic.  It is useful to bench mark good firings with cones and to save the cone results.  If you are experiencing unsatisfactory results, cones will quickly tell you whether the kiln is misbehaving or whether the glaze is misbehaving.  If the fired results of cones significantly differ to your benchmark then look to your kiln for the cause.  If the fired result of the cones is the same as your benchmark look to your glazes and or clay body for the cause.  It is useful to place cones next to your thermocouple and in areas where you get your best results, as these are your litmus tests.


How do I know if my kiln has reached temperature?

 

 

 

In modern times, cones are still a very important tool in a potter’s armoury.  It is the only definitive measure of heat work done on pottery.

 

With the current regime of power failures, without a cone in your kiln you will not know whether your pottery reached temperature unless it is badly under fired.  More recently we have had glazes from prominent suppliers, which have been problematic.  It is useful to bench mark good firings with cones and to save the cone results.  If you are experiencing unsatisfactory results, cones will quickly tell you whether the kiln is misbehaving or whether the glaze is misbehaving.  If the fired results of cones significantly differ to your benchmark then look to your kiln for the cause.  If the fired result of the cones is the same as your benchmark look to your glazes and or clay body for the cause.  It is useful to place cones next to your thermocouple and in areas where you get your best results, as these are your litmus tests.


How can I get some Gerstley Borate or Colemanite
Both of these products have been discontinued some years back and are no longer available.  Potters worldwide have been reformulating their glazes as a result.  More information can be found at : www.digitalfire.com/gerstleyborate/substitutes.html

Why do the raku shelves have holes in them?
In raku often one uses more than one shelf when packing the kiln.

When the time comes for removing the fired items while still red-hot, tongs are used. In order to remove a shelf quickly with tongs, holes are made in the shelf in order that the tongs can grip the shelf easily.

How do you mix up Batt Wash and apply it to shelves?
Mix up a bit of batt wash with water until it is like milk in consistency. Rather make it too runny than too thick, as, if it is too thick, it will crack off your shelf before you even put your shelves in the kiln. Brush this runny mixture of Batt Wash onto one side of your kiln shelf with a soft brush. Allow this to dry. You will still see some of the shelf colour through the Batt Wash. Apply another coat of Batt Wash to the shelf and allow it to dry. You will probably need to apply a third coat.

You will know when you have applied enough when you can no longer see the colour of the shelf through the Batt Wash and your shelf now appears white.

Once you have done this, leave your Batt Washed shelves to dry overnight.

The shelves will have soaked up water with each application of Batt Wash, and you do not want to fire your shelves in the kiln when they are wet, as they may crack. For the same reason, fire your shelves in a bisque firing, rather than a glaze firing – i.e. fire them slowly, rather than fast. You will be able to use your shelves for many firings - up until the time that the Batt Wash starts to crack off the shelves. When this happens, scrape it off, turn your shelf around, and apply new Batt Wash to the other side of the shelf.

Should you apply Batt Wash to the underside of your shelf, when it gets to the stage where the Batt Wash starts to crack off, it would land on your pieces in a glaze firing and ruin the glazed pieces.

How do I mix plaster of Paris for making moulds?

1. To mix plaster of paris you need a ratio of 1.2kg of plaster of paris to 1litre of water (or 1kg plaster of paris to 833ml water).

2. Pour water into a bucket, sieve plaster of paris quickly through your fingers into the water, to disperse powder evenly over the surface of the water.

3. When the plaster of paris forms a peak in the water let it stand for approximately half a minute (to allow the plaster of paris to absorb the water).

4. Stir mixture vigorously with your hand until all the lumps have dissolved.

5. Let mixture stand for a few seconds until it becomes a creamy liquid.

6. The plaster is now ready to pour into the mould.


Kaolin vs Talc - what's the difference?

The other day a client asked me what the difference was between kaolin and talc. Hmm, good question. Well before getting to the differences, let's first see what they have in common.

They are both fine white powders that come from digging a mineral out of the ground and refining it. If you take a microscope and look at the particles close-up, they both look like tiny flat plates, which are called (surprise, surprise), platelets. This platelet shape makes them good at cutting out the light, which is why we put them into paints and ceramic glazes to help with opacity.

