The United Nations has designated 2022 the International Year of Glass. Cooper Hewitt is celebrating the occasion with a yearlong series of posts focused on the medium of glass and museum conservation.

The creation of a beautiful dining tablescape has long relied on decorated glass, such as the two Irish decanters from the museum’s collection featured in this blog post. Our previous Year of Glass post focused on different techniques for creating those forms, from cut and engraved to pressed designs. These processes all rely on stable glass compositions, meaning that the glass itself is properly mixed, melted, and formed. Though various recipes have been used over the centuries to create such table wares, it could be argued that the addition of lead to form leaded glass, historically known as flint glass and referred to as lead crystal or simply crystal today, allowed glass workers to reach new heights in sparkle and brilliance.

The early history of crystal is associated with England, as British glassmaker George Ravenscroft received a patent to produce crystal glass in 1674, an innovation aiming to capture some of the market share for luxury glass that was dominated by Venice. Though the earliest Ravenscroft glasswares were not stable and were known even at the time to develop a fine network of cracks, called crazing, the workshop soon developed a more reliable recipe. By the end of the 18th century, Irish glass factories established themselves as a source of finely worked leaded glass. These factories were located in port cities such as Cork, as the coal needed to heat the furnaces arrived via ships. Over time, glassworks such as Waterford became known for their excellent quality wares.

A clear, colorless decanter with a bulbous bottom ascending into a narrow-necked top and a disc-like stopper next to another clear, colorless decanter in a blocky form with decorative details and capped with a faceted stopper.

Left: Decanter (Cork, Ireland), ca. 1783-1818; Glass; H x diam.: 20 × 10 cm (7 7/8 × 3 15/16 in.); Bequest of Walter Phelps Warren, 1986-61-138-a,b. Right: Whiskey Decanter (possibly Ireland), ca. 1820 (decanter), after 1820 (stopper); Glass; H x W x D: 21 × 8.6 × 8.6 cm (8 1/4 × 3 3/8 × 3 3/8 in.); Bequest of Walter Phelps Warren, 1986-61-167-a,b

These two decanters, both generously gifted by Walter Phelps Warren, were made in Ireland. The rounded example, with the flat, disk-like stopper, was mold blown to create the lower body fluted ribs and features wheel-cut designs with stars and cross-hatching, along with curved vegetal swags. “Cork Glass Co” is molded on the underside. Cork Glass was founded in 1783 and operated until 1818. The second decanter has a rectangular body with cut, faceted shoulders and the cut inscription “The Bushmills Old Distillery Co Antrim Ireland” surrounded by shamrock designs. Bushmills was founded in 1784, an origin date that tells us the earliest year this decanter could possibly have been made.

Our interest in the museum’s larger collection of Irish decanters prompted a deeper study, which included taking extremely small samples from the glass (think the size of a sharpened pencil tip!) from areas where the glass was either damaged or presented an edge that allowed for easy removal. We then sent these samples to our colleague Dr. Thomas Lam at the Museum Conservation Institute in Washington, DC. Dr. Lam put the samples on special stubs and looked at them in his scanning electron microscope, using a special feature to see what elements were present in the tiny flakes. We were thoroughly surprised when Dr. Lam reported back that all of the samples contained lead except the Bushmill whiskey decanter stopper, which, given the purported time period, should have contained lead. Though glass researchers have studied later-19th-century reproductions of the Bushmills whiskey bottle made with unleaded glass (see Francis, 1994, below), it is more likely that this stopper is not actually original to the whiskey bottle.

As we did not sample the whiskey bottle itself, we turned to a much more accessible means of examination—looking at the glass under ultraviolet (UV) light. UV light contains more energy than visible light (roughly 400–700 nanometers in wavelength), and it is this part of the electro-magnetic spectrum that causes sunburn (think sunscreen and UVA/UVB protection ratings). When viewed under UV light, with other ambient light removed, some materials fluoresce. Glass with leaded content usually glows a cool blue color when viewed under shortwave UV light (about 254 nanometers wavelength). As the Cork decanter was sampled directly from its pontil mark on the underside, it was useful to compare it to the Bushmills decanter. And voila! Both decanters presented the expected color for leaded glass—but the stopper, as expected, did not.

Two glass decanters glow electric blue in a dark black space with a decanter stopper, lit faintly pink, stationed between them.

The Cork Glass Company decanter (left, 1986-61-138) and Bushmills Whiskey decanter (right, 1986-61-167) glow blue under shortwave UV illumination while the latter’s stopper turns pink.

Though it is always best to confirm results discovered under UV light with other forms of laboratory analysis, for now we feel confident that the decanters contain lead and that the stopper belongs to a different bottle altogether. So even though these bottles themselves are transparent, the histories buried within each of them often take quite a bit of sleuthing to illuminate.

Sarah Barack is Senior Objects Conservator and Head of Conservation at Cooper Hewitt.



Dungworth, David and Colin Brain. “Late 17th Century Crystal Gass: An Analytical Investigation.” Journal of Glass Studies 51 (2009): 111–137.

Francis, Peter. “Franz Tieze (1842–1932) and the Re-invention of History on Glass.” The Burlington Magazine 136, no. 1094 (1994): 291–302.

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