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Author: Subject: More on PbO2 electrodes
MadHatter
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[*] posted on 18-9-2004 at 13:37
Specs


Cell volume is just under 6L by the measuring standards you're using.
At 15 amps the current density on a 3/8 inch diameter gouging rod is
1.07 amps per square centimeter for a 12 inch rod assuming that all of
the rod(I WISH !) was submerged into the brine. I can't be 100%
sure on this because that's the initial current until the rod starts to
deteriorate and of course the current rises as the rod gets smaller.

That's for the anode. I use a gouging rod for the cathode also. It
doesn't deteriorate over time so I guess that I have to say it's static.

Dichromates are often used to protect the cathode if it contains iron -
not a problem for carbon. I still like to use NaF to increase the efficiency
of the cell and here's an article that may be of interest:


United States Patent No. 3,493,478.


Handady V.K. Udupa, Srinivasa Sampath, Kapisthalam C.


Feb. 3, 1970

------------------------------------------------------------------------


Electrolythic preparation of perchlorates

------------------------------------------------------------------------


Abstract of the disclosure

Sodium perchlorate is prepared electrolytically from sodium chloride
solutions in one step by passing a direct current through the solution ,
in the presence of sodium fluoride. A mild steel cathode and a lead
dioxide anode are satisfactory.

This invention relates to the electroltic preparation of perchlorates
directly from sodium chloride using lead dioxide anode.
Hitherto it has been the practice to employ a two stage process for the
production of perchlorates from chlorides , the first stage being the
oxidation of chloride to chlorate using graphite, magnetite or lead
dioxide anodes and the second stage consisting of the oxidation of
chlorate to perchlorate, using platinum or lead dioxide anodes. This
conventional process has various drawbacks, such as:
(i) that in between the two stages the solutions have to be processed to
isolate the chlorate and recover the unconverted chloride:
(ii) that graphite gets desintegrated to a considerable extent and
magnetite to a lesser extent when used in the production of chlorates and
(iii) that there is an inevitable loss of platinum due to corrosion in
the perchlorate cell.
With a view to avoiding the intermediate step of the processing of the
liquor after the said first stage of the conventional process, it has
been proposed that the effluent from the chlorate cell should be treated
under different electrolytic conditions from thoses in which the sodium
chloride solution was treated in the chlorate cell during the first
stage of the process. But this proposed process also is a two stage
process and the processing of the effluent from the chlorate cell
introduces complications in the method.
This invention has for its main object an improved method which will
obviate the drawbacks of known methods, and whereby the perchlorates may
be prepared directly from sodium chloride in a single step of
electrolytic oxidation, the conditions of electrolysis being maintained
constant at predetermined levels throughout the period. Another object
of this invention is to avoid the use of graphite, magnetite or platinum
as the anode. With these and other objects in view , this invention
broadly consists of a process of preparing sodium perchlorate directly
from sodium chloride in a single step of electrolytic oxidation, which
consists in passing a direct currint through a bath of sodium chloride
solution containing fluoride ions.

The improved process according to this invention may be carried out into
practice under the following conditions:
(i) The said fluoride ions may be provided by adding sodium fluoride ou
hydrogen fluoride to the bath.
(ii) About 0.5 to 5 grams per liter of sodium fluoride may be added; but
about 2g/L is ok.
(iii) The electrolytic oxidation may be carried out in a cell having a
stainless steel cathode and a lead dioxide anode.Br> (iv) The said anode
may consist of a massive lead dioxide or a GSLD.
(v) The current density may be in range 5 to 40 amps per square decimeter.
(vi) The electrolyte oxidation may be carried out at a temperature of 30
to 60 C.
(vii) The process may be carried until all the chloride is converted
into perchlorate, the conditions being maintained constant at
predetermined levels throughout the process.
The following Table 1. gives the particulars of three different examples
illustrating the invention.


