Introduction
Among the condition issues that affect early English glass, crizzling is the most serious and the least well understood by new collectors. Unlike a chip or a crack — which are immediately visible and straightforwardly assessed — crizzling is an internal deterioration of the glass itself, progressive in nature and impossible to reverse. A crizzled glass is, in a fundamental sense, a glass that is slowly destroying itself, and understanding what crizzling is, how to identify it, and what it means for the value and long-term prospects of a piece is essential knowledge for any collector of early English glass.
What Is Crizzling?
Crizzling — also known as glass disease, sick glass, or in German Glaskrebs (glass cancer) — is a form of deterioration caused by a chemical imbalance in the glass formula. It manifests as a network of fine internal cracks — sometimes called crizzles — within the body of the glass, and as a progressive clouding or iridescence of the glass surface.
In its early stages, crizzling may be barely perceptible — a very slight haziness or iridescence visible only in certain lighting conditions. In advanced stages, the glass becomes heavily crazed with internal cracks, the surface loses its transparency and takes on a milky, opalescent appearance, and the structural integrity of the piece may be compromised. In the most extreme cases, crizzled glass can literally crumble.
What Causes Crizzling?
Crizzling is caused by an excess of alkali — typically potash or soda — relative to the silica and stabilising components in the glass batch. When the alkali content is too high, the glass is chemically unstable. Moisture — from the atmosphere or from contact with liquids — reacts with the excess alkali at the glass surface, leaching it out and leaving behind a silica-rich layer that is under stress. This stress produces the characteristic network of fine cracks.
The problem is exacerbated by fluctuations in humidity — cycles of wetting and drying accelerate the leaching process and the development of cracks. Glass stored in consistently dry conditions deteriorates more slowly than glass exposed to variable humidity.
Why Early English Glass Is Particularly Affected
Crizzling is most commonly found in English glass produced in the period from approximately 1673 to 1695 — the years immediately following George Ravenscroft's development of lead crystal. Ravenscroft's original formula, while revolutionary in its use of lead oxide to improve clarity and brilliance, was not immediately stable. The early batches produced at his Savoy glasshouse and at the glasshouse at Henley-on-Thames contained excess alkali, and the resulting glass was prone to crizzling.
Ravenscroft himself was aware of the problem — the Glass Sellers' Company, which had commissioned his research, complained about crizzling in his early glass, and Ravenscroft worked to reformulate the batch to address it. By approximately 1676–1677, he had developed a more stable formula, and the introduction of the raven's head seal — a small applied seal bearing a raven's head, used to mark glass produced to the improved formula — was intended to reassure buyers that the glass they were purchasing was of the stabilised type.
Despite this, crizzling continued to affect some English glass produced into the 1690s, as other glasshouses adopted Ravenscroft's lead crystal formula without always achieving the correct balance of ingredients. By approximately 1700, the formula had been sufficiently refined across the industry that crizzling became much less common, and the heavy balusters of the early 18th century are generally free of the problem.
Crizzling is also found in some Continental glass of the 17th century, particularly German and Bohemian glass of the period, where similar formula instabilities occurred. It is less commonly found in Venetian soda glass, which has a different chemical composition and a different pattern of deterioration.
How to Identify Crizzling
Identifying crizzling requires careful examination under good light. The following approach is recommended:
Transmitted Light
Hold the glass up to a strong light source — a window in daylight or a bright lamp — and look through the glass. In transmitted light, crizzling appears as a network of fine lines or cracks within the body of the glass, sometimes described as resembling the cracking of old varnish or the surface of a dried lake bed. In early stages, the network may be very fine and visible only in certain orientations; in advanced stages, it is immediately apparent.
Raking Light
Direct a light source across the surface of the glass at a low angle — raking light — and examine the surface carefully. Crizzling produces a characteristic iridescence or haziness on the glass surface that is most visible in raking light. The surface of a crizzled glass has a slightly milky, opalescent quality that is quite different from the clear, brilliant surface of sound glass.
Surface Texture
In more advanced cases of crizzling, the surface of the glass has a slightly rough or granular texture that can be felt by running a finger lightly over it. This roughness reflects the physical disruption of the glass surface by the leaching process. Sound glass has a perfectly smooth, polished surface.
Cloudiness
Advanced crizzling produces a general cloudiness or opacity within the glass that reduces its transparency. A glass that should be clear but appears milky or hazy when viewed in transmitted light is likely to be significantly crizzled.
