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Dense packing and filter pressing moist clay: Jeff Zamek discusses the options for clay mixing.

The question of How to make clay more plastic and workable has been discussed since clay was first discovered to be a useful medium for forming functional pottery, ceremonial objects and sculpture. While every period and culture has developed its own means to achieve this end, several clay mixing systems are now being rediscovered to achieve greater plasticity in clay bodies. (1) Today there is only one company, Matt and Dave's Clays, which has specialized in the dense pack/filter press method of clay mixing. Their efforts have resulted in significant improvements in clay body plasticity and a reduction of common clay body defects. While other clay mixing methods are currently in use by ceramics suppliers and individual potters, Matt and Dave's system of dense packing and filter pressing has produced technically improved clay bodies.

Every potter has suffered the negative effects of 'short' or non-plastic clay, no matter which forming method is used. The clay does not hold curves well and can split during forming operations or, when rolling out a slab, the edges begin to crack. While some clay body formulas are inherently more plastic than others (the term plasticity is subjective), at some point the clay just does not bend or do what you want it to do.


Why is clay plastic when wet? Why does the shape of clay influence its plastic properties? Clay under a microscope looks like a hexagonal plate having a ratio of roughly one part thickness to 10 parts surface area. The diameter of kaolin, a refractory, low iron, white firing clay can range from one to 100 microns (one micron equals one millionth of a metre; the diameter of a human hair is 80 to 100 microns). Other types of clays such as fireclays, ball clays, earthenware clays, stoneware clays and bentonites all have different size platelets, with bentonites having the smallest platelet size. (If the platelets in 2.6 grams of bentonite were placed edge to edge they would cover approximately 6000 square feet; 100 pounds would cover 2,400 acres). (2)



Imagine a room filled with basketballs, each one representing a large clay platelet. When the room is filled there are always spaces between the basketballs. Then fill those spaces with soft balls (a medium sized clay platelet). When those spaces are filled, place tennis balls in the room (small size clay platelets) and then golf balls (very small clay platelets) in the remaining spaces. There will always be some empty space, which water can fill to make the clay body plastic.

In addition to clays, other materials contribute to the total performance of a clay body. In high temperature formulas above cone 6 (2232[degrees]F) felspar and flint act in conjunction to control vitrification temperature, density and absorption. (3) Felspars and flint are large in comparison to clay particles and the clays pack around them. Other materials can also be used such as grog, silica sand and other fillers that are of different shapes and, in some instances, many times larger than the clay component.

Grog, molochite (white grog) and silica sand are all produced with varying mesh sizes. To further maximize particle size variation and physical interlocking, several mesh sizes can be used in the clay body formula. The larger the number, the finer the particle size of the material. Numbers below 100 mesh indicate a fine powder which can decrease clay body plasticity. For example, a 48/to fine mesh size grog can be used in throwing bodies while a larger mesh grog, 20/48 mesh or a 20+30 mesh material would be suitable for sculpture bodies. Silica sand is also produced in various mesh sizes. Some sands, however, are round in shape and will not add 'tooth' or stand-up ability to the moist clay in the forming operations. Sharp sand, having an angular irregular shape, is better suited in the clay body as its surfaces interconnect more efficiently with the adjoining raw materials and clays.

Felspars and quartz (silica) are giant grains in the multi plate-like structure of the clay body. Many small clay platelets, however, are able to control the movement of relatively few large grains of materials found in fillers. (4) Although these materials do not improve plasticity, they contribute workability to the clay body. (5) The selection of clays (especially their particle size distribution) and the total amount of clay used in the clay body are important factors in dense packing clay body formulas. Dense packing of clays contributes to an internal structure of the clay body characterized by more powerful bonding between the clay particles, making it stronger.

Surprisingly, aging of the clay and its moisture content are not as critical in determining plasticity as dense packing of the clay platelets. (6) The actual water content of the clay body will determine if it will be hard or soft but does not influence its performance. Some potters have a preference for softer or harder clay but this is a personal consideration that allows one to achieve a particular goal when making a form. In many instances potters blame a very soft clay (high water content) for warping or hard clay (low water content) for cracking when the actual cause is whether the clay body has achieved a dense packing configuration or not. If the clay body contains only one clay such as grolleg kaolin found in porcelain clay body formulas, the uniform sized clay platelets leave a lot of empty space which is filled with water, causing the 'floppy' feeling. When working porcelain on the wheel, potters frequently report that it slumps or falls back on itself. This is in contrast to stoneware clay body formulas that usually rely on several clays, each having a different platelet size, so there are fewer empty spaces between the plates to be filled with water resulting in a clay body less likely to slump during handbuilding and throwing operations.