TABLE I

EXAMPLE 1 2 3
Concentration of electrolite (initial) g/L NaCl 290.8 291 308.1
Sodium fluoride g/L 2 2 2
Anode GSDL GSDL GSDL
Cathode SS SS SS
Current density (amps /dm^2) 10 25 19
Current concentration (amp/L) 18.75 25 10
Temperature 40 40 45
Cell voltage ( volts) 3.0 to 3.9 3.7 to 4.5 3.9 to 4.5
Current efficiency % 52.2 53.2 57.5
Energy consumption (kwh/kg of NaClO4) 13.3 13.9 13.2
Current passed Amps 15 20 800
Duration (hrs) 105 78 223
Quantity of NaClO4 present in solution (gms) 544 546 61KG

___________________________________________________________________________________________
Table II shows other benificeal effects of using NaF.


Table II

Experimental conditions Without NaF With NaF
Anode GSDL GSDL
Cathode SS SS
Anode current density (amps / dm squared) 20 20
Current concentration (amp/L) 25 25
Temperature C. 3.5 TO 3.8 3.5 TO 4.0
Current efficienct % (with ref. to NaClO4 formed) 39.5 53.4
Energy consumption (kwh/kg of NaCl04) 16.2 12.3

Snip of most of the text repeating the claims above except:
Claim (3): Current density is in the range 5 to 40 Amps per dm^2.
Claim (4): Temperature is in the range 30 to 60 C.

Other patents sited:
2,813,825 11/1957 Miller et al
2,840,519 6/1958 Stern et al
2,872,405 2/1959 Miller et al
3,020,124 2/1962 Bravo et al

Now you know why I LOVE using NaF in my perchlorate cells ! It seems
that potassium persulphate can be used with a similar effect.


I realize that this is for NaClO4 but getting the less soluble KClO4 is
easy with KCl given the fact that KClO4 will precipitate out of solution
very easily.

Solubility of KClO4: .75 gr @ 0 C, 21.8 gr @ 100 C according to CRC
52nd Edition(1971-1972).

Sorry for the long post but I hope this helps you in some way.




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[*] posted on 22-9-2004 at 18:10
PbO2 electrodes


do it the usual way shone on web pages
use cheap welding rods
they last from one to more days
save the broken pieces of PbO2
when you get enough put them in a fiber type window screen in the shape of a tube (not metal) put a new annode in top down in the pieces for electrical contact
put this in a chlorate solution but don't let the new annode touch the solution just the pieces
takes some maintanence but worked for me
went through one pack of rods but made about 100 pounds of NaClO4
lotta trouble just depends on how bad you want it
takes time
solution would get thick as mud from failed annodes but it all settles out to perfecty clear solution
keep adding NaClO3 untill crystals start to form
let cool
O4 crystals are thin flat and look like an enlongated diamond
they come out first
cleaning the electrode and putting it back in the plating solution long enough makes a big solid monster of an electrde
get rid of ALL ClO3 with HCl OUTSIDE away from civilization
don't even think you have got it all until you use indigo test to make sure
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[*] posted on 23-9-2004 at 09:39


OK, going to update this as I go... to collect my thoughts in one place if nothing else...

I've prepared a plating tank equipped with a stirrer (submerged aquarium water pump), an alcohol thermometer and a dimmer controlled 300W immersion heater.

I've filled the tank with 400g of PbAc2 dissolved in 2.5 liters of 6% HAc. This equals a concentration of 0.5mol PbAc2 / litre HAc.

I'm waiting for the electrolyte to reach 55 degrees C, then I'm going to try plating a piece of plastic I've previously coated with solid PbO2 powder using PVC glue (THF).

The plastic is a floppy bay cover from an ATX tower I had in a box --- I haven't measured its dimensions yet.

Should be possible to keep a fairly constant temperature once I've determined the proper dimmer setting...

I'm going to use 5V from an ATX PSU, with a lead cathode as suggested.

50 degrees, lots of bubbles in the liquid... have to wait until it clears up before mounting the substrate...

