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Those Unlabelled Bags and Buckets

Description

Mike Bailey and David Hewitt provide detailed information on how to identify materials from the properties of their powders

Article

By Mike Bailey and David Hewitt.

There can't be many potteries without the odd bag or two of unidentified material tucked away in some dusty corner and even in the best organised set-ups materials can get put in the wrong bins and their labels lost (by someone else, of course). Suddenly a well tried and tested glaze goes wrong - was it weighed out correctly?, has a supply source changed?, is the bin labeled 'whiting' supposed to have a pale blue powder in it? Many of us will have been in this situation - chaos reigns and it never, ever happens at a convenient time. Is there anything that we can do when it does occur? Well, fortunately for potters, apart from making various observations of the suspect material, there is a potentially very useful diagnostic tool readily to hand for identifying materials; their kiln!

Of course, the only sure way is to send some of the material to a laboratory for an analysis. You will have to suggest which oxides to look for and, more importantly, be prepared to pay for the work, usually between £30 and £100. Naturally most of us would wish to avoid the 'paying' part of this process. Here we describe an approach where some observations and simple fired tests should be helpful in giving a diagnosis. This involves looking at; the colour and texture of the material, a fired 'button' test and a loss on ignition test. In case you haven't got time to do a fired test then a bulk density test could help identify some materials, especially if you've got some 'known' material to compare it with. All these parameters are summarised in Table 1. and we've also given a description of the fired results which acts as a basic diagnostic key.

While this article is aimed at the problem of the unlabelled bag, the methods detailed can equally be used to check that a new supply is what it says it is on the label.

Colour and Texture

This is one of the first things that you can look at and most potters will have a good working knowledge in this area. One may also be fortunate enough to have a correctly labeled bag with which to compare it. To say a material is white is over simplifying the matter. There is white and there is white so look closely for a start. There are obvious ones like Cornish Stone which has a blue tinge and Dolomite which is usually buff coloured. In fact many of the other minerals such as the feldspars and clays also have a characteristic slightly off white tinge. Colouring oxides, too, will often have more distinctive colouring although there are, admittedly, some where the shades of black are very close. Copper Carbonate and Chrome Oxide are both green but most potters would instantly recognise the difference.

Next you can look at the particle size. Talc, for example, is a very fine powder, Flint and Quartz are often lumpy as they are supplied damp. With the addition of water clays will make up into a plastic ball, whereas other materials are short and crumbly. Add these observations to the colour and you will certainly have narrowed down the field. .

The Fired Button Test

This is probably the most useful test that you can do and it can be done very quickly. Just mix a teaspoonful of the unidentified or suspect material with water into a thin paste. Try and get it to the consistency of a glaze. Put a blob of it on to a tile or chard and smooth it out with the back of the spoon getting some variation in thickness from a thin smear to a thicker 3mm+ area. Then fire it on the clay body in regular use and to your normal firing schedule, so that it can be readily compared with known fired samples which you have previously made. Obviously the clay body and the firing will have some effect on the colour of the fired button tests and so the samples shown below have been done on a white body and a darker body, and both oxidised and reduced, to indicate this. For the individual it is obviously best to use your normal clay body and firing for comparative purposes

Loss on Ignition Test

It can be quite amazing just how much weight some materials can lose during a firing. We've listed this in Table 1 as the % lost. For mathematical convenience put 100gms of dried material into a bowl and fire with your next bisque. This drying is very important as you don't want to weigh absorbed atmospheric moisture, or the water added to silica products to keep down the dust. A classic example is whiting when 100gms fired in a 1,000^OC bisque loses 44gms of CO₂ and comes out weighing 56 gms! . One could certainly use this information to tell the difference, for example, between a ball clay which will have some L.O.I. and feldspar which will have virtually none. For some materials the L.O.I. test is also helpful diagnostically because they melt at 1,000^OC! Hence it's sensible to use a spare bowl.

Underglaze Colour Test

This is also a very quick test to do and is particularly helpful at identifying the black powder forms of Cobalt, Copper, Iron, Manganese and Nickel. The material is painted on to a bisqued pot, or shard, with a consistency that is a bit thicker than a water-colour wash but not as thick as gouache or poster paints. This is then glazed and fired, ideally using either an earthenware or stoneware clear glaze, which will show the colour to its full effect.

A Comparative Test Using Bulk Density

We did wonder if the Specific Gravity of a material would be of any use as a diagnostic aid. After all, the Specific Gravity values are given in the reference books, e.g. in Frank and Janet Hamer's, "Potter's Dictionary of Materials and Techniques. Specific Gravity is the ratio of the weight of a substance with an equal volume of water. In the laboratory this is measured with a special flask filled with water at a set temperature. A known weight of the substance is added and some water is displaced. The displaced water is collected and weighed and so the SG ratio is calculated. The average potter is not really equipped for such a test and we found it difficult to get any degree of accuracy using 'kitchen' equipment, so the SG figures are rather academic in this context. However, we did have a bit more success using Bulk Density.