Kaolin and talc have a lot more in common: They are both inert, which means they take no part in chemical reactions. They are also both insoluble in water, so they do not dissolve (although it looks like they do when all the particles are dispersed and form a milky solution).

Although both are minerals, only kaolin is called a clay. In fact kaolin's common name is china clay. The common name for talc is talcum powder and it is the softest mineral on earth. That must be why it is chosen for use on babies bottoms!

Kaolin is also a soft mineral, though. It comes just above talc on the Mohs hardness scale of minerals:

 

Mohs hardness of minerals

Diamond

10

Silica

7

Kaolin

2

Talc

1

Looking at a pile of kaolin powder and a pile of talc powder, however, you will notice an obvious difference. The talc is much whiter (unless it is very impure). If you put your hand into the powder, you will find that although the kaolin makes your hand white and dusty, the talc sticks to it like blazes. And to your clothes, your face, the dog, etc.

That's the biggest difference, I think. Talc is hydrophobic which means water-hating. Hence it loves anything organic like you, your face, the dog.....you get my drift.

Kaolin on the other hand, loves water so it is hydrophilic. That makes it much easier to disperse into water than talc, which tends to sit on top of it. Talc may be water-hating but surprisingly, you can still disperse it into your water-based paint without too much of a problem. This water-hating property does come in useful in waterproofing products, where you need ingredients that help to reject water.

Open a chemistry book (does anyone still do that these days?), OK strike that, go to Wikipedia, and it will tell you that kaolin is a clay mineral and falls into a family called alumino-silicates. Talc, however is a magnesium silicate. This would explain their different behaviour. However sometimes when you are just using them as inert fillers, their differences are not important and you would just buy whichever is cheapest - usually kaolin. Like when you are using them in laundry soap bars, for example. For white toilet soap, though, you will need whiteness, so then you would chose talc rather than kaolin.

It gets even more interesting when you melt these two minerals, as happens when they are used in ceramics. Kaolin is a basic ingredient of most ceramic bodies and glazes and is needed for its alumina content, while talc is used mainly as an additive which helps to control thermal expansion, reduce crazing and improve the whiteness of the ceramic article or glaze.

Just like a brother and sister, these two minerals sometimes work well together and sometimes they compete with each other. But we love them both, just the same (OK, I admit it, I prefer kaolin, but that is only because it has been my baby for so long...)

THIS ARTICLE WITH THE KIND PERMISSION OF JENNY JAY OF SERINA TRADING : www.kaolin.co.za


Kaolin vs Calcium carbonate – what's the difference?
 wrote last month comparing kaolin to talc, which many found quite useful. I was asked to also compare kaolin to calcium carbonate. So here goes…

(Email me if you would like a copy of last months’ Serina Snippets)

Kaolin and calcium carbonate are both used as fillers or extenders, in other words they make something cheaper. By adding them the volume goes up and the cost per ton goes down. What music to our ears!

But add too much filler and you start to lose good properties, so it is always a trade-off of cost versus quality.

Fillers replace a bit of something that is much more expensive, such as titanium dioxide in paint, soap in soap bars, resin in plastic parts, pulp in paper or rubber in conveyor belts. Like it or not, there is a filler in nearly everything we see around us.

If kaolin is called china clay and the mineral is kaolinite, then calcium carbonate is called limestone, calcite or chalk, and the mineral is calcite. Limestone is the name of the sedimentary rock it comes from. However you also get calcium carbonate from marble, chalk and crushed sea shells.

The main difference in my mind between kaolin and calcium carbonate is that kaolin is a clay, while calcium carbonate is a salt. That means it can dissolve. Lucky for us, it won’t dissolve at high pH such as in paint or soap, or in organic media like plastics or rubber. It only really dissolves in acids.

You can be pretty sure that kaolin, though, will not dissolve whatever you do to it. (The only thing I know that can dissolve kaolin is a super-strong acid called hydrogen fluoride).

For many years, printing paper was made in acid conditions. This meant calcium carbonate could not be used as filler because it would dissolve. Now most paper mills have gone over to alkaline sizing. Thus calcium carbonate now beats kaolin as the main filler for paper.

