Claim: Old window glass flows and gets thicker at the bottom over centuries

First requested: July 5, 2026 at 8:02 AM
15%

IsItCap Score

Truth Potential Meter

Not Credible

AI consensusMedium

Grader consensus is moderate.
Range 5%–20% (spread Δ15).
The graders lean in the same direction but differ on strength. Skim the summary and sources.
Read analysis summary

OpenAI Grade

0%
20%
40%
60%
80%
20%

Perplexity Grade

0%
20%
40%
60%
80%
5%

Google Gemini Grade

0%
20%
40%
60%
80%
5%
Shareable summary
Verdict: Questionable
  • Glass is an amorphous solid, not a liquid, and doesn flow at room temp.
  • Thickness comes from handcrafting variation, not flow over time.
/r/fact-check-old-window-glass-flow-thicker-bottom

Analysis Summary

The claim that old window glass flows and becomes thicker at the bottom over centuries is mostly false. Mainstream scientific sources and experts argue that the thickness variation in old glass is due to manufacturing techniques rather than flow. Alternative sources perpetuate the myth, suggesting that glass behaves like a viscous liquid over time. However, this is not supported by substantial evidence or statistical studies. The graders agree on direction, but vary in strength. OpenAI comes in highest (20%), while Gemini is lowest (5%). While some sources claim that old glass panes appear thicker at the bottom due to slow flow, this view is largely debunked by scientific analysis. Critics argue that the observed thickness is a result of historical glass-making practices, such as the placement of thicker edges down for stability. This indicates that the claim lacks robust empirical support, and the notion of glass flowing over centuries is not widely accepted in the scientific community, leading to uncertainty about the claim's validity.

Source quality

Truth (from sources)2.00 / 10
Source reliability8.00 / 10
Source independence7.00 / 10

Claim checks

Fits established facts3.00 / 10
Logical consistency4.00 / 10
Expert consensus2.00 / 10

Source Analysis

Common arguments
Supporting the claim
  • Glass is technically a liquid, so it should flow over centuries.
  • Old windows are visibly thicker at the bottom, proving flow.
  • Teachers say glass flows, so it must be true.
Against the claim
  • Glass is an amorphous solid, not a liquid, and doesn flow at room temp.
  • Thickness comes from handcrafting variation, not flow over time.
  • Calculations show flow would take millions of years to be visible.

Mainstream Sources

Publication

ceramics.org

Title

Glass viscosity calculations definitively debunk the myth of observable flow in medieval windows - The American Ceramic Society

Summary

However, despite the low values, ... New calculations show that <strong>medieval glass windows, like these at Sainte-Chapelle in Paris, France, are not thicker at the bottom because of glass flow</strong>....

Source details

Type: Official
Primary Data

Publication

thefoa.org

Title

Does Glass "flow"?

Summary

And like the more traditional sort, ... (especially medieval church windows) is thicker at the bottom than at the top is because <strong>glass, despite its apparent solidity, is actually a liquid</strong>....

Source details

No Date

Publication

physics.stackexchange.com

Title

Do glass panes become thicker at the bottom over time? - Physics Stack Exchange

Summary

<strong>The observation that old windows are sometimes found to be thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a timescale of centuries</strong>.

Source details

Alternative Sources

Publication

guernseydonkey.com

Title

Are Old Windows Thicker at the Base Because Glass Flows Like Syrup ? - guernseydonkey.com

Summary

This is one of those urban myths that refuses to die. <strong>The idea that glass is really a viscous liquid, so thick that it takes centuries for it to flow</strong> and that is why Medieval glass is thicker at the bottom than than the top.

Source details

Type: Blog

Publication

historymyths.wordpress.com

Title

Revisited Myth #18: Panes of window glass in old buildings are visibly thicker at the bottom, proving that glass is a viscous liquid that has “flowed” over time. | History Myths D…

Summary

If one edge was thicker, the installer generally placed that edge down for stability. If you look closely at a piece of antique window glass, you may see an arc, not to mention bubbles or other imperfections, all of which are evidence of hand craftsmanship. Debunkers make the argument that if glass really did flow visibly over time, we would see more flow in ancient Egyptian glass than we do in, say, colonial American glass.

Source details

Type: Blog
Published: 2014-06-22
Low Transparency

Publication

glassnotes.com

Title

Does Glass Flow | Glass Notes, Version 4.0

Summary

Finally, Dr. Neumann relates the following. Read this carefully: &quot;In other words, while some antique windowpanes are thicker at the bottom, <strong>there are no statistical studies to show that all or most antique windowpanes are thicker at the bottom than at the top</strong>.

Source details

No Date

Analysis Breakdown

True/False Spectrum (2.0)Source Credibility (8.0)Bias Assessment (7.0)Contextual Integrity (3.0)Content Coherence (4.0)Expert Consensus (2.0)43%

How to read the breakdown

Weakest areas
Truth2.0/10Consensus2.0/10
  • Truth: how well sources support the core claim.
  • Source reliability: whether the sources have a strong track record.
  • Independence: whether coverage looks one-sided or recycled.
  • Context: missing details (timeframe, definitions, scope) that change meaning.
  • Tip: if graders disagree, rely more on the summary + sources than the single number.

Detailed AnalysisPremium Feature

Get an in-depth analysis of content accuracy, source credibility, potential biases, contextual factors, claim origins, and hidden perspectives.

Create a free account to unlock premium features.

Methodology