Glass process fundamentals: batch, continuous-tank melting, fire-finishing, and lehr annealing

Sources: SHA “Glassmaking & Glassmakers” and “Bottle Finishes” (Bill Lindsey, https://sha.org/bottle/glassmaking.htm, /finishes.htm); Wikipedia “Glass production” and “Annealing (glass)”; Kopp Glass “Basics of Glass Annealing”; EPA AP-42 §11.15; Corning Museum of Glass. Ingested: 2026-07-26 (agent research).

Batch — the soda-lime-silica recipe

• Silica sand — the glass-former (network builder), ~60% of the mix. (Three Rivers used local Carrizo quartzose sand.)
• Soda ash (Na2CO3) — the flux: pure silica melts near ~1700 C; soda cuts the working temperature into ~1400-1600 C.
• Lime / limestone — the stabilizer: makes the glass chemically durable/water-insoluble (soda-only glass would dissolve).
• Cullet — recycled/broken glass added to help the batch melt faster/cheaper (typically 14-30%); virtually every plant used it (the Three Rivers brochure simplifies and omits it).
• Batch-house sequence (matches the brochure): raw materials weighed -> hopper -> high-speed cylinder mixer for homogeneity -> charged into the furnace.

Melting — the continuous tank furnace

By 1922 bottle houses used continuous regenerative tank furnaces (not older pot/day tanks): batch is charged at one end (the “dog house”), melts, and flows continuously toward the working end. Regenerative brick “checkers” preheat combustion air with waste heat (Siemens principle) for efficiency; furnaces are gas- or oil-fired (Three Rivers touted local natural gas). The furnace runs 24/7 — reheating a 125-ton tank from cold is wasteful and damages the refractory — so Three Rivers’ “45 tons of molten glass drawn daily” is a steady-state flow. Between melting and forming the glass is fined (bubbles/seeds rise out) and conditioned (cooled to gob viscosity), which is why it takes “3.5 days from raw material to finished bottle.”

Fire-finishing

Every forming machine leaves a sharp/rough edge at the mouth. Fire-finishing (fire-polishing) reheats just that rim with a direct flame until the surface melts and rounds over — smooth and safe. (This is distinct from the mouth-blown era’s separately-applied/tooled finish.) Three Rivers’ “five blue-flame gas jets” melting “the sharp edge inside the finish ring” is a textbook description.

Annealing in the lehr (NOT tempering)

Glass cools unevenly — the skin hardens while the interior is still hot — locking in internal stress that makes a bottle crack spontaneously under minor shock. Annealing relieves this: a lehr (a long, temperature-zoned tunnel oven on a moving belt) holds the ware near the annealing point (~454-482 C) so stress relaxes, then cools it slowly and evenly through the strain point. Three Rivers’ ~2-hour lehr transit, with a “Hartford-Empire stacker” loading the belt, is standard 1920s-30s equipment. Terminology flag: annealing relieves stress; tempering deliberately adds a compressive-surface stress profile — the opposite. The brochure’s process is entirely annealing; its gloss “annealed … means tempered” is a period lay-misnomer.

Inspection & testing (QC)

Visual sorting by packers, plus a lab battery targeting residual stress/strength: thermal-shock immersion (146 F water), a finish/wedge-expansion test in the neck, a pressure/burst test (Three Rivers cited “>600 lb” vs ~90 lb capping force), and the polariscope — polarized light makes stressed glass birefringent, showing residual strain as colored fringes (“prismatic reaction”), a non-destructive check that the lehr did its job.