Several of our past updates have reported the continuing developments in the use of silver nitrate in saturation of ceramic filter media.  Unlike the Indian method, reduction of silver nitrate, prior to firing, our past purifiers have been without this prior reduction.  Four candle purifiers of this type, made without prior reduction, have indicated that silver nitrate candles are 100% effective.  At the same time four non silver candles had been around 99.5% effective. 

Following is the newly found reference, silver nitrate with prior reduction. The work is by the authors, S. R. Deb and R. V. Lele, A Note on the Manufacture of Self-Sterilizing Water Filter,  Central Glass & Ceramic Research Institute Bulletin, Calcutta, India.  Volume 19, 1972.

The authors go on to describe how it is necessary to fire to 600^oC, keeping the kiln atmosphere in reduction so as to insure the silver remains metallic and bonded to the ceramic.  The difference between our own prior silver nitrate work and that of the authors is that we did not reduce the silver salts prior to firing.  Now it is our intention to do our best in replicating this AgNO3 process, in hopes this alternative may prove to be of interest.  Economic considerations are important, no doubt.  For example, silver metal is said to be about ten times more effective in disinfection than is the case with silver oxide.  Still, in no case should the cost of silver per purifier  exceed US$0.20  (20 cents).  But to minimize the price for the poor the silver should cost no more than 2 cents.

Does the manufacturer of the purifier need to have a kiln?  In reality all silver ceramic purifiers have been fired, however, for bricks, as a raw material, these were fired at the brick factory.  Subsequently the bricks are crushed to particles at a mill, possibly an agricultural hammer mill.  But other than this pre-processing there is no need for a kiln at the factory.

Note that regardless of which different form of silver is used, there needs to be a common practice with respect to measuring the amount of silver per purifier/ filter.  Some of the filter producers, in other projects and countries, have followed the practice of referring to parts per million of CS in solution, then indicating the amount of this solution used.  While this method does allow replication of results by other producers it does not readily indicate a total amount of silver per purifier/ filter.  So in this listing of different silver processes we now refer to the total weight of silver per candle.  Check the calculation for silver weight, where using silver nitrate, as follows.

A.  Silver nitrate, with reduction prior to firing
The process is as indicated in the above, Indian reference.  In this form of silver ceramic treatment the silver nitrate is reduced with ammonia and acetaldehyde, the non metallic elements removed.  Then the purifiers are fired in a to 600C, the silver metal bonding to the ceramic.  This requires slightly reduced oxygen in the kiln.

B.  Silver nitrate, without reduction prior to firing
Recent trials with silver nitrate, where there is no prior reduction, have followed two, slightly different approaches.  The first is to add the silver nitrate to that water that is mixed with the clay composition, prior to forming.  The second approach is to apply the silver nitrate solution after the firing, then dry and re-fire.  Both of these approaches have worked, though the second necessitates an additional firing.  Following is a description of the second approach:
1.  For the large sized candles, 10.5 cms. dia. by 12.0 cms. high, each one requires approximately 300 mls. of solution for thorough saturation. In preparing to saturate four candles we use, 4 x 300 mls. = 1.2 liters of water.

Shown right is the first step in AgNO3 saturation of a candle.  With the help of a squeeze bottle the solution is being pushed into the inside of the candle. This gives 250 mgs. of silver per purifier. 

Problem:  What weight of silver nitrate salt must be prepared in order to saturate each of four large candles with 250 mgs. of silver metal?  Thus, 1.0 gram of silver is required.
Answer:  Where 62 = the nitrate portion of the mol, 1000 mgs. silver is given by the cross multiplication, 1000 / 108  =  x / 62.  Therefore, the nitrate portion, x = 574.  So, to get 250 mgs. of silver per candle, 1000 + 574 = 1574 mgs. of silver nitrate salt.  Stepwise, add 1.57 grs. silver salt to 1.2 liters of tap water.

The candle is here immersed into the silver nitrate solution, which is a milky white color.  Silver nitrate is highly sensitive to light, suggesting saturation in subdued light.

Alternatively it is possible to saturate the purifiers without immersing these, rather by brushing on the solution.  As the picture below shows the candle can be placed into a shallow bowl, then the silver solution is painted on.  In the beginning stages of this kind of saturation the porous structure quickly sucks in the solution.  Once saturation is complete it is evident that the candle will accept no additional solution.

3.  It is necessary to dry the candles, following saturation.  The is best accomplished by using a fan, thus requiring only a couple of days.  Otherwise, air drying without a fan can take about a week.
 

For any process in which a purifier is CS saturated the method of applying by brush can be used.  The solution is brushed on until the candle will accept no additional amount.

C.  How-to Salt, Silver Chloride Candles

In the method that results with a dispersion of silver chloride particles, within the purifier media, the purifiers are first saturated with silver nitrate.  The method of silver nitrate application is the same as that of B., above, however there is no subsequent firing.  Instead, after the candles are dried they are set within the containers of the water  purifier system.  Then salt water is run through the candles.  So,  AgNO3 + NaCl -->  AgCl + NO3 + Na 
As a result of the reaction the nitrate and sodium are flushed off, leaving silver chloride as the candle disinfectant.

D.  Concentrated Colloidal Silver

The concentrated CS generator shown on the home page is currently utilizing two flasks, of 2.4 liters each.  However additional flasks can be wired into the system, upto eight in all.  Given that each flask contains about 500 ppm of CS, this will saturate sixteen candles.  So, 8 flasks x 16 candles per flask =  128 candles, the daily production of concentrated CS.

Several people have asked whether the flask size can be increased, so as to get bigger volumes of CS, and in my view this may be tricky.  For example, if the flask size is doubled then the wet surface area of electrodes must be increased, quite clearly.  And the DC voltage would also need to be increased.  But at what DC voltage would the system require safety apparatus?  While we feel confident that the 24 volts DC for the generator is perfectly safe, at additional voltage would tend toward an unsafe system.  Would a 4.8 liter flask require double the voltage, something like 48 volts DC?  And would this require the enclosure of the leads to the circuit, so as to avoid a shock to the operator?

Post Script of 23 April:  If the producers are purchasing the concentrated colloidal silver, for saturation of their filters, then this should be carefully sourced.  It may be considered best to know the form of the silver, resulting within the filter, in case further studies on these are at some point considered desireable.  In general, however, silver is considered to be among the most benign of substances, no harmful health effects associated.

In Conclusion
The methods for the purifier treatment as per the alternatives listed are being resolved.  Current production is utilizing the concentrated colloidal silver.  And on an experimental basis, in the lab, several methods with silver nitrate and silver chloride have proven 100% removal of e coli.  Use of the Indian technique with silver nitrate and prior reduction is also about to go ahead.  For the silver alternatives our aim is to find those approaches that are cost effective, bearing in mind the availability of local resources.