Odd black, RAISED spots on sunshelf plaster in pool


Gold Supporter
Mar 21, 2015
I having being using TFP for quite some time and have never seen this and I searched around but could not see anything related... thus, the new post.

The sunshelf on my SWG pool is 9" deep and the spa waterfall does overflow onto it. Recently, I have found ~10 black, RAISED spots on the sunshelf (see photo). They are hard as a rock, including the black top. They are raised up about 1/4 inch, maybe a bit less, and are about 1/4 inch in diameter as well. Scraping the black top does not remove anything unless you really tear at it with your fingernail. I smeared it on white paper and it does not smear at all- thus unlikely to be black algae or any algae really.

When I did get a bit more to scrape off the substance under the black top appears to be plaster. It is like the plaster is rising up in these strange bumps. I showed them to my pool guys with 30 years experience and he has never seen or heard of anything like it. He says definitely not algae and advised me to call the company that built our pool. That will be expensive visit and my faith in them is LOW...

I have attached a photo but you can't see much here- they are the 3 larger black dots on the plaster. Does anyone have any ideas what these may be? The cause? and how to treat them?

I believe my chemicals have been in very good balance for a long time. Current chemical readings from my Taylor kit are:
Hardness: 430
FCL: 7.8
CCL: 0.4
pH: 7.5
Alk: 80
CYA: 65

Thank you so much!!Pool Spots.jpg


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
They sound like calcium nodules. But the black color is odd. Is it black or dark brown? Any chance of iron? Can you get a piece of it and see what it does in acid?


Gold Supporter
Mar 21, 2015
I have never heard of calcium nodules so if you know of any links either on TFP or outside- I would appreciate them. I have had calcium scale build up on the wall that the waterfall goes over from the spa into the pool over time but believe that rather normal in Texas especially with the waterfall splashing onto the wall.

It appears black to me and when i do get a piece off, it appears black or dark charcoal grey. Does not appear brown at all. I doubt it is iron as I have had iron stains before and it is not similar; plus it is raised which makes me doubt iron stain.

I was able to scratch off a few pieces today but they are tiny slivers when they come- these things are hard as a rock. I placed the slivers in a cup and poured in 20 baum muriatic acid and there was NO reaction at all. When I put that stuff on the calcium deposits on my wall, the reaction is severe with bubbles, etc and that is using diluted acid. No reaction today and the acid was not diluted.

Let me know if you have any further thoughts and thank you for your reply. Also, any links to calcium nodules would be appreciated, else I will search around.

Best- Warren


Gold Supporter
TFP Guide
Nov 12, 2017
Central California
and advised me to call the company that built our pool. That will be expensive visit and my faith in them is LOW...
Can you explain that? Did your pool and/or plaster come with a warranty? Is it still under warranty? If it is, why would it be expensive for their visit?

The reason I ask... I just had a weird spot on my pebble. Like you, I posted here about it, with all kinds of thoughts about it. Someone posted: just call the plaster guys and let them deal with it. Which I then did. And they did. Took 'em 20 minutes, problem is gone. They are 2.5 hours drive from me, no charge for the service call.

If it's possible someone else is responsible for this issue, then let them deal with it, and don't mess with it yourself until they do. Otherwise, you could compromise the finish in some way, and get blamed, giving them an excuse to not take care of it.

If it turns out to be something not covered under the warranty, then come on back and continue to work with everybody here to figure it out.