Wednesday, 30 January 2013

Post 70 Lime scale removal

I guess it is commonly called that, although I'm sure that's not what it is and there is no doubt that getting it off a pot is a challenging task.

I've bought a few second-hand pots at different times which have been afflicted with the problem but I have never had any scale develop like this on the bottom of any of my pots in service and that in itself may be instructive about what it is and how to avoid it. This is one of those unglazed second-hand pots.

I have however observed a very light deposit of scale around the upper rim of some of my pots - those flat surfaces exposed to the sun where water pools and evaporates. I usually clean this up in an annual cycle of spring cleaning with a vinegar and a soft cloth.
Apart from that I can think of two instances where I have observed heavier scale development. One is in bathrooms/laundries where water is allowed to collect and evaporate. The other is in any potted plant that has ever been grown with a saucer under it. I've seen a few of these in offices where folks are sparing with the watering so the saucer never overflows. So what's going on.

In the first case it is just the product of town water evaporation. All water has dissolved salts in it. Most domestic supplies have a very low level however as the TDS ( Total dissolved salts) level increases the water becomes hard. Regardless of the level as water evaporates whatever has been dissolved in it will be left as a solid salt. The most prevalent salt is Calcium Carbonate. CC is just sparingly soluble in water and this solubility increases in the presence on CO2, carbon dioxide. CC will react with dilute acids to produce salts which are soluble and so this is the best answer for the bathroom variety of deposit.

To digress for a moment to limestone caves - the ones that have stalactites and stalagmites. As rain falls through the air it dissolves/reacts with CO2 to form dilute carbonic acid ( just like soda water). With a pH in the low 6s (remember neutral is 7, acid <7 and base >7, on a 1 to 14 scale) this will dissolve CC from rocks as it moves through cracks and crevaces. When this salt rich solution reaches the cave the wet surface is exposed to air which evaporates the water and takes out the CO2 causing the CC to be redeposited. The structures sparkle because the CC forms small crystals. So there is a clue there about one dilute acid that may be useful in dissolving CC deposits - soda water. Suspending a pot in a rainwater tank may be a long term slow means of removal and certainly leaving it out in the weather and rain may be useful.

In process industries where closed cooling water systems are used, these commonly need to be cleaned of deposited salts which form on 'cold' surfaces. The acids of choice are Acetic Acid (vinegar) and Cirtic Acid (as in citrus juice). Both of these as available to the householder are safe dilute acids with pH in the 2.0 to 3.0 range. Citric acid is particularly effective in removing rust deposits and stains. Another acid which will dissolve CC is Hydrochloric Acid. That's the stuff brickies use to clean cement off bricks. You'll need to take care with this from a personal safety point of view as it's pH is about 1. The other one that gets raised is soft drink. Coke contains phosphoric acid - a food grade material that gives drinks and food that little zing. Phos acid is better at cleaning metals then reacting with CC.
All of these acids will be effective on deposited CC, but it is not highly reactive with dilute acid and is unlikely to be removed without some mechanical assistance or a long time frame.
As a cautionary note however, for glazed pots, anything with a pH below 3 has the potential to damage a glaze, subject to its composition. So be very cautious with glazed surfaces.

So how does all that apply to scale that can form on pots. Well it might as some part of such deposit will be CC. Unfortnalely is is likely that there will be other salts as well. The reason it can build up on a pot in a saucer and has not under my pots in service I believe is due to the fact that when I water I make sure the pots are sufficiently wetted to get a good flushing of the potting medium. This will be helped where the water has a pH less than 7. Rain water is just the thing to use if you can.

The plant reacts with the medium in the pot and salts are liberated as well as nutrient salts either included in the medium or added as fertilizer. The main nutrient elements are Nitrogen (N), Phosphrus (P), Potassium (K), and Sulphur (S), present as things like Ammonium Phosphate, Ammonium Sulphate, Urea, Potassium Chloride. Plant pots are reaction vessels and so you can get an interesting mix of salts in the water leaving the pot. A good flow will mean they are very dilute while small or no flow will build their concentration, increasing the chances of deposition on evaporation of the carrier water. Some of the things deposited as well as CC might be Calcium Sulphate (as in Plaster of Paris), or Calcium Phosphate or Potassium Carbonate.
The mixture of these deposited salts makes the deposit's removal more challenging because while Carbonates are reactive with acids, Sulphates are reactive with bases; things like caustic soda that you can buy as drain cleaner. Another readily available base is baking soda. A protective layer of sulphate for example may shield a layer of carbonate and vise versa from different chemical agents. Suffice to say one agent may not do the job and again a little mechanical disturbance of the deposit's surface will be useful too.

So the conclusion to all this is that if you want a quick result it is unlikely you'll get one from a single chemical agent, nor without some mechanical effort. Keeping some nice patina and getting rid of scale are going to be incompatible.

Another relevant scientific point is that medium to high fired ceramic will have a hardness of 6 or 6.5 on the Mohr scale. Mild steel will be around 5.5. What this says is that the point of a craft knife blade is not going to scratch the pot if you use it to abrade the surface of any scaly deposit. If it does then the pot isn't worth worrying about anyway.