Calcium carbonate is preferred for paper because it is much whiter than kaolin. Its ISO-Brightness is usually 96%, compared to around 83% for a typical kaolin. Quite a big difference!

Another big difference with these two fillers is the shape of their particles. Kaolin has plate-like particles (platelets) while calcium carbonate particles are irregular rhombohedral (or effectively round) in shape. This is why kaolin particles cut out the light better than calcium carbonate particles. In the paint industry we say kaolin has better hiding power than calcium carbonate.

Some clever scientists have now been able to make precipitated calcium carbonate with a platy shape for the paper industry (for more about this, click here).

I recently visited a lime factory and saw the age-old process of making lime from calcium carbonate. Not surprisingly, you are sure to find a lime factory next to a limestone deposit. The limestone is crushed and calcined to 930 degrees C to form what is called burnt lime/quicklime or unslaked lime:

CaCO3 + heat = CaO + CO2

The burnt lime or calcium oxide is reactive to water (it heats up and gives off steam). This process is called slaking and the result is slaked lime or calcium hydroxide – Ca(OH)2. That’s the stuff that is sold as builder’s lime. It is also used in water treatment and chemical plants for pH control.

Calcium carbonate is useful to us humans as a source of calcium and as an antacid. It is also used as the carrier in many forms of tablets and pills. Kaolin’s main medicinal use is in diarrhoea remedies.

So these two useful fillers are not only all around us, they may also be inside us!

This article with kind permission of Jenny Jay - Ph.D (Chemistry) ATSC - SERINA TRADING www.kaolin.co.za


Can I skip the “hardening on” step after painting my bisqueware to avoid extra firing costs
It is not advisable if you want to achieve a professionally glossed finish, as the glaze does not take well over underglaze that is not hardened on. The “hardening on” burns away dirt, dust etc. Liquid glaze can smudge fine details of an underglaze design which has dried to a powdery finish. “Hardening on” leaves the surface clean and the underglaze keyed into the bisque. Until the hardened on stage, underglaze can be washed off your bisqueware by gentle sponging, should you have made a mistake.

Can I paint underglaze onto greenware then to avoid that “3rd firing
You can if you do a low bisque firing, 900 – 950’C max as Claybright and some other underglazes become hard and start fluxing at 980 – 1000’C and will then not take a glaze. If you are not glazing then it’s not a problem, you can then just proceed with a high bisque firing.

How can I lift the readymade slab off my slabroller?
The best way to do this is to peel back the top canvas sheet and to place a wooden board or batt on top of your clay slab, then turn the boards over and your clay slab will now be on top of the wooden board or batt.   Remove the drive board, your clay slab is ready for cutting.

my square slabbed pots seem to warp and crack at joined areas even though I slipped them well, what else could be the matter?

Slabbing can be very tricky at times.  Here are a few tips to make the process a little easier.

1.  When scoring/ cross hatching the edges it needs to be relatively deep.  The clay slip applied to the area, scored again until it is mush.  The idea is that when the two pieces are pressed together the scored edges will mix.

2.  Take a tool and run it along in the corner, creating a shallow groove.  This action will help compress the clay in the joint.  Next is roll a thin coil of clay.  Whilst supporting the joint on the outside, reinforce the joint by smearing the coil into the groove.

3.  When it comes to drying your pieces, it is always advisable to dry it out slowly.  Keep it completely covered for the first 24 hours.  Thereafter keep it partly covered for +/- 3 - 7 days, only then open it up to dry naturally.

When rolling out a slab it is always advisable to roll in a cross hatch fashion, otherwise the clay particles align in one direction.  This causes the clay to shrink differentially creating stress on the joint.


How white is WHITE?

If you deal with white minerals and fillers you have probably pondered this question at some time or other. Comparing the whiteness of one white powder with another is not so easy. Especially if you only have the data sheets to go on!

Measuring the whiteness of minerals is a complex subject. What we need are a few simple guidelines that are easy to use in our day-to-day working lives.

Let's look at the simplest method first. If you have samples of the two minerals, you can do a comparison by eye. I suggest you pour out some powder on to a plain dark background. Press some flat with your finger. Wet a corner of it, then compare the two. Which is whiter? Is it a greyish-white or a yellowish-white?

