Many of us like glass or crystal door knob at house, it’s beautiful and apparently harmless. But solid glass door knob may act like a convex lens and focus sunlight on flammable stuffs which eventually may catch fire because of concentrated sunlight. So consider the glass door knob as a convex lens, a flammable object exactly at focal length of the lens and sunlight is coming from opposite direction. It’s like your childhood days when you set fire to paper with magnifying glass.
In September last year Clare Thomson’s family house in UK caught fire when a glass door knob focused sunlight on nearby clothing. No one was at home that time but their neighbors noticed the fire before spreading.
According to London fire brigade, fire triggered by crystal or glass door knob is not uncommon. Their advice is to keep crystal or glass door knobs and mirrors with concave surfaces away from sunlight. Even glass fish bowls may cause fire by focusing sunlight.