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Soluble salts in clay: Jeff Zamek details causes and solutions.

Most potters have experienced the effects of soluble salt migration in a clay body which can occur in the drying, bisque or glaze firing stages. Scumming (the industry term) presents itself as efflorescence, or white crystal powder forming on the surface of the clay. It can be observed on random parts of the clay body surface but is most often seen on edges or high points, which dry faster than other parts. Faster drying increases the 'wicking' action, bringing soluble salts to the surface of the clay with the evaporating water. Textured surfaces or the ridges on cup handles can reveal a hard discoloured crust on the fired ware. The same outcome is present with slip casting formulas that are over deflocculated with sodium compounds such as sodium silicate, soda ash, Darvan #811, or Darvan #7 (sodium polyelectrolyte), all of which are soluble.

In the bone dry or bisque state fingerprints can disturb the soluble salt deposit on the clay surface causing a noticeable flashing or discolouration on the fired clay. High concentrations of soluble salts on bisque clay surfaces can result in fused areas which retard absorption, resulting in uneven glaze deposits. Soluble salt migration can also disrupt the covering glaze surface. Essentially, it can create a mechanical disruption of the clay body/glaze interface which can result in the glaze crawling (the fired glaze rolls back revealing the clay body) or a series of small glaze blisters (sharp edged crater-like holes in the fired glaze) due to the fluxing action of the soluble salt on the clay body. (1) The relatively low melting point of soluble salts can prevent the release of organic material in the clay, resulting in carbon trapping at higher temperatures. The melting action of soluble salts on the clay surface can also fuse pottery to kiln shelves.

Glaze pinholes (a round edged hole exposing the clay body) can also occur due to soluble salt deposits on the underlying clay body surface. What makes the problem more difficult to solve is its intermittent nature. Any given clay body formula can produce a defect-free batch of clay many times and not the next. This is due to the specific level of soluble salts found in one or more of the clays used in the body. In most instances soluble salt deposits on the clay body surface do not come from water used in the clay mixing process, provided the water does not originate from a static source which through evaporation can concentrate salts found in the water. (2)

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WHERE SOLUBLE SALTS ARE FOUND

In clay mining, the overburden (the layer above the clay seam) and any possible water source nearby can contain organic material which, when decomposed, can produce soluble salts. The overburden and underlying clay seam can vary in the amount of soluble salts depending on location and topography. One marker for possible soluble material is the presence of calcite or gypsum, often observed as crystals in the soil. If the overburden is composed of permeable materials such as sand or loose gravel the rate of soluble material leaching into the clay seam increases. A high pH (acidic) level in the overburden can also intensify the transmission of soluble materials into the underlying clay seam.

The permeability of the clay seam also plays a part in overburden leaching. Some types of clays are more permeable to leaching from the overburden causing the top sections of the clay seam to become unsuitable for mining. Additionally, the clay itself can contain high levels of alkalis and alkali earths such as calcium, magnesium, sodium, potassium and iron sulphates which can be found at low levels in most clays. (3) As in the overburden, a high pH level in the clay can also bring more soluble phase ions into the system.

CLAY MINING

The most widely used method of clay mining involves drilling test holes through the overburden to reveal the contours and depth of the underlying clay seams. The overburden can be from six to 14 feet in depth with the clay seams ranging from five to 20 feet in thickness. The actual seams can contain different strata of clays for possible use depending on market requirements. At this point an economic factor comes into play: if too much overburden has to be removed, the cost of mining the clay and replacing the overburden decreases or eliminates the profit. The cost of clay recovery is also dependent on the energy required in man hours, equipment and fuel costs. Or, if the seam of clay is too thin and/or inconsistent, mining the clay is unprofitable. An exception occurs when the small seam is highly valued by the end point user who is willing to pay a premium price for its extraction and processing. Essentially, the clay company is looking for a large deposit of uniform quality clay which can be excavated with minimum costs.

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TYPES OF CLAYS WITH SOLUBLE SALTS

Bentonite clays can have high levels of soluble salts but, since their use in clay body formulas to increase plasticity is generally limited to 2% or less, their visible soluble effects on the surface of the clay body are less prominent. (4) Most ball clays and kaolins have relatively low concentrations of soluble salts but, depending on the specific levels of soluble material in the clay deposit, can exhibit various shades of discouloration. High content iron bearing clays which contain lime, such as Redart can periodically cause soluble deposits on the surface of the clay. Other high iron and/or organic bearing clays such as C-Red and Lizella clays tend to be higher in sulphate content and seem to generate intermittent excess soluble salts when used in a clay body formula. Iron sulphates of marcasite and pyrite can also be found along with lime in some high iron content clays. The dissociation of iron sulphides gives rise to sulphate in free radical form which can cause salt migration to the surface of the ceramic form.