EDIT1: Here we go, started plating... I decided to forgo making a lead cathode and go with stainless steel instead. The cathode has about twice as large area as the substrate. I've placed them 80mm apart and the temperature of the plating tank is 40C and rising... this is exciting...


[Edited on 2004-9-23 by axehandle]




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[*] posted on 23-9-2004 at 17:36
KClO4


gilbert pinkston, I've already made plenty of potassium perchlorate without
problems. As for indigo carmine, I have enough of that for a long time. Found
50 grams on eBay for $25. Not bad considering the higher prices I've seen
elsewhere on the net. My last batch of KClO4 was about 8 LBS. Very nice,
flat rhombic crystals in contrast to the KClO3's cactus needles which are
produced 1st during the electrolysis. BTW, I should have mentioned in 1 of
the earlier posts that the indigo carmine test can detect chlorates down
to 1 ppm. The test is sensitive enough to ensure that there shouldn't be
any problems when making NH4ClO4.




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[*] posted on 24-9-2004 at 00:59


Hmm, half of the electrode plated alright. I'll try making another with a more even (and thicker) layer of glued-on PbO2.

Looking good...

EDIT: Closer inspection revealed that the plating was OK in spots, mostly near the connection --- seems the main obstacle is getting the initial PbO2 layer conductive enough to carry the plating current. Next I'll try mixing graphite + PbO2 with PVC glue and smear that on the next substrate... but now I'm going away for the weekend so it will have to wait. I'm very optimistic though.

[Edited on 2004-9-24 by axehandle]




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[*] posted on 1-10-2004 at 11:07
Some developments...


Thought I should mention something of what I'm trying right now... I suppose many have read US patent 4008144 with the ceramic substrate PbO2 anode manufacturing process.

I decided to give it a go.

I made 4 substrates of clay, 3 of them cylindrical rods approximately 15mm x 200mm in size, the 4th a rectangular plate approximately 5mm x 150mm x 40mm. These I fired in my electrical kiln last night
(burning out the heating coil after 4 hours BTW --- which is horrible considering I have to wind a new one (gives you blisters and is boring) -- not important though but worth complaining about).

I've only begun processing 2 of the substrates; the plate and one of the cylinders; but here are my observations:

The substrates were soaked in a 52 degrees C, 0.5 mol/dm3 aqueous solution of Pb(Ac)2 (I dumped them in my plating bath, don't want to poison more glassware than necessary) for 1 hour.

Then they were dried in a 120 degrees C oven for approx. 1.5 hours.

Then I dumped them is a lukewarm saturated (at least I think so, there is undissolved stuff in the bottom of the jar) solution of Ca(OCl)2. This solution is supposed to be basified with a hydroxide for this to work but I reckoned the hypochlorite would be contaminated enough with Ca(OH)2 for me to avoid having to use OTS NaOH (contains sodium stearate and shit).

The substrates immediately turned light brown. I let them sit in the oxidizing solution for 2 hours, then I dumped them in warm tap water to wash them.

This is as far as I've got. I intend to let them sit in the washing bath for a total of 2 hours, them I'm going to dry them at 100 degrees C and repeat the process, as many times as necessary to get a good conductive layer of PbO2 for the electroplating to work -- probably nicely indicated by the "brownness" of the substrates.

More info coming as things happen...




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[*] posted on 2-10-2004 at 03:59


The process seems to works admirably -- I've finished 2 passes with all 4 substrates, and they get much "browner" with each pass. Stupid of me not to leave one of them unprocessed for color comparison though.

The question of how to make the plating connection arises. The obvious way would be a few turns of Ag wire around the end, if I had any Ag wire lying around. Blah. I suppose copper will have to do.

2 more passes on 2 of the substrates, 4 more on the other 2, then I'll try plating and record my observations.

Anyone knows whether a cheapo multimeter will allow current to pass through even when it's turned off? (I want to measure the plating current during the entire operation; simply including the multimeter in the circuit would be easiest.)