So here is the 'after' picture. As you can see still a little more to do on it but sooooo much better. The program I followed was:
  • Soak in vinegar for 24 hrs.
  • Used a kitchen scourer (green plastic thing) to scrub the surface.
  • Used a knife blade to chip the surface and try to get under the edges of any small flakes.
  • Scoured again in dilute caustic soda.
  • More scratching with the blade.
  • Scoured in vineger.

The take away messages from all this are:

  • The best way to remove 'lime' scale is to not let it deposit in the first case.
  • Flush through the pots when watering.
  • Use pH <7 water to water plants. Rainwater is the best as it also has no dissolved salts and will not leave any deposits.
  • If your circumstances encourages scale development then treat early and often.
  • If you have to remove a heavy scale deposit quickly, then you can't do it gently.
  • One chemical agent is unlikely to do it all.
  • Vinegar and caustic soda will assist removal. Better not to go to lower pH agents.
  • Use only very weak acid solutions on glazed surfaces.
  • Don't be afraid to use a knife blade, steel wool, abrasive pads etc on unglazed surfaces - you are unlikely to get a result unless you do.
  • If all of that is too much then leave it in the weather exposed to rain or immerse in a rainwater tank. A long term weak vinegar bath may also be effective for unglazed pots  but unlikely to be the whole answer.

Sunday, 27 January 2013

Post 69 Rock Pot

In late December I made a freeform mock rock pot, inspired by and loosley styled after the sort of devil's causway basaltic crystalline formations you sometime find along an ocean headland waterfront.
This picture was taken as it was finished and still quite wet. It is actually arched and designed so that when finished it only makes bench contact at the left and right hand ends.


At this stage it is supported by a arched clay wedge in the middle.

This is a photoshop compilation of a potential final composition. I'm thinking about a glaze that is basically basalt black/grey in the crevices with lighter browner tones on the projections to reflect a weathered look and a number of trees in a group windswept style. I have a couple of matte glaze recipes that will achieve that result with different depth of application. I might even wind back the silica a little further and if this makes the surface more prone weathering as a result then all the better for the appearance in this case.

The pot is now dry and in the kiln with 5 other pots for a bisque firing at some time over the next few days.
We are enjoying some nice cyclonic weather here at the moment as a tropical low tracks south bringing much needed rain after an unusual and very prolonged summer dry. A good day to be indoors mixing glazes for a stack of pots sitting there waiting to be finished.

Wednesday, 23 January 2013

Post 68 And finally two more

Just to close out the pot discussion here are another couple from my collection, both currently in service but just tilted on their sides for the photos.

The detailed picture of the second pot shows a quite grogged - gritty - clay body. They are an attractive buff beige/brown colour and about 350 to360mm long. The workmanship is quite good and as you can see the pots are quite clean. Nothing more than a light wipe with a damp cloth for the photo. This says they are likely to be high fired and relatively impervious. I think they are Chinese.

 I've had them for many years and they've been constantly in use. These sort of pots, both in colour and shape, are good for a wide range of trees and styles. As I recall, I probably paid around $80 each for them at the time. Looking at them now they were a good buy and if you like a purely unglazed pot, albeit of unknown origin, they are a good choice.

I've just about got things back in order and ready to get back into the real world of making pots. Over ther last three weeks I've had a number of pots dry off and will have a bisque fire for 5 pots over the weekend.

Monday, 21 January 2013

Post 67 My Indian Adventure

I'm afraid blogging has taken a back seat for a couple of weeks. I've been away enjoying a little run around Rajasthan from Mumbai to Delhi. It was certainly a different sort of enjoyment from any other trip I've been on. Certainly more a case of 'isn't that amazing / surprising' rather than any pure enjoyment alternative. Coming home in one piece and healthy was an achivement in itself.


The best place to start is with a picture of a 10th C carving of Lord Ganesha, Hindu patron of the arts and sciences and lord of beginnings and obstacles - both placement and removal. He might be a good kiln god but I'm not sure about needing any new obstacles in my potting.

There were a few things of ceramic and bonsai interest on the trip and certainly much of decorative art. This post covers a visit to a small village pottery. Much of the more remote areas which are purely agricultural is a bit like the middle ages with mobile phones. People live and work close to the ground using their hands.

The potter we visited used a wheel which was a big flywheel that spun on a point pivot. He used a stick in a hole in the flywheel to get it spinning and then it operated like any other wheel, with the exception that because it used a point bearing it could take on a bit of an orbital action, resulting in the top of any work being turning in a tight circle rather than stable. This required a practiced skill to finish off the top of any vessel.

In this pipcture on the left you can see another wheel and pivot and the spinning stick. Indians work hard and long hours to survive. Our potter friend worked in this position for long periods and when he needed to reach something, he just sort of crabbed around on his haunches. My hips and knees screamed just watching him down there.

These spherical pots seemed to be the major product made. All very uniform in production and thin walled. The clay is earthenware and only fired to about 600C and no doubt had a half life of about 3 weeks. That's probably why there were so many potters in the business.

