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Properties of porous ceramics

Physical properties of porous ceramics
Formulas or mixes (m) and associated Specific (B) Bar Values
Understanding porous ceramics
Glazing
Washing
Temperature ranges

Table 1. Physical properties of porous ceramics

Air Entry Value	Bubbling Pressure (PSI)	Approximate Porosity (% vol.)	Saturated Hydraulic Conductivity (cm/sec)	Maximum Pore Size (μm)	Flow Through 1/4-Inch Plate (ml/hr/cm²/14.7 psi)
0.5 Bar (-B0.5M2)* High Flow	7 to 9	50%	3.11 x 10^-5	6.0	180
1 Bar (-B01M3)* High Flow	19 to 28	45%	8.6 x 10^-6	2.5	50
1 Bar (-B01M1)* Standard Flow	20 to 30	34%	7.56 x 10^-7	1.7	5.0
2 Bar (-B02M1)* Standard Flow	38 to 45	32%	6.30 x 10^-7	1.1	4.2
2 Bar (-B02M2)* High Flow	32 to 42	38%	6.93 x 10^-7	1.3	4.6
3 Bar (-B03M1)* Standard Flow	46 to 70	34%	2.5 x 10^-7	0.7	1.6
5 Bar (-B05M1)* Standard Flow	80	31%	1.21 x 10^-7	0.5	0.7
15 Bar (-B15M1)* Standard Flow	220	32%	2.59 X 10^-9	0.16	0.015

* Part number suffix corresponding to the appropriate air entry value and mix number. See Formulas or mixes (m) and associated Specific (B) Bar Values for more details.

Formulas or mixes (M) and associated Specific (B) Bar Values

B0.5 - 1/2 Bar (7.4 psi or 50 KPa) Air Entry Ceramics
Ceramic - B0.5M2
This ceramic is developed from a high fired, Alumina body. The resulting ceramic is an excellent material which is extremely porous, inert to most all solutions, possesses hard exterior and interior surfaces, and is pure white in color. This material is recommended for low pressure differentials not exceeding 7.4 psi. The tremendous porosity and high conductivity of fluids or gases make it ideal for quick extractions or in creating, monitoring or extracting pulse hydrological events. The material is ideal for liquid or gas sampling as the Alumina material has almost no ionic exchange sites or "leachable" mineralogy. This is a truly superior ceramic for both industrial and scientific work where high volume transfer or testing at low pressure differentials is necessary.

B01 - 1 Bar (14.7 psi or 100 KPa) Air Entry Ceramics
Ceramic - B01M1
This ceramic is developed from a moderately fired largely Talc body. The resulting ceramic material, made from a time proven formula, is a utilitarian ceramic having good porosity, tough exterior and interior surfaces, and is ivory white in color. This material is recommended for general purpose uses that involve pressure differentials under 15 psi. This ceramic is an excellent choice where cost and precise content of fluids or extracts are not at issue. This material has been applied successfully to tensiometers, pressure plate assemblies, suction tables and the like for nearly 50 years. This ceramic is not recommended for precision fluid sampling work as it has some ionic exchange sites and a mineralogy that is leachable with strong acids over the years. This is the perfect choice for those needing a low cost industrial ceramic product or where the science requires minimal fluid content determinations.

Ceramic - B01M3
This ceramic, like the B0.5M2, is developed from a high fired Alumina body. The resulting ceramic is an excellent material. It is extremely porous, inert to most all solutions, possesses hard exterior and interior surfaces, and is pure white in color. This material is recommended for standard pressure differentials not exceeding 15 psi. The tremendous porosity and ability to conduct large amounts of fluids or gases makes it ideal for quick extractions, and creating, monitoring or extracting pulse hydrological events. The material is ideal for liquid or gas sampling as the Alumina material has almost no ionic exchange sites or "leachable" mineralogy. A great material for most any application involving sampling, testing, monitoring or infusion where precision and actual liquid contents are of importance.