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[*] posted on 2-10-2004 at 11:29


Ooo-kay, here goes..... I've submerged the better part of one of the rods in the plating solution, which is heating up now (I'm gonna plate at 50 degrees C)... I'm using 3 SS cathodes surrounding the rod as plating cathodes.

The connection to the substrate is simply a strip of Al foil wound around the top...

Plating current is currently 85 microamps (at 5V) and rising... looking good.

EDIT: couple of minutes later: Has jumped to 106 microamps... should increase over time as the voids in the ceramic are filled with deposited PbO2.

EDIT2: I don't know how much time has passed --- 2 hours? Plating current was 450 microamps, I've turned the substrate upside down now, the part that was previously submerged should conduct much better now and indeed it DOES! 1.39mA and rising! :)
Painful having to plate half the electrode at a time, but if it works this anode should last forever..


[Edited on 2004-10-2 by axehandle]




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[*] posted on 3-10-2004 at 07:14


No, no, no. This is much too slow, I don't fancy plating for a week.

I'll get a much more porous clay tomorrow and start over. The clay I used was probably too fine-grained.

In the meanwhile, I'm going to try a plastic substrate again. Right now dissolving PMMA in acetone, gonna mix with PbO2 and cover a strip of PMMA. Observations coming.

[Edited on 2004-10-3 by axehandle]




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[*] posted on 3-10-2004 at 13:50


Aaah, yes yes yes! This works much better! Plating current is 0.1A.

What I did was make a paste comprised of plexiglass drill shavings dissolved in acetone (dissolved in about 30 minutes with intermittent stirring) mixed with powdered PbO2. Don't have any exact recipe for the paste, it's probably not critical...

The paste I smeared on one side of a piece of plexiglass, then I pressed the plate upside down in more powdered PbO2, then I forcibly evaporated the acetone for 2 hours in a 75 degrees C oven.

The conductivity of the pasted layer seems very good, my multimeter gave me something like 1k as compared with 4 orders of magnitude higher for the ceramic substrate I tried plating yesterday...

Anyways, there are some pictures in http://species8472.dyndns.org/pbo2/pics/PMMA/
in case anyone wants to take a sneak peek...




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[*] posted on 3-10-2004 at 14:05
PbO2


Mixing some PbO2 in the paste probably helped quite a bit ! It should
ensure longer conductivity even if the coating starts to deteriorate. My last
PbO2 rod consisted of layers of PbO2 held onto PVC by plumbers
cement. The rod did deteriorate, but not nearly as fast as the
gouging rods. Probably due to the plumbers cement. On the bright
side, I was able to recover most of the PbO2 that had fallen off.




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[*] posted on 3-10-2004 at 14:17


Yes! Yes! Hooray! Victory at last!! I just checked the half of the substrate I plated at 0.1A for an hour, and it's covered with shiny blackish electroplated PbO2!!! I'm now plating the other half... Once that's done it should only be a matter of deciding how thick a layer of PbO2 I should aim at... :)



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[*] posted on 3-10-2004 at 15:48
Behold, and be impressed!


Right now thickening the layer even more by additional electroplating...

With a couple of strips of Al foil wound around both end of the anode, my multimeter gave me... tadaaa: Negligible resistance! Even the "connectivity beep" test beeped.

Ooooh, can't wait until I get a big bucket and a few kg of NaCl tomorrow --- perchlorate cell, here I come!

Oh, and BTW the substrate dimensions are 40x250x8mm.

PMMA_anode1.jpg - 29kB




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[*] posted on 3-10-2004 at 17:41
Anodes


ROCK N' ROLL ! With all of the hard work you've put into your anode,
you're very deserving of a HUGE quantity of perchlorate ! I wish you
great success ! Be sure to let us know how it works out.

When you're electrolysing, chlorates will be the predominant compound
at least in the 1st few days. Easy to spot - nice thin cactus
needle-like crystals will form. Make sure you add enough water to
dissolve them. What you're looking for is the rhombic crystals of
perchlorate.

