In the P&SN article “Carrying the Torch: How to Fight Plaster Discoloration with Fire” there is some incorrect information and some important information is missing.
Torching and heating plaster to drive out the water content will likely convert calcium hydroxide into calcium oxide, not calcium carbonate (carbonation) as the article suggests. Calcium oxide is the form when manufactured into dry Portland cement.
Carbonation of a new plaster surface takes place naturally by simply being submerged in water. Balanced pool water contains both CO2 and bicarbonate that will convert calcium hydroxide into calcium carbonate. It is the carbonation process that forms a thin protective layer of calcium carbonate.
Torching can cause expansion of the plaster matrix leading to micro-cracking, increased porosity, and cause friability (crumbing) of the plaster. That detrimental process can also result in lightening of the color of plaster.
Also not mentioned in the article is that one common type of plaster discoloration, known as gray mottling may be caused by improper troweling practices such as performing overly late and excessive hard troweling. The resulting increased density produces a darker plaster shading where the excessive hard troweling took place. And yet, in the privacy of backyard residential pools, that plaster discoloration is (incorrectly) blamed on water chemistry by some plaster people.
On that front, the article correctly mentions that an acid bath treatment helps lighten discolorations. Does that not provide some evidence that aggressive water does not cause gray mottling discoloration?
Que Hales, Doug Latta, Kim A. Skinner
onBalance