 The process started with the pots in the foreground being turned up on the wheel. Then the guy in the background gets to do his part. The white 'mould', which looks like a big egg shell is then used to support the pot while it is struck on the outside with a flat timber batter against a curved piece of stone held against the inner surface. Repeated hits and rotation turns out the spherical finished pot. Very clever. He could do about 20 a day but summer when the temperature gets close to 50 can't be good for productivity. I think they do it this way because the clay may not have the strength to turn with thin walls; labour is cheaper than almost anything.

Here are some recently fired pots. The kiln is a brick cylinder about 2m in diameter and 1.5 high. The dry pots are placed in the kiln and covered with a layer of broken pots, then fired with timber/coal. There is no stack or chimney so everyone close by gets to 'share' in the firing.

Brickmaking was another revelation and the subject of a post to come.

Saturday, 5 January 2013

Post 66 Another pot

Hers's another high fired pot from my collection. It's very like the Tokoname one, a buff unglazed rectangular pot about 350 long. Like the Tokoname a good durable surface that comes clean and as quite impermeable. Once again just a damp wipe for the photo.

As you can see from this picture is has a bit of an arched curvatur

e which is a little more pronounced, but not so much to be a problem aesthetically.

Here is the makers chop. I'm not sure of its origin or what I paid for it. I think it is older than the Tokoname so at least 15 years in service. It has an interesting symbol next to the chop - again any ideas would be gratefully received. I am aware of a Japanese maker who uses a chop with that framed design but I wouldn't be surprised if it was higher end Chinese.

This close up shows the surface. The clay also has quite a bit of granularity. The 'window' relief pattern on the pot wall is not a great piece of workmanship, much like many of the finer details all over the pot, but wholistically from a distance an attractive and useful design.

Here is a detail of the footwork, adequately executed.
All in all not quite the workmanship of the Tokoname. It seems that time was more important than getting the best finish. The arched body could have been managed better, and probably suffered for the same reason. But time was put into some decoration and an attractive foot design. It has been competently executed and ticks of the boxes of timeless design, long term keeper potential and high fired impermeable clay body. This is one that will not be going onto the next society exhibition sale.
Another and final thought for the day is about keeping pot records. Most enthusiasts keep progression photos of their trees which is great to see where you come from. I've done that for years but didn't keep pot records - whatever information available about their source and cost - and now wish I had.

Tuesday, 1 January 2013

Post 65 Tokoname Pot

Last post I said we'd look at my Tokoname pots. Well I pretty sure one is. I've owned it for perhaps 15 years and it has been in service all that time. That shows how long I've been struggling with the pot problem. This is the most expensive pot I've ever bought and looking back I think I must have scored a Christmas bonus or the like. Easy come easy go. Buy on emotion and defend on logic; I guess I may have also just been keen to be a Tokoname owner.

This pot is an unglazed buff coloured rectangular pot about 360mm long. As in the last post the only cleaning of the pot I've done for the photograph is a wipe with a damp cloth.

As you can see from this and the next picture the surface really is as good as new. You just wouldn't imagine it's been out in the weather for 15 years. As you would expect from Tokoname it is clearly a high fired very impervious clay body fired to maturity. This is the unglazed surface standard to aspire to and it can only be achieved in Stoneware.

Other evidence of the high firing to maturity is the very slight arch the pot has taken up. This as I well know can occur during the vitrification transformation as the material becomes plastic, shrinks and closes up the spaces between the particles, ensurng impermeability.

This once again reinforces the Earthenware / Stoneware dichotomy. As another clear indicator when buying pots  - those that are absolutely square and flat and smooth will be highly likely to be low fired or heavily grogged and not fully vitrified. Do not shun pots that show a little dimensional imperfection as this is an indicator of material characteristics that will give you the better long term performance. The skilled maker can do various things to minimse these artifacts of vitrification, as he has with this pot, but if you look carefully you will always find the evidence, if it is there to be found.
Here is a more detailed shot of the corner of the pot. There is quite a bit of granularity in the clay which gives the surface a little texture.

Here is the underside with the original price tag on it and the chop. If anyone can precisely identifyy this please let me know. Just above the drainage hole you can see the word JAPAN impressed into the clay.

And from the underside a couple of shots of the feet. This is the first time I've really looked critically at them and got a bit of a surprise. I have to say that I'm a little underwhealmed. They look great from the viewing front but from under here they are a little less shipshape than I would now expect. I like to think that ALL the pot should be finished to the same high standard as the outsde; including the underside and inside.

That minor point aside I'm still pleased to own this pot. Aesthetically it wins and that impervious surface offers a pot ready to bring inside for tree display with no more than a wipeover with a damp cloth.
Acquiring bonsai pots is a bit like buying furniture. You can buy function very cheaply; but that's the furniture you often see on the footpath after a couple of years when there's a Council cleanup. I've heard it said many times that you should, when you can over time, buy a few high quality, timelessly designed, well built pieces that will last a lifetime, age gracefully but always look good and can be handed on to the next gereration. Good advice.