Venetian Mirrors: Murano Glass, Facets, Silvering

A Venetian mirror is a sheet of glass whose edges have been ground and polished into sloping facets, then backed with a reflective metal coating, usually a tin and mercury amalgam in older pieces and a thin silver or aluminium layer in modern ones. The cutting, bevelling and faceting are done on the glass itself, in Murano, before any reflective backing is applied. Silvering is the last step, and it is the step that fails first in an old mirror.

What happens to the glass before it is silvered?

Murano glass for mirrors starts as a flat sheet, historically blown and flattened, now usually drawn or floated, and the decoration lives almost entirely in the edge. A cutter scores the sheet and breaks it to size, then a grinding wheel takes the raw edge down at an angle. That angled band is the bevel. Its width is measured in millimetres and it is what throws a bright line of light back at the room.

Faceting goes further. Instead of one continuous bevel, the edge is ground into a series of flat planes meeting at shallow angles, so the border reads as a row of small windows rather than a single frame of light. On elaborate pieces the facets are cut into the mirror face itself, around the border or in a crest at the top, and each one catches a different part of the room.

Grinding leaves the glass grey and opaque. Polishing with progressively finer abrasives, traditionally with felt wheels and rouge, clears it back to transparency. A polished bevel is not a coating and cannot peel; it is the same glass, thinner at the edge and shaped.

The vocabulary of the trade is worth knowing because it is what sellers use. The Venetian mirrors: glass, facets and silvering process runs from cutting to bevelling to faceting to silvering, and a piece described as faceted has had planes ground into it, while a piece described as bevelled has only the edge taken down at an angle. The two words are not interchangeable, and the difference shows in the price and in how the mirror behaves in daylight.

How is the reflective backing applied?

Silvering is a chemical deposition, not paint. In the older method, tin foil and mercury were laid against the glass and left to form an amalgam; the mercury was later driven off, leaving a reflective film bonded to the surface. The method was slow, and the mercury was dangerous to the people doing it, which is one reason the trade was regulated and eventually abandoned.

The modern method, in use since the mid nineteenth century, is wet silvering: a dilute silver nitrate solution is poured onto the cleaned glass with a reducing agent, and metallic silver deposits directly onto the surface. A protective coat of copper, then of lacquer or paint, goes over the back to stop the silver from being rubbed or oxidised. Aluminium vacuum coating is cheaper and is what most plain household mirrors use.

What matters for an old mirror is the edge of the silvered area. Silver tarnishes from the rim inward, and the dark speckled border that appears on an antique is the coating failing where moisture has reached it. This is called desilvering or foxing, and it is not dirt on the surface. No cleaner removes it, because the damage is behind the glass.

What are the main styles and shapes?

Venetian mirror styles are named mostly by their outline and by what sits on top of the frame.

• Rectangular with a crest: the classic form, a tall rectangle with a shaped glass crest at the top, often a fan, a shell or a pair of scrolls.
• Cushion or octagonal: the outline is softened at the corners, and the bevel follows the curve.
• Oval and round: common in smaller sizes, with a continuous bevel and sometimes a separate glass border.
• Cartouche: an irregular, lobed outline, usually symmetrical, built from several pieces of glass joined at the edges.
• Overmantel: a wide, low mirror made to sit above a fireplace, often in three sections.

Decoration on the frame is usually glass too: twisted rods, rosettes, leaves and small drops, cut and polished separately and pinned or glued to the border. On older pieces these elements are held with wire or with hide glue, and both fail with age. A missing rosette is a common repair, and a replacement is ground to match the surviving ones rather than bought as a standard part.

How is an old mirror kept?

Dust with a soft, dry cloth, and use a barely damp cloth only when the glass is genuinely soiled, then dry it immediately. Water that sits at the edge of a bevel can wick behind the glass and start or worsen desilvering. Ammonia based glass cleaners attack the backing coat if any reaches the rim, so a mild pH neutral cleaner on a cloth, not sprayed onto the mirror, is the safer habit.

Keep the mirror out of damp rooms and off exterior walls that run cold. Condensation on the glass is the main enemy of the silvering, and a bathroom or an unheated hallway will shorten the life of an antique. Direct sun is a lesser problem, but it will fade a lacquered back and dry out old glue holding the decorative elements.

Hang from a proper batten fixed into studs, not from a single hook, because these mirrors are heavy for their size and the glass frame is brittle. Check the wire or the original hanging rings once a year. If a decorative piece works loose, take it off and store it rather than letting it hang by one pin.

Can a failing mirror be repaired?

Resilvering an antique is possible but it is a decision, not maintenance. The old coating is stripped, the glass is cleaned, and a new silver layer is deposited, which removes the foxing and also removes the age. Most conservators advise against it on a piece with historical value, because the worn silvering is part of what the object is. On a plain twentieth century mirror with a damaged back, resilvering is straightforward and a local glass shop can usually arrange it.

A broken bevel cannot be repaired, only replaced. If a corner has chipped, a restorer can grind the whole edge back to a new line, which shortens the mirror slightly, or fit a new piece of glass cut to the original profile. Both are visible on close inspection, and both are preferable to a mirror that is unsafe to hang.

For a mirror that is only dusty and slightly foxed, the honest answer is to leave it alone. Clean the glass, keep the room dry, check the fixings, and let the edge do what it does.

What to check before buying

Look at the bevel from the side, not straight on. A true ground bevel has depth and a clean line where it meets the flat face; a printed or glued imitation border has neither. Look at the back if you can: a paper or felt backing that is intact and dry is a good sign, and a back that is stained or smells of damp is not.

Check the joints between separate pieces of glass by running a fingernail lightly along them. A gap that has opened means the mirror has been stored badly or has been dropped. Check that the crest and the applied decoration are all present and firm, and ask what has been replaced. A mirror with two or three replaced rosettes is normal for its age; a mirror with a replaced crest is a different object.

Finally, measure the wall before you fall for the outline. Overmantel and cartouche shapes are wide, and a mirror that looks modest in a shop can dominate a small room once it is up and reflecting the window.