By Richard LaMotte
When a sea glass collector finds a section of a bottle base, their opportunity to date that shard and potentially learn who produced the original vessel is far greater than if the shard is from an unmarked side wall of a bottle. Telltales include the thickness of the shard, bubbles within the core glass, kick-ups, pontil rod scars, and various other key markings. Such markings include a bottle maker’s mark, text, numbers, suctions scars, valve marks, stippling and knurling. Color also plays a vital role in judging the age of each shard—as always.
Early bottles had virtually no specific markings on the bottom, but later glassmakers began to include simple letters and then evolved into making complex patterns, the latter to increase the speed of 20th-century production. Colonial glassmakers often designed their bottle bases for simple stability, to avoid tipping and breakage. As transportation improved in the early to mid-1800s, the bases became thicker so that filled vessels could make numerous trips along rough roads and arrive safely. Some even had bottom thicknesses of 3/4 of an inch as purveyors hoped to get bottles returned for repeated uses. This was especially the case for mineral waters and beers of the early to mid-1800s. History would repeat itself in the early 1900s, as milk bottles with rather thick bottoms were designed to be sturdy, in hopes of getting at least 20 or more returns for refills on average.
Most bottle bases have at least a slight concave dome from the outer wall into its center. Some rise up rather abruptly toward the center of the vessel from the sidewall. Many wine bottles today still have this feature commonly referred to as a “kick-up.” The one exception to an indented base, of course, is the protruding rounded bottom on “torpedo-style” bottles designed to lie on their sides and never be placed upright. This was to keep the cork moist, so vessels with effervescent contents did not pop their corks during transport. The torpedo-style bottles are rare finds, and so are their shards. If a shard from the bottom of a glass vessel is virtually flat, then it was most likely from a drinking glass or vase.
In the 1700s, bottle production within our U.S. colonies was in its infancy, and many bottles were still imported from Europe with their highly valued contents in place. The initial designs were relatively simple, and most were in black or deep olive-green glass. The glass blowers of that era would inflate a “gather” of glass using a blowpipe and also use a pontil rod to indent the base to enable it to sit flat on its outer rim. A simple “Dutch Onion”-shaped bottle was common in the early 1700s. It soon gave way to more cylindrical shapes as the glass blower would roll the sides back and forth across a “marver,” a hard, flat table surface, such as marble, to give height to the outer walls. In the latter half of the 1700s, “Dip molds” were used more often to form the bottle bases more efficiently. The glassblower would inflate his gather of glass using his blowpipe, but do so while the bottom half and wall were held within a wooden or metal Dip Mold resembling a modern-day wine chiller. Thus, most of the vessel was inflated into a defined shape.
During the beginning of the 1800s, many bottle bases were still being formed with Dip molds. Soon after, larger hinged-molds grew popular, forming most of the bottle’s shape. Some bottles had bottoms at least 3/8 of an inch thick, and several, especially mineral water bottles, had bases closer to a half inch in thickness. Also, in the mid-1800s, bottoms were often left with “pontil scars” in the center of the base from the iron pontil rod or had a small blob of glass attached to the bottom from being held in place by the pontil rod. By 1870, most of the crude-style bottle bases were disappearing, and full-sized bottle molds were taking their place. During the second half of the 1800s, more colors were being introduced as production quantities increased. While vessels with mold seams and the addition of text and numbers increased after 1870, most mass-produced bottle bases at the turn of that century were unmarked.
By the early 1900s, bottle-machine automation took hold and never let go. This led to thinner and more uniform bottle bottoms, often with thicknesses of 1/4 inch or less. Most included numbers and bottle-maker marks. Mold numbers were added to identify which cavity within a larger mold might need attention in case of problems. Machine-made bottles could be produced with very few bubbles in the glass, and the vessels had a uniform wall thickness on the side and bottom. Thus, finding a bottle base with bubbles in the core of the glass or a variable thickness usually indicates a pre-1900s shard. In the early 1900s, some bottle bottoms had faint circles on their base. A large circle that spans most of the diameter of the bottom is known as a suction scar and is frequently off-center. A small circle of about 1/2 inch in diameter is a valve mark. Both are indicators of early mold marks between 1910 and 1950.
By the mid-1900s, bottle bases commonly featured a variety of textures so that mass production could move faster than ever. By the 1950s, many bottle bottoms were made with tread-like curved ribs on their outer edge known as “stippling” or a uniformly textured pattern across most of the bottle bottom known as “knurling.” These helped bottles move faster from forming stations into the “lehr,” a temperature-controlled kiln for slowly cooling glass, without sticking to conveyors.