The lighting in the room can affect this method a lot, so it is only a very rough test.

Another plan is to keep a standard and compare your new sample by eye to your standard - a quick and easy way to check how different batches compare with each other.

I am always impressed how good paint chemists are at this. They make up a paint sample with the standard and one with the new filler, and then they do a drawdown. This compares two films of paint against a black and white striped background and is a quick way to check whiteness as well as hiding power (opacity).

A more accurate approach is to measure the whiteness with a spectrophotometer. There are several types and makes of these; the one we use is the L&W Elrepho spectrophotometer.

The measurements you get from a spectrophotometer are usually taken at different wavelengths. These are also manipulated to give further parameters. A list of these reads something like this: X, Y, Z, L*, a*, b*, x, y, Rx, Ry, Rz, R457, CIE Whiteness/Tint-D65 resp. C, Fluorescence, ISO-Brightness, D65-Brightness, Yellowness, Opacity, Transparency, K/N-value, IE, Residual ink, DW, pe, s, k, sR457, kR457, Metamerism, ΔE

So which of these parameters should be used for comparing whiteness? More than one from this list can be used - the confusion arises because we don't all use the same one! It doesn't matter so much which one we use, but for comparing apples with apples, we should use the same parameter for the comparison.

The paper industry has standardised on ISO-Brightness, which is the same as the R457 Reflectance measurement, measured at 457nm. So if you find one mineral's data sheet reports ISO-Brightness and another says Reflectance R457, you can use these as one and the same and easily compare one mineral to another.

Whiteness Ry is the other common parameter used as a measure of whiteness.  You will often find this parameter on talc and calcium carbonate data sheets. However it gives a different (usually higher) result to the R457 measurement. 

Some mineral suppliers, however, just report the Y-value as a whiteness measurement.

So read the data sheet carefully. If you need a different parameter to make a particular comparison, ask the supplier for it - he has probably measured it but has just not reported it on his data sheet.

If you are given the L*, a* and b* values (sometimes called Lab colour) these can tell you a bit more about the colour of that mineral than just whiteness.

Lab colour was designed to approximate human vision. The L component closely matches human perception of lightness and is given on a scale of 0 to 100.

The b-value gives a reading between -1 and +1 on the b-axis or the yellow/blue scale. The a-value is similarly a measure of greenness or redness.  

Look out for a high b-value, for example - it means the mineral is yellowish. Similarly, a positive a-value indicates reddishness while a negative a-value means it is a bit greenish.

Hope this helps you. If you are in the Southern hemisphere, you are probably looking a bit blue-ish yourself right now because the middle of Winter is upon us!

This article with kind permission of Jenny Jay ATSC. Ph.D. (Chemistry)
Marketing Director  SERINA TRADING Website: http://www.kaolin.co.za/


The particle size question: Mesh vs Microns

I find that much confusion with data sheets arises because not everyone measures things the same way!

Last month we discussed the different ways that whiteness is measured (if you missed that article, email me and I will send you a copy).  

This month we look at particle size. Sometimes it is reported in Mesh and sometimes in microns – what is the difference?

What is Mesh?

What is Mesh?

This month we look at particle size. Sometimes it is reported in Mesh and sometimes in microns – what is the difference?

What is Mesh?

What is Mesh?

The Mesh system is the old imperial standard for measuring particle size and is still widely used in countries like India and China where they still use screens to determine how fine something is.

Originally it was the measure of a woven filter cloth's ability to remove contaminant particles. This system simply counted the number of strands or yarns per inch of woven media. Hence, a 100 Mesh media has 100 yarns per inch of media.

Thus the higher the Mesh size, the smaller the particles that are able to pass through. The symbol used for Mesh is #.

This system is not exact because it depends, for example, on how thick the strands are. Over time, various Mesh systems were developed such as the Tyler, the American ASTM and the British BSS systems. These are similar ro each other but with some variations, so for the purposes of this article I will be using the ASTM system (to see how they compare, click here). 

What is a Micron?

The micron is the more modern metric measurement of particle size. A micron is also called a micrometre, with symbol µm or simply µ.