B02 - 2 Bar (29.4 psi or 200 KPa) Air Entry Ceramics
Ceramic - B02M1
This ceramic is developed from a variety of ball clays into a moderately fired ceramic body. The fired product has an effective porosity and good hydrologic flow capability. Bubbling pressures for this ceramic are pressure differentials less than 29.4 psi. This general use ceramic is most often used on specialized plates for soil water retention or in unique oil and gas industries for reclamation studies. The material is moderately hard and creamy white in color. This ceramic is recommended for specialized applications using plates.

B02 - 2 Bar (29.4 psi or 200 KPa) Air Entry Ceramics
Ceramic - B02M2
This porcelain ceramic is developed from a high fire Silica body. The resulting ceramic is an excellent material for slightly elevated pressure differentials not exceeding 29.4 psi. The material has a somewhat grainy texture and pure white appearance. A good material for sampling fluids and gases as porcelain has few ionic exchange sites or "leachable" mineralogy. With the good porosity and hydrologic characteristics, this ceramic provides a material that can be used by oil companies or agricultural research scientists. This ceramic is an excellent choice for those needing the added capacity of elevated pressure differentials and precise fluid content measurements.

B03 - 3 Bar (44.1 psi or 300 Kpa) Air Entry Ceramics
Ceramic - BO3M1
This ceramic is developed from a complex mixture of ball clays into a moderately fired ceramic body. The resulting ceramic has good porosity and good hydrologic flow capability. Bubbling pressures for this ceramic are pressure differentials exceeding 44 psi. This ceramic is generally used within pressure vessel equipment for the determination of soil water retention or in oil and gas industries for reclamation studies. The material is moderately hard and tannish-white in color. This ceramic is recommended for specialized applications where the differential pressures will be less than 44 psi.

B05 - 5 Bar (73.5 psi or 500 Kpa) Air Entry Ceramics
Ceramic - B05M1
This ceramic, like the "B03M1", is developed from a complex mixture of ball clays into a fired ceramic body. The resulting ceramic has good porosity and good hydrologic flow capability. Bubbling pressure or air entry values are pressure differentials exceeding 74 psi or greater. It is generally used within pressure vessel equipment for the determination of soil water retention or in oil and gas industries for reclamation studies. The material is very hard and brownish-white in color. This ceramic is recommended for specialized applications where the differential pressures will be less than 74 psi.

B15 - 15 Bar (220.5 psi or 1500 KPa) Air Entry Ceramics
Ceramic - B15M1
This ceramic is developed from a proprietary mixture of ball clays fired to a ceramic body. The resulting ceramic material is pinkish-tan in color, moderately hard and will withstand pressure differentials of 220 psi. This unique ceramic, incorporated into 0675 pressure plate cells, has been used in Agronomy for many years in water retention studies to a theoretical wilting point of 15 bars. It has also found use in the oil and gas industries in studies of reclamation and production techniques. It remains the worldwide choice of experts when they need to know the behavior of liquids to a 3 dimensional porous material that mimics soil and stone. The B15M1 is still the only ceramic in the world that, when wetted, can withstand a pressure differential of 220 psi and not leak or bubble. The unique characteristics of the B15M1 ceramic make it the selection of experts and scientists who are involved in liquid movements and transfer conditions at elevated pressure differentials.

NOTE: Formulas and Mixes
The above formula descriptions for our ceramics are keyed to the "bubbling" (B) or air entry value of a ceramic. There may be one or more "mixes" (M) associated with a particular air entry value. M1 will denote the first formula, M2 the second, and so forth. An example of this non-relationship: M1 mix for a B1 (one bar air entry value) is not in any manner related to the mix or formula M1 for a B5 (five bar entry value ceramic).

Understanding porous ceramics

Porous Materials: Materials containing a passage or channel, a small opening for absorbing or discharging fluids.

The Closed Pore Structures
As the name would imply, this is a material in which the pathways or channels are blocked and do not provide the fluid a continuous and interconnected network of channels to connect one surface of the material to an opposing surface.

The Open Pore Structures
An open pore structure denotes a material that permits a fluid to move from one surface to an opposing surface in the material through a convoluted pathway of interconnecting networked channels. Soilmoisture ceramics are an example of an open pore structured material.

Hydrophobic Material - Water Repelling
This term refers to materials such as porous plastics that have no affinity to wetting by water. These substances may have pores and channels but have no surface ability to wick water into the pores by the action of capillarity. Such materials are generally made from long chain organic molecules which have no substantial surface charges. Without a charged surface, there is no attractive or bonding effects on the polar molecules of water, and such substances must be specially treated.