We all know from picking up bags of different materials that they appear to vary quite a lot in their density. Some appear to be relatively heavy and others relatively light. What we are experiencing is Bulk Density and which one would express as gram/ cubic cm. A little experimentation will quickly show that measuring this is not as simple as it might sound. The materials are easily aerated and may have lumps in them, so that trying to measure a volume and weighing it can give variable results. It is however a measure that is routinely used in industry and the key to its use is to have a standard test method which is reproduced exactly each time it is used. The Fig 1 shows one such method based on ASTM E153-59T and used for Calcined Clay. The main idea is that you are controlling the filling up of the measure.

The funnel is placed at a set distance from the top of the measuring pot (we found a 100 cc volume to be convenient). A finger is held under the funnel which is then filled with the material. Funnily enough most fine pottery powders don't suddenly run out of the funnel when your finger is removed and one has to gently coax the material by poking it with a pencil. The measuring pot is allowed to fill to overflowing and a knife is then gently drawn across the top of it. The contents are then weighed and the weight calculated per cm³ (which in this case just means moving the decimal point two places). It's best to do at least 5 measurements and take the average.

If, therefore, in the process of identifying this unlabelled bag you have come to the point of hazarding a guess at what it might be from the other observed factors, and/or not having time to do the fired tests, then comparing its bulk density with a known material might just clinch the matter. Your figures probably won't be the same as ours although they should be in the same sort of order. Certainly the alkaline earths, some frits and metal oxides stand out as being significantly heavier, and the clays lighter, than the average.

Diagnostic key:

Below is a key based on the button test and which then links in with the other factors that we've been discussing. Of course, what you are looking at is not just how this mineral or oxide looks when fired on its own, but how it reacts at these high temperatures with clay. In general, the materials we think of as 'fluxes' form some sort of bonding or melt, feldspars a stiff melt and frits a very shiny melt.

1. Button test forms a very (too) shiny, clear transparent melt.

This probably means that you've got a frit or an earthenware glaze. The frits come as pure white powders and if you've tried a L.O.I. test at 1,000^OC. the material will have fused. Their similar bulk density figures don't help but the degrees of crazing and brown halo can narrow the field.

2. Button test forms a satiny rather stiff greyish semi transluscent to transparent melt. (Almost a glaze.)

This fired description matches most feldspars. Unfortunately L.O.I. and bulk density figures are similar for all and therefore aren't much help.

Cornish Stone and Petalite. A slightly stiffer melt compared with other feldspars. Unfired powder usually has a bluish tinge.

Potash and Soda Feldspars. Hard to tell apart in the button test but can often be used interchangeably anyway! Potash often has a pink/buff tone in the dry state.

FFF Feldspar and Nepheline Syenite. These are usually the most fluxed of the feldspars. FFF often has a pinky buff tinge as a powder and the Nepheline Syenite button test usually develops a brown halo.

Wollastonite also falls into this category where thin, but is rougher and less fused where thick as in 4 below.

Or, of course, it could be a mixed, very quiet glaze in its own right!

3. Button test forms a corrosive, greeny-brown glaze large brown halo.

Probably one of the alkalis, lithium or soda. Soda ash is soluble in water, both melt in the L.O.I. test.

4. Button test forms a corrosive, greeny brown glaze, opaque matt when thicker with only a slight brown halo.

Probably one of the alkaline earths containing; barium, calcium or magnesia. The colour of the powder may be helpful, Barium Carbonate is a purer white, Dolomite usually has a buff colour and Whiting is a greyish off-white. Plasters would also come into this category. Their low L.O.I. and, of course, setting properties would be diagnostic.

5. Button test forms a very stiff, matt coat.

If tinged greyish or tan could be a Ball Clay, Bentonite usually has a cracked fired surface. If fairly white is either China Clay or one of the less reactive fluxes. Here the colour of the powder, comparative bulk density or L.O.I may help. Clays will mix to a plastic ball.

6. Button test unfused, little reaction with the clay surface.

Indicates a refractory flux containing magnesia; magnesium carbonate/oxide or talc.

7.. Button test remains unfused, flakes off easily.

The material is probably a purer form of alumina or silica.

Flint/quartz. No loss on ignition, middle range bulk density. Nowadays is often supplied damp and lumpy.

Alumina. High L.O.I. if alumina hydrate, high bulk density.

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BULK DENSITY TEST PROCEDURE

The important thing for the user is to be able to repeat the tests using the same equipment, funnel, cylinder, distance between end of funnel and top of cylinder and to follow the same filling procedure on each occasion.