We now have instruments like the Sedigraph (which uses sedimentation and X-rays) to give accurate micron readings of particle size. This is especially important for very fine minerals where screening becomes difficult, like those below 20 microns.

To get an idea of the size of a micron, I find it is useful to remember that there are 1000 microns in one millimetre, or half a millimetre is 500 microns.

Pluck a piece of hair out of your head (ouch!) and look at it. Your hair is about 100 microns thick. If it was only 40 microns, it would be invisible – humans just cannot see something that small without using a microscope.

The following chart gives the micron size of some common particles:

Micron

Particle

0.3

Smoke, TiO2

0.5

Bacteria

0.7

Lung Damaging particles

1.0

Atmospheric Dust

1.3

Moulds

2

Flour mill dust, kaolin

4

Cement dust

5

Pulverized coal, CaCO3

7

Commercial dust

10

Pollen

75

Silt

1000

Sand

Source: http://www.liquidfiltration.com/principles.htm

I notice that Western mineral producers have stopped using Mesh sizes in favour of the more accurate micron sizes. However Mesh sizes are still widely used in the East. So to compare one mineral to another, we need a conversion table (Note: this is one of many conversion tables that are available on the Internet).

325 Mesh

325 Mesh is a common size used, especially on kaolin data sheets. You may be offered a “325 Mesh kaolin”, for example. What does this mean?

Well, if you go to the conversion table you will find out that 325 Mesh is 45 microns (some tables say 44 microns). So a 325 Mesh kaolin means that most of the particles are smaller than 45 microns - simple as that!

Particles that remain on a 325 Mesh/45 micron screen are usually called the “Residue”, which should be a very small amount (less than 1%). In our kaolin operation, we often called this the grit content. It can be written on the data sheet in many different ways, such as: 

% > 325 Mesh

0.2

Sieve residue 325 Mesh

0.2%

325 # (% Max)

0.2

Residue (> 45 micron)

0.2%

+ 45µm

0.2%

Retention on BSS 350 Mesh

0.2%

You will notice that 325 Mesh on the ASTM system equates to 350 Mesh on the British BSS system.

In another example, if you are offered a 400 Mesh talc, you can assume that 99% of the talc particles would be smaller than 37 microns (400 Mesh = 37µ).

D50

The D50 (also written D50) is another measurement we look out for. It is the mean or average particle size of a mineral. D50 = 2 microns means the average particle size of that mineral is 2 microns. It also means that half of the particles are smaller than 2 microns and half are bigger.

D98 = 2 micron on the other hand, means that 98% of the particles are smaller than 2 microns.
 
As technology advances we are starting to mill minerals to finer and finer particle sizes, even down to nanoparticles (less than 0.1 micron). But that’s a topic for another day!

As a parting shot, I couldn’t resist sharing this with you:

Q. What do you do with a dead chemist?
A. Barium.

Written by Jenny Jay, of Serina Trading -  Website: http://www.kaolin.co.za/

As a parting shot, I couldn’t resist sharing this with you:

Q. What do you do with a dead chemist?
A. Barium.

Written by Jenny Jay, of Serina Trading -  Website: http://www.kaolin.co.za/


KILN WASH FOR GLASS

KILN WASH

Kiln forming techniques require separators between the glass and the shelf

or mould on which it rests during the heating process. These separators have

different generic names - kiln wash and batt wash are two.

There are a number of brands of kiln wash. All of them contain two main

ingredients - alumina hydrate (sometimes called slaked alumina) and kaolin

( also called china clay). Different producers use these ingredients in

various proportions. A number also include a colourant that changes when

fired above certain temperatures to indicate the material has been dried.

An important thing to remember is that the kaolin changes its composition

once it is fired over 700C. This change causes it to stick to the glass on

subsequent firings. Thus, it is essential to change the kiln wash after

every firing that reaches that temperature or higher.

It is possible to apply a fresh coat of kiln wash over the old one to save

time. However, as soon as the kiln wash flakes you must scrape off all the

old kiln wash and apply a new coat to the bare shelf or mould.

Posted by Stephen Richard on http://glasstips.blogspot.com/2009/07/kiln-wash.html


 
©2007 Claybright. | Kiln Contracts | The Clayman | Cape Pottery Supplies