Masonry Magazine March 1971 Page. 21
REMOVING VANADIUM STAIN
Since it is impossible to predict whether or not clay masonry units contain sufficient vanadium to cause staining and, if so, in what form the vanadium is present, it is important that the effect of any acid wash on the masonry units be determined by applying it to a sample wall area before it is applied to the entire project.
If, following the acid wash, green staining appears on the sample, the following procedure, which provides for neutralization of the acid, should be followed:
1. Immediately following the acid wash, as recommended for cleaning new construction, wash the wall with water.
2. Wash or spray the wall with a solution of potassium or sodium hydroxide, consisting of 12 lb hydroxide to 1 qt water (2 lb per gal). Allow this to remain on the wall for two or three days, in order to neutralize the acid which causes green staining.
3. The white salt left on the wall by the hydroxide may be hosed off the wall after two or three days or it will be removed by the first heavy rain.
To date, research has not developed any single method for removing green stain that can be recommended as best for all conditions. The following recommendations of SCPRF-SCPI are based on information currently available:
For the removal of only green stain, the first effort should be with sodium hydroxide. A successful practice has been to apply it with a paint brush (scrubbing is probably wasted effort); simply put a liberal application on and give it plenty of time to work. Pass no judgment on it for at least three days.
A convenient way to use sodium hydroxide is in the form of Drano. The mixture that has been used successfully in field trials is one 12-oz can per qt of water.
The sodium hydroxide, or Drano, will leave a white salt on the wall which can be washed off with a hose after three days.
Various proprietary cleaning compounds, such as Nos. 5, 6 and 7, Table 1, have proved successful in some instances. Their effectiveness on a particular project can best be determined by trial.
REMOVING MANGANESE STAIN
This form of stain usually occurs on the mortar joints between manganese grey or brown brick. Also, in some cases, it appears on the brick. It has a brown oily appearance and seems to run down from the brick-mortar interface.
This stain has been traced to the manganese used to color the brick. Some of the manganese which is added to the clay is converted during firing to a form which is soluble in a mildly acidic solution. Rain water acidified by carbon dioxide or sulphur in the air is sufficiently strong to dissolve some of these salts. When the solution reaches the mortar joints, it is neutralized by the basic nature of the cement or lime in the mortar and the salts are deposited there.
Manganese stain is difficult to remove, since it is not particularly soluble in hydrochloric acid. Theoretically, it is soluble in sulfuric acid, but such a strong solution is needed that the mortar joints would be eaten away, to say nothing about the danger of white efflorescence forming later.
The quickest, easiest method of removing the stain so far discovered is to use a solution of 1 part, by volume, of acetic acid (80 per cent or stronger), 1 part hydrogen peroxide (30-35 per cent) and 6 parts of water. This solution, when sprayed onto the wall, removes the stain very rapidly. The chief difficulty is that the stain often returns in a few days.
One of the proprietary cleaning compounds (No. 8, Table 1), investigated on several jobs, appears to be effective in keeping the stain from reappearing, although it is not always as rapid in removing the manganese stain initially as the acetic acid-peroxide solution mentioned above. Therefore, the best method found to date for the long-term removal of this stain is:
1. After wetting the wall, brush or spray it with the acetic acid-hydrogen peroxide solution described above.
2. After the reaction is complete, rinse the wall again with water. Brush or spray the wall with a solution of 1 part by volume of the proprietary compound, No. 8, Table 1, to 3 parts water. This solution is allowed to remain on the wall and is not flushed off.
REMOVAL OF EXTERNALLY CAUSED STAINS
These are stains caused by something being spilled on and absorbed by the brick. Each is an individual case and must be treated as such.
A large number of such stains can be removed by scrubbing with a good kitchen cleanser. Others can frequently be removed by bleaching with a household bleach. A combination, such as is found in some of the new kitchen cleansers, may prove most effective.
Table 2 lists sources, other than chemical supply firms, of some of the materials recommended below.
Poultice. A poultice, as included in some of the following recommendations, is a paste made with