1. Place the tared, calibrated cylinder directly below (3.8 cm) the powder funnel.

2. Block the bottom of the cylinder and pour in approximately 150 ml of sample.

3. Quickly remove the blockage and allow sample to flow (use of a small saptula may be needed to obtain flow).

4. Allow the cylinder to over fill by a samll amount.

5. Use spatula to scrape the sample off even with the top of the cylinder.

6. Wipe the cylinder off and reweigh to obtain the weight of sample contained in the cylinder.

Bulk Density (gram/cm cubed) = grams in cylinder / 100

 

POTTERY MATERIAL IDENTIFICATION
RAW MATERIAL	Unfired Colour	Specific Gravity	Bulk Density	% Loss on Ign.	Oxidised fired colour and description
Alumina Hydrate	white	3.8	1.15	33	Refractory, remains white, powdery.
Ball Clay AT	light tan	2.6	0.59	9	Greyish buff, very matt.
Ball Clay HVAR	off white	2.5	0.44	7	Greyish buff, very matt.
Ball Clay TWVA	grey	2.5	0.49	13	Creamy buff, very matt.
Ball Clay HYPLAS 71	cream	2.5	0.58	5	Greyish buff, very matt.
Barium Carbonate	white	4.4	0.68	0	Grey green satin to unfused white matt,slight brown halo.
Bentonite	off white/brown	2.5	0.67	14	Buff, very matt, cracked surface.
Bone Ash	white	3.1	0.61	0	Grey green satin to unfused white matt,slight brown halo.
China Clay N50	creamy white	2.5	0.33	12	White, very matt.
Colemanite	off white/grey	2.3	0.65	melts	Clear, very shiny.
Cornish Stone W/G	pale blue	2.6	0.68	0	Grey, satin matt.
Dolomite	off white/buff	2.8	0.82	46	Grey green satin to unfused buff matt,slight brown halo.
FFF Feldspar	off white/pink	2.6	0.59	0	Grey, semi-transluscent, shiny/satin.
Flint	white	2.6	0.69	0	Refractory, remains white, powdery.
Lithium Carbonate	very white	2.1	0.49	melts	Grey green, satin, reactive with large brown halo.
Magnesium Carbonate	white	4.5	0.64	52	Refractory, remains white, can be scraped off.
Nepheline Syenite	off white	2.6	0.6	0	Pinky grey, semi-transluscent, shiny, often slight halo.
Petalite	white	2.4	0.62	0	Grey, semi-transluscent, shiny/satin.
Potters Plaster	white		0.74	61	Grey green satin to unfused white matt,slight brown halo.
Potash Feldspar	off white	2.6	0.62	0	Grey, semi-transluscent, shiny/satin.
Quartz	white	2.7	0.68	0	Refractory, remains white, powdery.
Soda Feldspar	white	2.6	0.62	0	Grey, semi-transluscent, shiny/satin.
Talc	white	2.7	0.57	7	Refractory, remains white, can be scraped off.
Whiting	off white	2.8	0.81	43	Grey green satin to unfused white matt,slight brown halo.
Wollastonite	white	2.8	0.74	0	Grey, semi-transluscent.
Zinc Oxide	white	5.7	0.51	0	Greyish white, matt.
FRITS
Lead Borosilicate	white	3.1	0.74	fuses	Clear, shiny.
Calcium Borate Frit	white	2.5	0.74	fuses	Clear, shiny.
High Alk. Frit	white	2.5	0.67	fuses	Clear, shiny, large brown halo.
Standard Borax	white	2.5	0.68	fuses	Clear, shiny, brown halo.
Standard Alkaline	white	2.5	0.67	fuses	Clear, shiny, large brown halo.
P2955 'J' Frit	white	2.5	0.69	fuses	Clear, shiny, brown halo.
Lead Bisilicate	white	4.5	0.901	fuses	Clear, shiny.
COLOURING OXIDES					IN GLAZE
Antimony Oxide	white	5.5	0.82	0	White
Chromium Oxide	green	5.2	0.72	0	Green
Cobalt Carbonate	mauve	4.1	0.85	37	Dark blue
Cobalt Oxide	black	6.1	1.15	0	Dark Blue
Copper Oxide	black	6.4	1.41	0	Green
Copper Carbonate	green	5.1	0.91	36	Green
Ilmenite	black	4.8	1.1	0	Brown
Iron Oxide Black	black	5.7	0.72	0	Dark Brown
Iron Oxide Red	red	5.3	0.62	0	Dark brown
Manganese Dioxide	black	4.9	1.08	0	brown
Nickel Oxide	black	6.7	1.95	0	Brown
Rutile	brown	4.3	1.32	0	Tan
Tin Oxide	white	6.8	0.42	0	White
Titanium Oxide	white	4.2	0.44	0	white
Vanadium Pentoxide	ochre	3.4	0.73	0	brown/grey
Zirconium Silicate	White	4.8	0.77	0	White

Note from a reader: An awesome way to help narrow down possible choices of materials is adding a few drops of dilute hydrochloric acid or vinegar to a sample of the dry powder. Fizzling (effervescence) indicates that it is a carbonate; the amount it fizzles can also lead you to which carbonate it is. For example dolomite will not fizzle as violently as whiting.

Related Information

Links

URLs	http://aqua-calc.com/calculate/volume-to-weight Mineral volume-to-weight calculator

By David Hewitt
Mike Bailey

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