Introducing ever smaller particles into the clay body, however, can cause the moist clay to become too stiff for handbuilding or wheel throwing due to less room for water in the clay body structure. The overly packed moist clay is almost unbendable but suitable for Ram pressing or jiggering which apply greater force to form moist clay.

Diversity in clay platelet size creates increased contact between platelets and enhanced electrostatic attraction of clay platelets. The faces of the platelets attract each other as the positive and negative poles of a magnet draw together. Additionally, the attraction effect of water clinging to the individual clay platelets serves to increase plasticity. Greater clay surface areas of different sizes are used by both reactions. Clay body formulas with a limited particle size distribution have less plasticity.


Traditionally, porcelain clay body formulas are composed of 50 percent of a single kaolin, 25 percent flint and 25 percent felspar and do not handle well, resulting in a 'flaccid' condition and 'rubbery' feel in forming operations. This is due to a low clay content and the similar platelet size found in its kaolin. Using only one type of kaolin limits the particle size distribution and creates a less than dense pack condition. Using a plastic kaolin such as grolleg, EPK or Tile #6 in conjunction with other kaolins such as Velvacast, Kingsley or Helmer can improve overall plasticity as compared to stoneware clay body formulas which can contain approximately 80 percent of clays of various particle sizes.

Matt Katz states that dense packing of clay bodies depends on three conditions: the clay platelet sizes used and their subsequent surface areas, the particle size distribution of the clay(s), and the morphology or shape and aspect ratio of the particles introduced into the clay body. Dense packing and mixing the clay as a liquid slip (slurry mixing) increases green strength, making cracking less likely when leather hard or bone dry. It also minimizes packing voids in the clay body resulting in less warping in the drying and firing stages.


Traditionally, clay body formulas mixed by commercial ceramics suppliers and individual potters involve weighing out the dry materials (which might be mixed in random order) then adding 21 to 25 percent water to achieve a plastic moist clay. The moisture content differential between soft and hard clay, however, can be as little as three to five percent. The clay body can then be sent through a de-airing pug mill which further mixes and compresses the clay while removing excess air. This has proven to be a low cost, high volume method. The mixing operation is usually performed by bread dough mixers, Muller type roller mixers where the clay is kneaded but not crushed by the wheels, or ribbon mixing machines and pug mills. While economical and labour saving, plastic mixing does not produce the optimum plasticity. It also does not promote green or fired strength in a clay body.


When the clay becomes plastic after additions of water, there are clumps of felspar, quartz and clay. These clumps (or agglomerations) cause inconsistency in performance, inducing many forms of body failure; such as bloating, warping and slumping in the fired clay. Over time, water does wick through the entire mass of clay, increasing its plasticity. But the normalization of water does nothing to disperse the agglomerates. Many potters have experienced this type of delayed plasticity when they first mix their clay and roll small coils around their fingers and the coils crack. A few days later, the same diameter coil of moist 'aged' clay rolled around the finger does not crack due to increased moisture saturation of the clay body, with a water film surrounding a greater number of clay platelets.

Plastic clay mixing can cause several defects, one of which is marbling, which produces darker or lighter discolouration in the moist or fired clay body. Marbling is due to incomplete blending in the mixer. Where coloured clays are used in the clay body and are marbled they can cause areas of varying fired colours. If a low temperature darker coloured clay is added for clay body colour in a high temperature clay body, it can cause over-fluxed areas due to its lower melting point. When felsapar or other raw material particles are not distributed equally they can stick together yielding highly concentrated areas of flux in the clay body that can cause bloating or bubbles in the fired clay. The formation of felspar clusters due to agglomeration (clumping) of felspar particles can be stopped by the addition of clay to the mixing process which coats the felspar. (see The Slurry Mixing Process)

What makes the dense packing and the filter press method of mixing clays unique is the efforts of a small company located in Alfred, New York: Matt and Dave's Clays, LLC. Founded by Matt Katz (clay artist and ceramics instructor at Alfred University), Dave Finkelnburg (ceramics engineer and potter) and Bill Carty (whitewares professor at New York State College of Ceramics at Alfred University). Their pioneering efforts have given potters superior quality clays. They have used their engineering skills and artistic talents to develop clay bodies in the mid to high temperature range. They mix specific blends of clays and raw materials to formulate densely packed clay bodies and then use slurry mixing and filter pressing to ensure wider firing ranges with fewer defects. Potters can now obtain clay that meets or exceeds industry standards.


