26 April 2004
What grog is best - for good porosity & strength?
Considering the high content of grog, a pre-fired powder, the pottery purifier candles are highly processed ceramics.  The production is not difficult, however, it is given to a lot of detail.  details follow
Processing Materials for Solid Purifiers- Candles or Disks.   see below
It should be understood that  the use of grog makes the pottery purifier a highly processed ceramic ware.  This is due to the need to prepare a lot grog in advance and fire this, then crushing to a powder and sizing.  This routine is followed because:  1.  the grog gains a high percentage porosity,  that is to say, water of absorption, by comparison with other kinds of pottery,  and,  2.  there is a separation by shrinkage of grog and clay particles, in the processes of drying and firing.  This brings about a network of voids, which facilitates a greatly improved rate of flow.
This silver grog, sized between 20 and 100 mesh, is now ready for insertion into a particulate canister. 
Go to the Workshop Needs link for details.
 
We now look at the use of crushed brick as a grog, alternative to the use of candle grog.  Bricks are one type of ceramic ware, a few others being:  tableware, tiles of all kinds, sanitaryware, etc.  But of all types of ceramic wares, in general, it is only brick that has a lot of porosity, in the range of 20 to 50%.  Other types of ware have a lot less porosity, less than 20%. 

For any brick used in production of pottery water purifiers a higher percentage porosity is best.  A good amount is 40% or more, while grog tending to 20% would be less acceptable.  The problem is that the lower percentages porosity, for both candles and particulate purifiers, can contribute less surface interior area for entrapment of bacteria, and there is less active surface area for disinfection by silver.  There is also less water retention within a purifier of low percentage porosity, indicating a lower rate of flow/ capacity. 

As to strength, improved with higher temperature, a leading reason for users to stop using a candle or system is because they break the ceramic.  Higher strength helps reduce the amount of breakage.  Also good strength may better preserve the silver ceramic bond.  For differing silver treatments, if purifiers were fired at very low temperature the ceramic and the silver may degrade more quickly.

Question:  Under what circumstances would it be important to make ones own grog, rather than simply using crushed brick?  Answer:  The quality of red brick available in the market can vary widely, some having inadequate porosity, as well as questionable strength.  And for good brick of any particular brand the amount of porosity and strength can also vary, brick to brick.  For this reason the experimenting ceramists/ potters may prefer to make their own grog.  Or they should work with two parallel systems, those made with candle grog and those made with brick grog.

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Why not make filters without using grog?

Some producers of silver ceramic filters will choose not to use grog at all, and that's okay.  These producers should still feel they could pursue any of the approaches to silver treatment that are presented here.  All such worthy producers of silver ceramic filters should be encouraged.  But at what cost to the efficiency of the filter systems would the producers decide not to use grog?  It seems that for non grog filters, in order to achieve appropriate flow rates it may be necessary to pursue big variations in differing parameters. 

For example doubling the water column height of the system may provide an adequate amount of filtered water. Or to achieve a very high percentage porosity the use of 60 to 70% sawdust to the composition may lead to a great increase in flow, upto the amount needed by a family.  But for such filters, with lots of big voids between particles, there may be big limitations as to which form of silver treatment is possible.  A key to adequate, virtual 100% removal of bacteria may be in increasing the active surface area of the ceramic medium.  This suggests, 
1. smaller ceramic particles, and, 2. smaller voids/ air spaces, of the type leading to intermediate size air spaces.

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Processing Materials for Solid Purifiers-  Candles or Disks

The following describes the methodology to be pursued for production of purifier grog.  Though there is a bit of new content, much of this is a reiteration of prior information on grog and candle compositions and processing.
1.  Procure grog for the systems, whether from candles or from brick.  Candle grog is that which has been produced for candle production, however it is also possible to use crushed, reject candles as grog.  The percentage of factory reject candles is typically about 10%, the crushed material quite appropriate for grog of additional candles or for particulate systems.
2.  A good rule of thumb is that porosity should be not less than 40%, but upto 60% or more is also quite

An inexpenisve spring scale may not give accurate indication of weight, better in determining percentage composition.  For the critical tests, however, more accurate scales are needed.