Hydrophilic Material - Water Accepting
This term refers to those materials, such as porous ceramics and metals, that have a natural ability to fill pores by capillary action. This means the pores and channels of a ceramic have a highly charged pore surface that attracts and bonds the polar molecules of water and other polar fluids. The net effect is called "wicking" -- the ability to pull fluids into the material and transport that fluid by capillary forces. Soilmoisture ceramics are all hydrophilic.

Pore Size

The pore size of a porous ceramic is of critical importance because it directly affects the ceramic's air entry value or "bubbling pressure" and hydraulic conductivity. The air entry value is the pressure at which air will break through a wetted pore channel. The hydraulic (liquid) conductivity of a porous ceramic is a measure of the rate at which a ceramic material of known thickness may conduct liquid from one surface to an opposing surface under a known pressure. The hydraulic conductivity will vary with the type of pore fluid used which is generally water, but can be oils or other natural and artificial liquids. The effective pore size is determined by the minimum orifice within a channel or pore. These properties that are determined by pore size are intrinsic to ceramics and to all other porous materials. How a porous material behaves in any application is directly related to these pore properties and the material from which it is made. Soilmoisture assures consistent pore sizing by testing each batch and many individual porous products. See Table #1 that describes these properties.

Glazing

All glazing used in conjunction with the fabrication of standard or special products is of highest quality and contains no lead or heavy metals that could leach into solution samples. Glazes come in several firing temperatures and can be applied in one or two step processes to meet the most demanding requirements. Glazing can also be applied to one or more surfaces to create the wicking or capillary pathways of desired width and direction.

Helpful Hints
Acid Washing a Ceramic? - It's generally not needed

There are some who believe that all ceramics need to be washed with a strong acid to "clean" pores of possible ionic exchange sites prior to use in liquid sampling. This misconception is based upon using the older ceramic mix formulas (1960s) of moderately fired ceramics such as our B01M1 which has a talc component with potential ion exchange sites. In those early cases, some ionic exchange did occur during the first sampling periods as weak mineralogical lattice sites exchanged larger ions for more highly charged smaller ions. In reality, however, after several weeks in situ the transfer of ions was completed and the saturated lattice sites within the ceramic were at equilibrium with local soil waters. Acid washing cleaned most of the susceptible lattice sites in pore areas of these moderately fired ceramics leaving new holes within the ceramic lattice. As before, the first samples lacked uniformity with pore water content as the highly charged ions found these preferential "cleaned" mineralogical sites within the ceramic. After several weeks of installation and use, the cleaned sampler ionic sites were saturated and at equilibrium with the surrounding soil waters.

For solution sampling we recommend only the use of high fired, very stable and clean porous ceramics like the B0.5M2, B01M3, B02M2 that provide not only the hydraulic properties and air entry values that far exceed older formulas, but also possess the levels of cleanliness and stability designed for liquid content sampling. Even these extremely clean ceramics may, upon first use, attract some cations that have an affinity for the generally negative surfaces within the ceramic. All naturally hydrophilic materials will have these tendencies to affect initial ionic content in some slight manner no matter how pure and inert the hydrophilic material. The rule of thumb is "pore structure and pore liquids reach an equilibrium state after 5 to 8 pore volumes have traveled the pore spaces". This is the reason why the first several samples should be ignored as the samples will probably not precisely represent the actual pore fluids.

If you must wash

If you must consider solution sampling using our moderately fired materials such as B01M1, B03M1, B05M1, B15M1 (not recommended for solution sampling), we would suggest a simple acid wash as described below:

Prepare a 1 Normal solution Hydrochloric Acid (HCl) and pass 30 pore volumes of the acid wash through the ceramic by gravity or slight pressure. Rinse with 20 pore volumes of deionized or distilled water.

Again, as pointed out earlier, the washing process will result in a ceramic that has a fair amount of empty lattice sites waiting to snag the unwary ionic traveler. Once in situ, these sites are filled and a state of equilibrium will exist between soil solution contents and pore solution contents.