Tip: Keep your brine to no more than 10% chloride by weight to
start out with. According to Wouter, perchlorate production
doesn't begin until the chloride content is 10% or less by weight.
From personal experience, IMHO, Wouter hit the nail on the head !

BTW, your picture is impressive ! Check out your U2U later on.

[Edited on 4-10-2004 by MadHatter]

[Edited on 4-10-2004 by MadHatter]




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[*] posted on 4-10-2004 at 17:31
A couple of observations


that I've made while electroplating PbO2 on top of "glued-on" PbO2 are:

  • It can be so slow that you'd think is doesn't work. Raising the voltage helped for me, going from 5V to 12V *tripled* the current.
  • Maintaining the connection from the power supply to the substrate is tricky as it's near impossible to keep fumes from the heated fluid in the plating tank away from it -- I noted that the Al foil I used "stuck" to the PbO2 layer on the substrate, leaving a residue that scared me but which proved easily removed by the HAc in the plating bath. Making sure the "connection spot" on the substrate is completely dry prior to wrapping it in foil and wrapping it tightly to avoid water vapor coming between the foil and the PbO2 layer helps as well. Using another metal as the connector would probably be better -- Au, Ag or Pt perhaps...
  • Even when plating at 12V, the operation WILL take an enormous amount of time: Getting the topmost 50mm of the substrate satisfactory thickly plated to serve as the connection for plating the rest took me upward of 6 hours! I suspect plating the rest will take even longer.
  • Keeping the plating bath heated makes a BIG (non-linear) difference with regard to the current. I've kept the temperature at between 50 and 55 degrees C, higher would probably be better but I don't want to boil the stirring pump that's submerged in the tank.
  • For the next anode (I'm going to make a bunch to have as spares, also planning to give away a couple) I'll definitely add graphite to the acetone/plexiglass glue. The conductivity is everything!
  • Electroplated PbO2 looks very different from glued-on PbO2. The former looks like shiny black metal, the latter like a dull, non-shiny brown. However, they look just the same when wet! This means that to see if the plating has actually worked, the anode has to be washed and dried.



[Edited on 2004-10-5 by axehandle]




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[*] posted on 5-10-2004 at 09:21


is you newly plated PbO2 hard and adhering or is it more like powder that has been clad into place?

good work!

/rickard
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[*] posted on 5-10-2004 at 09:44


Quote:

is you newly plated PbO2 hard and adhering or is it more like powder that has been clad into place?

It's hard and glossy and seems to adhere very well. It looks a bit like shiny black rock. Lightly scraping one of my multimeter probes along it felt like scraping unglazed porcelain and made a similar sound. I'm not going to risk smashing the very tediously obtained layer performing any tensile strength tests though...
Quote:

good work!

Thanks.

[Edited on 2004-10-5 by axehandle]




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[*] posted on 5-10-2004 at 13:14


no, we dont want you to risk anything :D

I was thinking about wether it could be considered a "robust" electrode that would survive many hours in a plating bath.

but it sure looks like it. A much more simple solution to the PbO2 electrode compared to
what otherwise is seen on the ┬┤net.

I wouldnt worry to much about the Al. Some PbAc2 could splash onto it and therefore some Pb deposit, but any Al in solution will of course stay there. Just watch out with the pH so that Al(OH)3 doesnt preciptate.
I dont remember exact pH but usually around 6-8 if I remember it correctly.
Al(OH)3 preciptate will flocculate and probably mess upp everything.

/rickard

[Edited on 5-10-2004 by rikkitikkitavi]
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[*] posted on 6-10-2004 at 16:55
Doing final plating sequence!


I've obtained a good looking shiny PbO2 layer over the entire anode. I've done it by repeatedly plating 1/2 the anode at a time with the connection made from wrapping Al foil around half the anode's length, tightly wound tight with cotton string.

Plate, wash, dry, turn, wrap foil+string, plate, repeat (ad nauseam).

