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.