The clay bodies being produced are more responsive to handbuilding, throwing and slip casting methods of construction due to scientific testing and analysis resulting in increased performance for potters with a wide range of skills. Most causes of clay body failure are due to inconsistent mixing of the clays and raw materials. Matt and Dave's Clays are designed to avoid the most common clay body failures due to slumping, bloating, cracking, warping and discolouration of the fired ware. The company is unique in that it brings together a systematic approach to choosing the appropriate clays for a range of clay body formulas along with a mixing method that ensures better performance and fewer ceramics failures.


The slurry mixing process uses excess water, a specific sequence of adding materials and high shear blending to ensure a thorough mix in the clay/water structure. This combination assures that each clay platelet is surrounded by a water film, resulting in greater clay body plasticity. The process begins with water and bentonite, an extremely plastic, small platelet clay that is added to increase plasticity. Bentonite, however, expands when it comes in contact with water. Therefore, it should be thoroughly mixed in water for at least 24 hours to ensure its complete dispersion. In high shear mixing operation, bentonite can be added first to ensure hydration of the particles. The process then requires that 20 percent of the clay component be added to the water resulting in a dilute clay/water soup. The suspension encapsulates and disperses the felspar, which is added next, preventing it from clumping together and resulting in over-fluxed areas in the fired clay body. Flint is then added, followed by the remaining 80 percent of clay.

While the slurry mixing procedure is time consuming and labour intensive, it does produce greater plasticity, increased green strength and homogeneity in the moist clay, resulting in lower loss rates when moving green pots. The process also reduces failures caused by potter's wheel torque where the moist clay is twisted by the rotation of the wheel and the pot can unwind and warp in the drying or firing process. Slurry mixing also enables sturdier attachments such as spouts, handles and knobs to the moist clay. (8)


Since slurry mixing introduces excess water to the process, the next step removes it while leaving enough to achieve a plastic clay body. Water removal can be accomplished in several ways. In the private studio, a container of absorbent material such as plaster can wick out the excess water. In industry it is accomplished by a filter press where the slurry is piped between a series of plates with a fabric coat and pump pressure forces out excess water through the fabric, leaving the plastic behind. The resulting sheets of clay are then fed into a pug mill for compressing and de-airing. Significantly, dense packing clay, slurry mixing and filter pressing combine to make stronger and more plastic clay bodies than the plastic mixing process. This has been proven by using the same clay body formula and processing it using the plastic mix method versus the slurry mix, filter press method which resulted in a significant improvement in the handling qualities of the moist clay body.

Dense packing and filter pressing makes the clay more workable. For example, porcelain clay bodies can be used on the wheel and for handbuilding operations, while filter pressing of the clay enhances wider firing ranges due to the consistent distribution of felspar throughout the clay mix.



The cost of labour, raw materials and production time required to mix the clay body all play a part in determining why the plastic mixing process is so popular. The system is used by ceramics suppliers who sell moist clay and individual potters who mix their own clays. The efficiency and low operating costs are accepted industry standards. For professional potters who mix their own clays, saving time in any part of the pottery production operation is critical in keeping costs low. The plastic mixing method allows them to produce large quantities of moist clay faster than would be possible with slurry mixing and filter pressing. Almost all industrial clay manufacturing operations utilize the slurry mix and filter press operations. They find that this improves quality of their clay bodies and drastically reduces the losses they face in manufacturing.



The extra expense generated in dense packing clay body formulas requires ordering, inventorying and mixing an increased number of clays to enhance platelet size variation. It also demands greater time to produce a slurry mix and filter press a clay body. For ceramics suppliers, such steps would necessitate higher pricing, increasing the cost of a typical porcelain clay body by as much as 25 percent.