Soluble sulphur in clays similar to Goldart stoneware can give off a sulphur smell during firing from 600[degrees]F to 1200[degrees]F when the kiln is relatively cold and moisture from the clay saturates the atmosphere and condenses on the ware. (5) It can also leave a yellow efflorescence on the bisque fired clay surface. Generally, this deposit does not seem to interfere with the fired glaze.

Soluble salt migration is found in common red building bricks. For economic reasons low grade shales (containing clay, quartz, mica, organic matter and ferric oxides) are the most widely used clays in the brick industry and can contain high levels of soluble salts. This effect is most noticeable after a rain storm. As the surface water evaporates from the brick it draws out or 'wicks' soluble salts from the interior to the surface. This reaction deposits a white efflorescence on the surface, indicating the clay water system is saturated and the salts have not bound with anything in the clay body.

EXAMPLES OF USEFUL SOLUBLE SALT MIGRATION IN CLAY AND GLAZES

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There are instances where soluble salts are encouraged, either in the clay body or the glaze. Egyptian paste, as the name implies, is thought to have been developed in Egypt approximately 5000 BC. It is an extreme example of a clay body with intentionally high levels of soluble salts. As the soluble salts migrate to the surface, they form a glaze covering the entire exposed clay surface. Interestingly, areas such as the bottom of the piece resting on a table and not exposed to air while drying do not develop a soluble salt deposit and do not form a glaze. The Egyptian paste clay body lacks plasticity and is limited to low temperature firing conditions from cone 010 (1657[degrees]F) to cone 04 (1945[degrees]F). Metallic colouring oxides of cobalt, copper and iron or ceramics stains contribute colour to the glaze.

TYPICAL EGYPTIAN PASTE CLAY BODY FORMULA

In most Egyptian Paste clay body formulas soluble materials such as sodium carbonate, sodium bicarbonate, soda ash or borax can range from three percent to 18 percent. Soluble Salt Migration in Glazes

The most prevalent example of soluble salts occurs in carbon trap glazes. The addition of soluble materials such as soda ash (sodium carbonate), baking soda (sodium bicarbonate) or salt (sodium chloride) to the glaze can range from three percent to 17 percent. (6) As the glaze dries, soluble materials migrate to the glaze surface which then melt early in the firing cycle, sealing in the underlying glaze layer. As a reduction atmosphere (greater ratio of fuel to air in the combustion process) is achieved in the kiln carbon is deposited on the glaze surface. The melting soluble material will then trap carbon resulting in spotted random grey or black glaze surfaces notably on a white porcelain clay body.

ELIMINATING SOLUBLE SALTS

There are several options that can eliminate soluble salt migration. When developing a clay body or choosing a pre-mixed moist clay whenever possible, select clays low in iron content and/or low in soluble salt content. Often this information can be obtained from ceramics suppliers who mix clay. Or, if clays high in soluble salts are required, try to use the lowest possible percentage in the clay body formula.

The most widely used correction for soluble salts is the addition of barium carbonate, 1/16 percent to two percent based on the dry weight of the total clay body formula. When thoroughly mixed into the clay body, barium carbonate will precipitate soluble salts from solution yielding barium sulphate which is insoluble. (7) Brick industries are the largest users of barium carbonate where low grade, high iron content shale type clays are used in the production of bricks.

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Another alternative for eradicating soluble salts is the use of Additive A in the clay body. Additive A (Type 1 and Type 3) is a blend of lignosulphonates and barium carbonate. Produced by Ligno Tech, USA, Additive A derived from the paper pulp production process. It has been used in the brick industry since 1955. Aside from the benefits of stopping soluble salts it also increases the green strength and plasticity of the moist clay. It can be used in clay body formulas from 1/10 percent to 5/10 percent based on the dry weight of the clay body. It has the added advantage of lubricating the moist clay through any extrusion process. (8,9)

Some success has been reported with washing the exposed bisque or high fired exposed areas of clay with vinegar, a weak acid, or products such as Muriatic Acid, a solution of 31.45 percent hydrogen chloride and 68.55 percent inert ingredients, mostly water; the latter product requires careful use in its application and storage. If the clay's interior still contains soluble salts, however, they will again migrate to the surface. In such instances a sealer containing silicone or polymer solutions are used to close the clay pores. (10)

Sources

Simon & Schuster's Guide to Rocks & Minerals, Simon and Schuster Publisher. 1978.