The Loupe
Examining the glass surface under a 10x loupe in good light will reveal fine surface crizzling that may not be visible to the naked eye. The characteristic network of fine cracks is clearly visible under magnification even in early stages of the condition.
Crizzling and the Raven's Head Seal
Glasses bearing Ravenscroft's raven's head seal — a small applied glass seal on the bowl or stem, bearing an impressed raven's head — are among the most historically significant pieces in early English glass collecting. They date from approximately 1676 to 1681 and represent the first generation of stabilised English lead crystal.
Despite the seal's intended guarantee of quality, some sealed pieces do show crizzling — evidence that the formula, while improved, was not yet perfectly consistent. A sealed piece in good condition, free of crizzling, is exceptionally rare and of very considerable value. A sealed piece with significant crizzling is historically important but of reduced practical value as a collecting object.
The raven's head seal itself should be examined carefully on any piece purporting to bear it. Genuine seals are applied glass, slightly raised from the surface, with a clearly impressed raven's head design. Later additions or fakes are occasionally encountered.
Can Crizzling Be Treated or Stopped?
This is the question most frequently asked by collectors who discover crizzling in a piece they own or are considering buying, and the honest answer is: not effectively.
Various treatments have been proposed and attempted over the years, including consolidants applied to the glass surface to stabilise the crizzle network and slow further deterioration. Some museum conservation departments have experimented with such treatments, with mixed results. For the private collector, the practical options are limited:
- Controlled storage environment — the most effective intervention available to the private collector is to store crizzled glass in a stable, low-humidity environment. Fluctuations in humidity are the primary driver of crizzling progression, and a consistent relative humidity of approximately 40–50% will slow deterioration significantly. Silica gel in a display cabinet can help maintain stable humidity.
- Avoid washing — crizzled glass should never be washed in water, as contact with moisture accelerates the leaching process. Dust with a soft, dry brush only.
- Avoid temperature fluctuations — as with humidity, temperature fluctuations stress the glass and accelerate deterioration. Store away from radiators, direct sunlight, and other sources of temperature variation.
These measures will slow the progression of crizzling but will not stop it entirely. A significantly crizzled piece will continue to deteriorate over time, regardless of storage conditions.
What Crizzling Means for Value
The impact of crizzling on value depends on the severity of the condition and the significance of the piece.
Early Stage Crizzling
Very early stage crizzling — a fine network visible only in transmitted light, with no surface haziness or texture change — has a moderate impact on value. For a piece of significant historical importance (a sealed Ravenscroft glass, for example), early crizzling may be accepted as an inherent characteristic of the period and the piece may still command a substantial price. For a less significant piece, early crizzling will reduce value noticeably.
Moderate Crizzling
Moderate crizzling — visible surface iridescence, a clearly developed crack network, some reduction in transparency — significantly reduces value across all categories. A piece with moderate crizzling is a compromised collecting object, and its value reflects this.
Advanced Crizzling
Advanced crizzling — heavy crazing, significant cloudiness, surface texture change — reduces value very substantially. A heavily crizzled piece, however historically significant, is primarily of interest as a study object or museum piece rather than as a collecting object in the conventional sense.
The Collector's Perspective
For the collector, the key question is whether the historical or aesthetic interest of a crizzled piece justifies its price at the reduced value that crizzling implies. For a sealed Ravenscroft glass with early crizzling — one of the rarest and most historically significant objects in English glass — the answer may well be yes. For a more common piece with significant crizzling, the answer is usually no — a sound example of the same type, even if it costs more, is a better collecting investment.
Always factor the progressive nature of crizzling into any assessment. A piece with early crizzling today will have more advanced crizzling in ten years. The trajectory of deterioration is one-directional, and the value of a crizzled piece will not improve over time.
Disclosure and the Market
Crizzling should always be disclosed by sellers, and its absence from a condition description should not be assumed to mean its absence from the piece. Always examine early English glass — particularly pieces dating from before approximately 1700 — carefully for crizzling before purchase, regardless of what the condition report says.
At reputable auction houses and with established dealers, crizzling will be noted in condition reports. In less formal selling contexts — general antiques fairs, online marketplaces, private sales — it may not be, either through ignorance or omission. The buyer who knows how to identify crizzling is protected; the buyer who does not is vulnerable.
The Wonder Room describes all pieces honestly, including any condition issues. Browse our current collection or explore further guides in Collector's Insights, including our practical guide to authenticating antique glass and our guide to buying at auction.