 acceptable.  Porosity is measured as water of absorption =  [(wet wt. - dry wt.)/ dry wt] x 100. 
To determine this a simple piece of the pottery, broken brick chips, or reject candle, should be weighed dry, then immersed in water.  For very porous pottery this piece will saturate all possible water within a short time, perhaps minutes.  By contrast a whole brick may require twenty four hours or more to saturate all possible water.  In fact for brick that is too dense this will not fully absorb the water.  The dry and wet pottery/ brick samples are weighed before and after saturation.
3.  While porosity can be measured, readily affording good determination of appropriate grog, the same kind of clear approach may not be worth pursuing for determination of strength.  Rather, the determination of fracture strength may be empirical, by observation.  It may not be worth the pursuit of the subjecting of brick to the tests for three-point-bend, and calculation.  of fracture strength.  Rather, observe that brick of good quality is that which keeps it's integrity as a whole, not readily breaking or crumbling.  However, brick for grog may be somewhat prone to scratching or abrasion than would be the case with other ceramic ware, tableware, floor tile, sanitaryware, etc.  If bricks can resist being subject to abrasion by sand paper or scratching with a metal implement then they are appropriate, no doubt.  But small amounts of abrasion 
may not disqualify a brick.  Good practice is to determine the situation empirically, with a small number of differing brands of brick.
4.  The selected brick or candle grog should be milled, as per the article at the Workshop Needs link,  'Here are the steps of making a particulate grog system.'
Concerning the Grog Composition
5.  For production of candle grog, as opposed to brick grog, it is first necessary to mix the composition and water, and we suggest the ratio 63% clay with 37% combustible,  i.e.  wheat flour, then 20% water.   The following indicates the order of mixing, to begin with in the ratios indicated:  63 parts clay/  20 parts water.  For example, for every 630 grs. of clay add 200 mls. of water.  This will form a sticky mass and should be placed in a plastic bag overnight, so as not to do any drying.
(N.B.  In early experimentation with the processing of purifier compositions a lot of hand mixing and screening will be necessary, and this will be a bit time consuming for aspiring producers.  Once the real production is to begin, however, the amount of manual work in mixing of materials will be greatly reduced.  What is needed in order to streamline this processing is the construction of several simple pieces of equipment.  A rotating screen device will aid greatly in sizing the damp composition particles, and a simple mill will help in mixing.) 
6.  The next morning the 83 parts of clay/ water mix should be combined with the 37 parts of flour.  So, add 370 grs. of wheat flour to every 830 grs. of clay/ water composition.  Not yet having the simple machinery indicated in the note of #5, this will involve a lot of hand mixing, individually breaking the lumps of flour/ clay/ water.  For the time being this is a laborious step, seeing to it the mix is homogeneous.
7.   Disks are next pressed, then dried, then stacked in a kiln for firing, as per three links, all to be found at the pasticons link .  These past  icons are for:  Forming Candles, Kilns and Material Processing.
Concerning the Candle Composition
8.  Following processing of fired grog we are ready to prepare the purifier composition, needed for the pressing of candles or disks.  Composition and methods above pertain to grog production, which is preliminary to this new task of preparing candle composition:  adding grog, clay and combustible, then water, readying this for the press.  As in the Forming Candles link, the article 'Step-by Step Pressing,' we suggest for a purifier of maximal flow rate, a starting composition should be:  1.  45% clay,  2.  40% grog,  and,  3.  15% charcoal powder  These three materials constitute the 100% of the dry mix, and all can be sieved at 30 mesh.  (In a future article, grog materials of 40 to 60 mesh may also prove impart valuable attributes to the purifiers).  Percentage water for the composition should be around 15%, taken with respect to the weight total of the dry composition.  Also to be found at the Forming Candles link is the article,  Observations on Water Content of the Clay Composition.'
9.  The mixing of the dry materials for the candle composition, as in #8 is a simple matter, with nothing resembling the labor intensiveness of the hand mixing of the grog composition.  However care should be taken to add in the water.  This should be done by periodically sprinkling small amounts of water into the dry mix.  Then hand mixing insures this low percentage of water is properly combined in, to the point of homogeneity.  Care should be taken that individual clumps of the composition are neither too dry nor too wet.  Following this step the candle composition is ready for pressing.
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Question:  How do the voids of particulate systems differ from the voids of solid, candle systems? 
Answer:  They vary in shape, since the permeable network that forms the more important voids of the candle, solid purifiers are an irregular, planar shape, of intermediate size.  The shapes of the voids of the particulate system have more to do with particle packing distribution, how the various sizes of material fit together. 

For the candle/ disk purifier medium the planar shaped voids provide an interconnectedness of the airs spaces, pores, voids, fissures, etc.  Particulate media also have good interconnectedness, according to the particle packing and size distribution  Consider along with these points of particles and voids the important question of water retention.  If the particles do not have a good 40%, what will happen to the flow?  the situation of silver?  the disinfection?  These may all be inhibited.
Voids of intermediate size help facilitate the flow of a pottery water purifier.  To begin with pottery is not an especially appropriate medium for water filter. Use of grog brings about intermediate sizes of air spaces thus making good flow rates possible.
Briefly speaking there seem to be several different sizes of voids/ air spaces implicit to several silver ceramic filters:  a.  The small pores of pottery wares in general-  3.0 to 5.0 microns  (as opposed to 1.0 micron in a kaolin white filter).  b.  The intermediate voids/ air spaces- of  the pottery purifier, candles or disks, starting at 10.0 microns thickness for adjacent grog and clay particles of the smaller sizes.  For separation of bigger particles, grog to clay, the distance is thought to be about 50 microns, and a small percentage of voids are given by the much larger particles that that result from the burnout of 60 mesh charcoal.  c. The large spaces-  for other kinds of filters, where the pores are contributed by 60 mesh combustible  (or a comparable sized combustible).  Sawdust is a popular combustible for use in some other types of pottery filters.  At 60 mesh these combustibles burn out to give voids of about 0.25 mm. diameter. 

It's a great problem that some other types of filters do not have intermediate sized voids/ air space, so these have only the small pores and the large spaces, and there is only poor interconnectedness of spaces, of the type required in giving a good rate of flow.  There is less flow for these filters than would be the case with the use of grog.  Still, there may be other, new ways of improving flow rates, so as to get smaller, highly efficient candles, both for adequate flow and effective bacteria removal.  Further experimentation, from whatever capable quarter should be encouraged and commended.

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Address questions or observations to:
Reid Harvey <pottery@wlink.com.np>

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