Stains From Organic Compositions
Algae can create black stains as well. Algae needs moisture, and brick walls located underneath roof drainage or in shady, humid areas can quickly develop algal growth. Different types of algae can create different colored stains. Brick stains can result from black, green or red algae.
Mold is a type of fungus that develops in warm, moist conditions. It can look black, white, green, blue, brown, purple, or gray in color. Mildew—another type of fungus— often appears powdery and white, but it may look black, brown, yellow, or gray.
When black spots and stains appear on exterior brick walls, more often than not they're caused by carbon buildup, dirt and pollution – especially in urban areas. Water leaks, for example from a drainpipe, can also cause black stains to appear on brick. Sometimes, the cause is mold or algae.
Black staining on the exterior of buildings
That dark discoloration on the side of your house can be one of three things: algae, mold or moss.
Contrary to popular belief, exterior walls are porous, meaning they will absorb moisture over-time. This moisture is then transferred into the property causing high levels of moisture. This process is known as penetrating damp and is one of the most common forms of black mould growth on walls.
Mold is often found on brick because it traps moisture after it rains or there is flooding. Some places to check for mold include the physical structure of your home or patio if they are brick.
To remove mold, it is not always easy. Safety is key; you will want to wear goggles that do not allow air in for ventilation and rubber gloves to prevent the mold from getting on your skin. Black mold, dark green mold, and virtually every other type of mold is able to grow on brick surfaces.
If a side of your home doesn't get much sunlight, moss, mold or mildew can grow on the bricks. Mix a cup of bleach into a gallon of water and apply to the wall with a sponge. Use a natural or nylon-bristle scrub brush to remove the growth.
Types of Brick Stain
Water-based stains are easy to apply and dry quickly. Oil-based stain requires a primer first and are more difficult to apply. Keep in mind that when staining a vertical surface, mineral and acrylic stains are always the most appropriate. Horizontal surfaces do much better with acid staining.
Mold is usually the colors of green, white, gray or black.
Vinegar (For Basic or Interior Cleaning): Alternatively, some people skip the dish soap and mix a solution with equal parts vinegar and water, then pour it into a spray bottle. This solution can be surprisingly effective at removing basic stains from brick.
Brick staining is a permanent finish made out of mineral-based pigments that seep and bind into the brick (rather than remaining on the surface as paint does), act as a moisture barrier, and allows the brick to breathe. Stain works best on clean, porous, and unpainted brick and won't peel, chip, or flake off.
Brick is particularly susceptible to water absorption and subsequent damage. Caution should be used when pressure washing brick with liquid cleaning products, as they can penetrate porous masonry products (including mortar and brick) which can lead to material breakdown and deterioration.
Simply put, black stains on flagstones are caused by certain types of mold and mildew. These fungi find their way onto the stones either from their spores landing on the stones or from simply extending onto the stones from patches of mold or mildew right beside them—a pretty common occurrence for garden pavers.
Black mold, a type of mold that can potentially pose health risks to humans. It produces mycotoxins, which are toxic substances that can be released into the air when the mold is disturbed or when it grows on damp or decaying materials.
As the brick heats up, the minerals in its clay can often start to react with the air or other parts of the clay. This leads to the clay switching from its original color to a different one. This is because the hematite, a bright red mineral, reacts to become magnetite, a black mineral.
Mix one part bleach with three parts water in a bucket. Using a scrub brush or heavy-duty sponge, vigorously scrub the mold-affected wall with the bleach/water solution until the mold spots have disappeared.
Mold Grows on Bricks for the Same Reason Mold Grows Anywhere
Mold needs three things to have a very successful breeding ground: moisture, food, and stagnancy. As far as humidity goes, mold loves anything above 60%. The food mold will eat is anything that is considered organic matter.
Most mold is unmistakable, but sometimes small or largely hidden growths just make a surface look dirty. A quick test for mold can be done when you dip a swab in diluted bleach (1 part bleach, 16 parts water) and dab it on the wall. If the spot quickly lightens (or keeps coming back after cleaning), assume it's mold.
Older homes use older materials – in the presence of water intrusion and mold spores, mold can grow as quickly as 72 hours, so the older the house, the more chance for previous water intrusion and possible mold infestation.
Black mold, or Stachybotrys chartarum, surfaces as blackish-green spots or blemishes. It lives on moist, cellulose-rich surfaces like fiberboard, drywall, and gypsum board. Black mold is most attracted to moisture, which is why it lives in previously-flooded homes, leaky pipe areas, or anywhere with water damage.
If it's not mold
If mold is not present, the problem is mostly likely dust, dirt or soot. The solution here is to clean the dirty areas with a good household cleaner degreaser and warm water before repainting.
I frequently receive this same question: what causes those black verticals stains on walls along the stud lines? To make it short and sweet, the answer is soot, and it's usually caused by candles and thermal bridging.