I must admit I was a bit nervous about the conductivity up until now. Not anymore.

For the final round I've wound many turns of ordinary stripped copper wiring around half of the anode, with 40mm of the other end submerged in the plating solution. Those 40mm will be the "connection area" when the anode is used in the perchlorate cell. Once I deem the layer in that area thick enough, I'll plate the other 210mm of the anode. That part will be submerged in the brine in the perchlorate cell.

The plating current never got higher than 100mA average during the "aluminum foil" steps. It's up at 2.2A with the copper wire! And rising! (Started with a cold tank.) Yes, precious, yes!




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[*] posted on 6-10-2004 at 17:12
ARRGGHHHHH!!!!!


Back to square 1.... damned thing caught fire.

Had to put it out with water. Fizzled like black powder. I suppose that's what I deserved for plating a combustible substrate (plexiglass) with an oxidizer (PbO2)...

I must admit I had not foreseen this possible scenario.

My theory is that the PbO2 coating heated up in some extra-thin spot and reached heat of initiation with the substrate. Fuck. Well, better now than in a finished perchlorate cell, I guess.

PbO2fire.jpg - 28kB




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[*] posted on 7-10-2004 at 01:03


That sucks :( But I agree - better it catch fire now than later :o

Great work nonetheless :D




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[*] posted on 7-10-2004 at 13:41
Plastic


What about teflon ? Would this be less reactive than plexiglass ?
I've also been thinking about ordinary glass rods but I'm concerned
about the plating staying on. On Wouter's page there was also a
DSA anode using TiO2 and RuO2 on a Ti substrate. Like, Wouter,
I'd be interested in knowing if anybody has any experience with these.
Otherwise, it appears I'm stuck with gouging rods and PbO2.




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[*] posted on 7-10-2004 at 14:20


You'd have to melt the teflon to get it to retain the PbO2 and it would be hard to stick it to a solid object, but it would be interesting to try. Axe, how fast does material deposit on the electrodes in the plating bath? I'm wondering if it would be possible to build up a thick enough layer to simply remove it from a sheet of teflon as a self-supporting plate.
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[*] posted on 7-10-2004 at 15:08


Teflon might do the trick, but how would I make the PbO2 stick?

Quote:

Axe, how fast does material deposit on the electrodes in the plating bath? I'm wondering if it would be possible to build up a thick enough layer to simply remove it from a sheet of teflon as a self-supporting plate.

The speed of deposition depends on the current density when plating. Anything from hours to weeks depending on the power supply's voltage, it's power, the distance between the electrodes, the anode surface area, the temperature of the plating bath, the concentration of the Pb(Ac)2, the phase of the moon, etcetera :).

I like the removable substrate idea. Then again, PbO2 is very brittle. I think I'll go with a ceramic substrate for my next try. If the PMMA+PbO2 ignited from a measly 2.2A, there's no telling how many seconds it would last in a perchlorate cell running at 30A...

Now I only need to come up with a type of ceramic that's porous enough to allow a decent impregnation with PbO2 prior to plating. I'm not a potter. Ideas? A bench grinder grinding wheel would be excellent, but the shape is very wrong.

A plastic substrate anode has one advantage over a ceramic one: No corrosion at the connection from brine creeping up (which it undoubtedly will with a ceramic substrate). Though this, in theory, can be worked around by using silver as the connection metal in the finished perchlorate cell (corrosion products are conductive)...

One good thing came out of this: It opened my eyes to the fact that the PbO2 layer must be thick enough to avoid I2R heating. Wouldn't want the PbO2 to decompose from being too hot.

EDIT: I doubt it would be easy to obtain a DSA. I've previously looked for those but haven't found any... perhaps I looked in the wrong places. A DSA made from Ir or Pt on Ti would be sexy.


[Edited on 2004-10-7 by axehandle]




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[*] posted on 7-10-2004 at 19:46


Terracotta is a nice porous ceramic. Maybe pieces of terracotta plant pot might work.
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