If you have ever stood in an old house and looked through a window pane that bends the light just a little, you know the feeling. That soft distortion is not a flaw. It is a signature of the craftsmen who made glass before the twentieth century industrialized the process. Collectors and renovators pay real money for that character, and fakes have flooded the market to meet the demand.

When I started restoring a 1790s farmhouse in Connecticut, I needed to replace two broken panes. The local glass shop sold me "restoration glass" that looked wrong under scrutiny. A friend pointed me to https://mrglas.org/, a site run by a retired glazier who documents the subtle differences between genuine old glass and modern reproductions. That source changed how I look at every window I touch.

Why old glass looks wavy

You can trace the waves back to how the glass was made. Before float glass, workers made panes by spinning molten glass into a disk (crown glass) or by blowing a long cylinder and flattening it. Both methods left their marks. Crown glass has concentric ripples radiating from the center. Cylinder glass shows long, parallel undulations. Neither surface is perfectly flat, and that is exactly what you are looking for.

The ripple pattern is not random. It tells you the size of the original gather, the skill of the blower, and even the era. Early crown glass is often thick at the center and thins toward the edges. Cylinder glass tends to be more uniform but carries faint lines from the flattening table.

The crown glass method

Crown glass production survived for centuries because it required no large machinery. A blower gathered a glob of molten glass, blew a bubble, then transferred it to a pontil rod and spun it flat. The result was a disk, often called a roundel, from which panes were cut. The center knot, where the pontil attached, was usually discarded or used for small panes. Pieces taken near the edge show the strongest ripples.

I have a friend who collects salvaged crown panes from demolished buildings. He says you can identify them from across a room because they throw a shimmering band of light across the floor when the sun hits them. That shimmer comes from the changing thickness, which bends light like a weak lens.

Cylinder glass and the difference in distortion

Cylinder glass came later and became the standard for larger windows. A blower made a long bubble, swung it in a trench to elongate it, then cut off both ends and split the tube lengthwise. The flattened sheet was annealed slowly. The resulting surface is flatter than crown glass but still shows a gentle waviness along the length of the cylinder.

Here is a practical difference: crown panes look wavy in both directions, while cylinder panes wave mainly in one direction. If you hold a pane and look at a straight line like a window frame through it, crown glass bends the line in curves, while cylinder glass shifts it in long gentle arcs. That one-directional shift is a reliable clue when you are handling a pane that is still in its frame.

Quick field tests for authenticity

You do not need laboratory equipment to separate genuine antique glass from modern lookalikes. I use a few simple checks every time I am at an auction or a salvage yard.

  • Hold the pane edge-on to a bright sky. Real old glass shows a greenish or faint blue tint from iron impurities. Modern reproduction glass is often clear and colorless.
  • Run your fingertip across the surface. Antique panes rarely sit perfectly flat; you will feel the ripples. Float glass feels dead smooth.
  • Look for knife marks along the edges. Old panes were cut with a diamond or a steel wheel, leaving thin trailing scratches. Lasers and modern cutters do not leave those marks.
  • Check the corners of the pane. Hand-cut squares often have slightly rounded or uneven corners, not sharp perfect angles.

These tests are not foolproof. Skilled forgers have learned to tumble new glass to fake surface wear. But they catch most of the fakes, especially the cheap ones that flood online markets. If you are buying a pane for a historic restoration, ask for provenance. A documented salvaged window is worth more than a box of "vintage style" replacements.

Once you learn to read the waves, every old window becomes a small history lesson. The glass in your grandparents' house might be crown glass from England or cylinder glass from a midwestern factory. It survived two centuries of weather and one or two clumsy kids with baseballs. That is a good reason to keep it, and a better reason to replace it faithfully when you must.