Parmelee, CW. (1922), "Soluble Salts and Clay Wares". Journal of the America Ceramic Society, 5:538-553. doi: 10.1111/j. 1151-2916.1922. tb17439.x.

Hammer, Frank and Janet. The Potter's Dictionary of Materials and Techniques, 4th Edition, 1997. University of Pennsylvania Press, Philadelphia.

Chappell, James. The Potter's Compete Book of Clay and Glazes, Watson-Guptill Publications, New York. 1977.

Acknowledgements: Ken Bougher, Technical Director, Old Hickory Clay Company, supplied information on clay mining operations and testing procedures for soluble salts. Amy Waller provided images of her Egyptian paste pendants, Amy Waller Pottery. (amy@amywallerpottery.com) Jim Cutright, Technical Director, Spinks Clay Company, offered information on overburden and soil pH levels as well as commercial considerations in mining clay. Mike Schoenherr, Area Business Manager, LignoTech USA, Inc. www.lignotech.com , supplied information on Additive A. (ceramics@borregaard.com) Jim Fineman, professional potter and technical editor.

FOOTNOTES

(1.) Zamek, Jeff. The Potter's Studio Clay & Glaze Handbook. Quarry Books, Beverly, Massachusetts. 2009. p 109.

(2.) Fraser, Harry. Ceramic Faults and Their Remedies, 2nd edition, 2005. Gentle Breeze Publishing. Oviedo, Florida. p 45.

(3.) Fraser, Harry. Ceramic Faults and Their Remedies, 2nd edition, 2005. Gentle Breeze Publishing. Oviedo, Florida. p 45.

(4.) Digital Fire http://difitalfire.com/4sight/tests/ceramic_test_soluble_salts.html.

(5.) Parmelee CW. Ceramic Glazes, 3rd Edition, 1951. CBI Publishing Company, Inc. Boston, Massachusetts. p 585.

(6.) Britt, John. The Complete Guide to High-Fire Glazes, Lark Books, Division of Sterling Publishing Co., Inc. 387 Park Avenue South, New York. 2004. p 82.

(7.) Digital Fire http://difitalfire.com/4sight/tests/ceramic_test_soluble_salts.html.

(8.) Additive A produced by LignoTech USA, Inc.

(9.) Zamek, Jeff. What Every Potter Should Know. Krause Publications, Iola, Wisconsin, 1999. p 86.

(10.) Digital Fire http://difitalfire.com/4sight/tests/ceramic_test_soluble_salts.html.

Jeff Zamek walked into a pottery studio 45 years ago and started his career as an amateur potter. After completing a degree in business from Monmouth University, W Long Branch, New Jersey he earned BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. While there he developed the soda firing system at the college and went on to teach at Simon's Rock College and Keane College. During this time he earned his living as a professional potter. In 1980 he started Ceramics Consulting Services, a ceramics consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. He works with individual potters, ceramics companies and industry offering technical advice on clays, glazes, kilns, raw materials, ceramics toxicology and product development. He is a regular contributor to several ceramics magazines and technical journals. Zamek's books, What Every Potter Should Know and Safety in the Ceramics Studio, featuring the safe handling of ceramic materials; and The Potters Health & Safety (Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. Zamek is currently working on several ceramics research projects and is making pots as an amateur potter. His latest book, The Potter's Studio Clay & Glaze Handbook, was published in June, 2009.
Amy Waller Turquoise
Egyptian Paste Clay Body
(cone 010 to cone 04)

Flint 325 mesh 85
Sodium bicarbonate 6
Kentucky OM #4 ball clay 5.2
Whiting 1.9
Custer feldspar 1.9
Copper oxide 1.0

Carbon Trap Shino
(cone 9 reduction)

Nepheline syenite 270 mesh 40
Spodumene 30
EPK kaolin 5
Kentucky OM # 3 ball clay 17
Soda ash 8
Bentonite 2
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Author:Zamek, Jeff
Publication:Ceramics Technical
Date:Nov 1, 2012
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