Can Garden Mirrors Cause Fires? (How To Make Them Safe)

When thinking of home and garden fire hazards, it’s easy to list the obvious, e.g., gas geysers, electric toasters, dried leaves, trees that extend over the house’s roof, etc. However, few people consider the fire risks posed by decorative garden items like sundials or garden mirrors.

Garden mirrors can cause fires when they are concave and direct the sun’s rays onto a combustible material with a low flashpoint. Paraboloid mirrors are better fire-starters than spherical concave mirrors. Plane and convex mirrors won’t cause fire as they don’t concentrate the sun’s thermal energy.

Garden mirrors are rapidly gaining popularity because of their ability to grant small gardens the illusion of space, dark spaces the gift of stolen light, and lush vegetation the magnification of beauty. However, how safe are these aesthetic additions to your garden? Or is it true when people say that danger lies in beauty and beauty lies in danger?

Why can a garden mirror cause fires?

To start a fire, three things must be present:

1. Thermal energy
2. Oxygen or an oxygen-rich compound
3. Fuel, i.e., combustible material

Thermal energy can be from concentrated sunlight, a match, or even generated through friction in which kinetic energy is transformed into thermal energy. The thermal energy must exceed the flashpoint (minimum temperature) of the fuel-oxygen mix to start a fire.

The flashpoint differs between different types of fuels, e.g., dry leaves will ignite at a lower temperature than fresh green leaves.

Once the fire has started, a chain reaction is triggered. The fire generates enough thermal energy to maintain combustion as long as sufficient oxygen and fuel are present.

What kind of mirrors can start a fire?

There are three types of mirrors:

1. Concave (converging) mirrors
2. Convex (diverging) mirrors
3. Plane (flat) mirrors

Mirrors are reflective surfaces that “rebound” the incoming light rays, i.e., they redirect the light rays’ path. The path upon which the reflected light ray is directed depends on the following:

1. The angle of the incident light ray, i.e., the light ray, before it hits the mirror
2. The angle of the mirror’s surface

Incident light rays that connect with the mirror surface at a 90° angle are reflected at a 90° angle, i.e., turned around to retrace its original path. Whereas light rays that hit a mirror surface at an angle are also reflected at an angle, the angle of incidence is equal to the angle of reflection.

Are flat mirrors fire starters?

Flat mirrors are highly unlikely to start a fire. The incident light ray is reflected along its original path, or at an angle to its original path, i.e., the reflected light is no more concentrated than the incident light rays.

Another way of understanding this is:

Sunlight rarely causes combustion, as the thermal energy is too low to reach the flashpoint of most fuel-oxygen mixes. Flat mirrors don’t concentrate the sun’s energy and thus do not cause fires.

While the flat mirror is innocent of pyromaniac tendencies, the same cannot be said for concave mirrors.

Are concave or convex mirrors fire risks?

Concave mirrors reflect the incident light rays onto a focal point in front of the reflective surface. The convergence of light rays upon a single point increases the thermal energy and will start a fire in the right conditions.

There are two types of concave mirrors; spherical, which follows the arc of a true circle, and parabolic, which follows the arc of a circular paraboloid.

In a sphere, all points in a circle are equidistant from the center. In a paraboloid, the intersection of two parabolas is further from the center than any other point within the paraboloid. The two shapes can be seen here.

Parabola mirrors are even better at concentrating light energy on a single point and are thus more efficient fire starters than spherical concave mirrors.

Unlike concave mirrors, convex mirrors disperse light rays away from the original axis, i.e., they reduce the concentration of light. Because of the dispersion of light rays, convex mirrors are even less likely to cause a fire than a flat mirror.

What material is best for an outdoor mirror?

The best material for an outdoor mirror is the one that best suits your needs. The two most common materials used in garden mirrors are acrylic and glass.

Glass Garden Mirrors	Acrylic Garden Mirrors
Better clarity and the aesthetics are more pleasing	The reflected images are blurred
Heavy	50% to 80% lighter than glass
Will shatter if hit with enough force – tempered glass is the most shatterproof glass available but also the most expensive	17 times more shatter-resistant than annealed glass
More expensive	Cheaper
Longer lifespan	Shorter life span
More scratch-resistant	Scratches easily
Won’t warp	Prone to warping

Acrylic mirrors’ imprecise reflection of light makes them a slightly lower fire-risk than glass mirrors. However, the difference is so minuscule as to be negligible.

How to make your garden mirror safe?

The fire risk that using mirrors in the garden pose can be minimized by using a few simple strategies based on the physics of fire and light reflection.

Placing a garden mirror in a shaded area of the garden prevents the concentration of light rays onto the reflective surface, lowering the thermal energy reflected by the mirror. Thermal energy may also be combated by placing the mirror in a well-irrigated part of the garden.

The irrigation system will naturally cool the surrounding areas, and more thermal energy will need to be concentrated to raise the ambient temperature to the fuel’s flashpoint.

Using a flat mirror instead of a concave mirror prevents the concentration of light rays on a specific point. Natural sunlight rarely has sufficient energy to ignite a fire, unless the light rays are concentrated to a single point.

Lastly, when positioning your mirror, consider the surrounding vegetation. Evergreen plants contain more moisture than dead, dying, or dried-out vegetation; thus, their flashpoint is much higher than desiccated plant matter.

For example, it’s better to position your mirror amongst a bevy of evergreen plants than in the shade of an oak tree that litters the ground with piles of dried-out fall leaves.

How do you weatherproof a mirror outside?

Weatherproofing your garden mirror is essential for longevity as it prevents moisture from getting between the mirror layers. Mirror edge sealants prevent the mirror edges from warping, blackening, or becoming moldy.

1. In a well-ventilated dry area (preferably outdoors), lay down a plastic tarp.
2. Thoroughly clean the mirror with an alcohol-based cleaner and allow the mirror to dry; pay special attention to the mirror edges when cleaning.
3. Always wear gloves when applying mirror edge sealant.
4. If you’re using a liquid sealant, use a wide-head brush and apply an even coating along the mirror’s edge.
5. If you’re using spray-on sealant, hold the can approximately 6″ to 12″ away from the mirror when applying the sealant. Try to apply a smooth, even layer of sealant along the mirror edges. Never spray the can towards yourself.
6. Depending on the manufacturer’s instructions, a second coat may be necessary.
7. Allow the sealed mirror to dry for 3 hours before moving it.

Do garden mirrors break in cold weather?

Garden mirrors don’t break in cold weather. Neither glass nor acrylic mirrors are vulnerable to cracking in cold weather.

Neither glass nor acrylic mirrors are vulnerable to cracking in cold weather.

Glass can break because of thermal stress, i.e., rapid temperature fluctuations cause expansion and contraction, resulting in stress fractures occurring on the glass surface. However, environmental thermal fluctuations are unlikely to be sufficient for causing cracking.