Suncatchers: rainbows, feng shui, and the fire risk nobody mentions

The physics of the rainbow

Light slows when it enters glass and speeds up leaving it, and at any angle other than straight on this bends the path. That is refraction.

The colours appear because the amount of bending depends on wavelength, blue bends slightly more than red. A single flat pane bends all colours by so nearly the same amount that you notice nothing. A cut facet at a steep angle exaggerates the difference until the colours separate visibly, which is dispersion.

This is why the shape makes the difference more than the material. A well-cut leaded crystal with sharp facets throws vivid spectra; a smooth round bead of the same glass throws almost none, because it has no steep angled faces to exaggerate the separation. It's also why lead crystal was traditionally preferred, a higher refractive index bends light more, so the effect is stronger.

The rainbows moving across a room through the day are simply the sun changing angle. Nothing about them is unusual, and they're one of the more available demonstrations of optics in an ordinary house.

The fire risk, which is real

This is the part that belongs in an article about suncatchers and almost never appears in one.

A solid glass or crystal sphere is a lens. Sunlight entering it converges to a focal point a short distance beyond, and at that point the energy that fell across the whole sphere is concentrated into a spot a few millimetres across. On a dark surface (a wooden sill, a curtain, a sofa arm, paper) it can char, and given time and a still sun it can ignite.

Fire and rescue services in the UK, Ireland, Australia and elsewhere have issued repeated warnings, usually after house fires traced to a crystal ball, a mirrored make-up mirror, or a glass ornament left on a sunny sill. The sequence is unremarkable: a bright afternoon, a focal point resting on something dark, nobody in the room.

Faceted suncatchers that are thin, hollow or irregular are far less dangerous, because they scatter light instead of converging it. The hazard belongs specifically to solid spheres and strongly curved solid glass.

Move the crystal off the windowsill in summer. I've seen the scorch marks, and no rainbow justifies them.

What feng shui claims

Faceted crystal spheres have a defined role in feng shui practice, and the actual role is more specific than the vaguer version that circulates.

They're used to disperse chi where it's thought to move too fast or too directly: a long corridor, a door aligned straight with another door or a window, stairs facing an entrance. The sphere is held to break up and redistribute the flow rather than to add anything. They're also hung to soften what is called a poison arrow, the sharp corner of a wall or a beam pointing into a room.

This is a genuine tradition with internal rules, and no invention of the crystal shop. It's also not testable in the form it's stated: chi has never been measured, and the claim is about the movement of a quantity no instrument detects, the same situation described in energy and energy healing.

What can be said is that the placements often coincide with real spatial problems. A corridor that feels unpleasant to walk down usually has something identifiable wrong with it (sightlines, lighting, proportion) and hanging something at eye level in it changes how the space reads.

How hot the spot gets

The arithmetic is small and alarming. Full sun delivers about a kilowatt per square metre. A solid glass sphere ten centimetres across presents roughly eighty square centimetres to it, so about eight watts pass through the glass. A sphere is a poor lens and does not bring all of that to one point, but it brings a good fraction of it to a spot a few millimetres wide, a short distance behind the glass. A few watts into a few square millimetres of dark fabric or paper is enough to blacken it in minutes and to reach ignition in a still room with the sun high.

The hazard scales with the size and clarity of the sphere and falls off fast with anything that breaks the shape. A sphere behind a net curtain, a sphere with a flat base, a sphere of frosted glass: all safer. A clear ball on a stand in a south-facing window in June, with a cushion beneath it, is the case fire investigators describe.

Test your own window

Glass, crystal and 'crystal'

Nearly every suncatcher is glass, and the word crystal on the label describes a grade of glass, not a mineral. Lead crystal contains a large fraction of lead oxide, which raises the refractive index and gives the sharp, bright dispersion the old chandelier drops are known for. Crystal glass in current European labelling is a lower-lead or lead-free formulation, and the major Austrian maker moved its whole range to a lead-free composition in 2012 while keeping the brightness. Chinese pieces are often K9, a borosilicate optical glass that disperses less and costs a fraction as much.

Quartz is the one natural crystal you will meet, usually as a rough point on a cord. It refracts, but it is not cut to disperse, so it throws almost no rainbow, and the claims made for it belong to vibrational therapy. If the point of the piece is the colour on the wall, buy cut glass. If the point is the stone, know that the stone is not doing the rainbow.

Where feng shui hangs them

The practice has rules that a shop rarely passes on. A faceted sphere is hung on a red cord, traditionally cut to a multiple of nine units, and placed at the point where the chi is thought to need slowing: a third of the way down a long corridor, in the line between two facing doors, at a window that looks directly onto a road or a sharp roofline. It is not hung in every window for luck, and a sphere placed where there is no such alignment is decoration, which the tradition is content to call it.

What that gives a sceptic is a set of placements that happen to be sensible. The spots the rules name are the ones where a hanging object at eye level changes how a space reads: the long corridor gets a focal point, the aligned doors get a pause. The mechanism is not chi. The placements were arrived at by people paying attention to rooms, and attention to rooms is the part that transfers.

What a suncatcher does

None of that requires anything to be redirected. A thing can deserve hanging because it makes a room better to be in.

How do suncatchers make rainbows?

Through refraction and dispersion. Light bends as it enters and leaves glass, and because different wavelengths bend by slightly different amounts, a steeply angled facet separates white light into a spectrum. Sharp facets and a higher refractive index produce stronger colours, so cut lead crystal works better than a smooth bead.

Can a suncatcher start a fire?

A solid glass or crystal sphere can. It acts as a lens, focusing sunlight to a hot point that can char wood or fabric and, given time, ignite it. Fire services in several countries have issued warnings after house fires traced to crystal balls and glass ornaments on sunny windowsills. Flat or hollow suncatchers scatter light instead.

What do crystals do in feng shui?

Faceted crystal spheres are used to disperse chi where it's held to move too fast or directly, such as a long corridor or two aligned doors, and to soften the sharp corner of a wall pointing into a room. It's a defined traditional practice, though chi has never been measured and the claim isn't testable as stated.

Where should I hang a suncatcher?

A window that gets direct sun for part of the day gives the best rainbows, ideally one where the light falls across a wall instead of onto fabric or wood. If the piece is a solid sphere, keep it out of direct sunlight entirely because of the fire risk, and check what is below it at the brightest part of the day.

Do suncatchers stop birds hitting windows?

A little, but far less than purpose-made solutions. Birds strike glass because it reflects sky and vegetation, and a single hanging object leaves most of the pane unmarked. Dense patterns of dots or stripes spaced a few centimetres apart across the whole window are what reduce collisions.

Is a quartz point a suncatcher?

It refracts light, but it is not cut to disperse it, so it throws almost no rainbow. Cut glass with sharp facets does the colour. A quartz point on a cord is decoration, and the further claims made for it belong to vibrational therapy.

How does feng shui use a crystal sphere?

Hung on a red cord at a point where chi is thought to move too fast, such as a long corridor or between facing doors, to slow and disperse it. The rules do not call for one in every window, and the placements happen to be where a hanging object improves how a space reads.