The shortcuts taken by most companies is something that artists must consider carefully. The composition and mixing used by those companies does produce low cost clays. Low cost clays come with many potential problems that are corrected for in Filterpress mixing. Although the cost of filterpress clays is higher, the success of one's work in the studio and kiln are the most valuable asset a maker can have. Potters as a group are very sensitive to the cost of their materials, especially premixed clays, since this is one of their largest expenditures. The cost of raw materials, however, is small compared to the expenditure of time and labour required to make the pottery. This fact is still not widely recognized and, as a result, ceramics suppliers who make premixed clays are reluctant to incur additional expenses to change their mixing operations.


A further disincentive for commercial ceramics suppliers who mix clays is the trend to use less expensive raw materials, as the cost of transportation steadily increases and in many instances equals or is greater than the cost of the actual material. For example, transporting a west coast clay to an east coast ceramics supplier might incur prohibitive transportation costs, no matter how superior the quality of the clay. The trend over the years is to produce clay body formulas that work reasonably well for the greatest number of customers. For the relatively few potters requiring a specialized clay body formula, an extra charge is incurred.

As for individual potters mixing their own clay bodies, once aware of the benefits of dense packing, slurry mixing and filter pressing, it is an open question as to whether they will take advantage of this technical improvement in clay plasticity and strength. The extra cost and time and the additional equipment must be balanced against an improved clay body. It does seem that a percentage of potters are satisfied with their moist clay no matter how it is mixed and do not have the time or money to consider an alternative. Potters who realize the cost of clay is only a fraction of their production costs, however, may turn to dense packing, slurry mixing and filter press mixing to reduce defects and possibly to expand aesthetic choices. The greater appeal of this method might be found in college ceramics programs or craft centres where economic considerations are not as critical and there is more time to investigate different aspects of clay mixing.

Today there is only one company that uses the dense packing and filter press system of mixing clay: Matt and Dave's Clays. (


I would like to thank Matt Katz and Dave Finkelnburg of Matt and Dave's Clays who contributed technical content and images used in this article. John (Benny) Benedict supplied images of clay mixing machines and pug mills.

Frank Tucker, President of Tucker's Pottery Supplies, Inc., ( was helpful in supplying information about filter pressing of clay bodies.

John Britt, professional potter, supplied information on filter press clay mixing operations.


(1.) Clay bodies are composed of clay(s), various glass formers such as frits and felspars (depending upon the clay body temperature range) and fillers such as sand, grog and molochite, to achieve a precise handling characteristic, firing range, texture, shrinkage, absorption, colour and glaze compatibility in a specific kiln atmosphere.

(2.) WG Lawrence, Ceramic Science for the Potter, Chilton Books, 1972, pages 38-39.

(3.) Finkelnburg, Dave and Matthew Katz, NCECA Journal 2010 Vol. 31, "Sexy Bodies--In The Mix", page 95.

(4.) Katz, Matthew. NCECA Journal 2007 Vol. 28, "Clay Selection and Performance, or Performance Art", page 38.

(5.) Finkelnburg, Dave and Matthew Katz, NCECA Journal 2010 Vol. 31, "Sexy Bodies--In The Mix", page 95.

(6.) Katz, Matthew. NCECA Journal 2007 Vol. 28, "Clay Selection and Performance, or Performance Art", page 38.

(7.) Matt and Dave's Clays Porcelain produced by Matt and Dave's Clays, LLC.

(8.) Finkelnburg, Dave and Matthew Katz, NCECA Journal 2010 Vol. 31, "Sexy Bodies--In The Mix", page 96.

Jeff Zamek walked into a pottery studio 36 years ago and started his career as an amateur potter. After completing a degree in business from Monmouth University, W Long Branch, New Jersey, US, he obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. 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 and industry, offering technical advice on clays, glazes, kilns, raw materials, ceramic toxicology and product development. He is a regular contributor to Ceramics Monthly, Pottery Making Illustrated, Pottery Production Practices, Clay Times, Studio Potter, Ceramics TECHNICAL and New Ceramics. Zamek's books, The Potter's Studio Clay & Glaze Handbook, 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. (
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Author:Zamek, Jeff
Publication:Ceramics Technical
Date:Nov 1, 2010
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