COE 33 Boro Annealing Temperature and Soak Time

Short answer: The two big boro color makers publish the same working anneal: 1050 °F. Northstar Glassworks’ annealing chart is built on it and puts the strain point at 960 °F; Glass Alchemy’s color pages, such as its Peacock working notes, give a 1050 °F anneal and a 975 °F garage. For soak time, Northstar publishes the rule most boro workers use: about one hour at the annealing temperature per 1/4 inch of thickness, measured at the thickest part, followed by stepped holds at lower temperatures on the way down. The glass manufacturers’ datasheets read differently (Schott’s DURAN and Corning’s 7740 both give a 560 °C annealing point; Corning adds a 510 °C strain point and Schott’s sheet lists none), because they define a physical property rather than a studio soak. Use your glass maker’s schedule, and do not invent a shorter soak because a piece looks small.

This page is the boro-only companion to our general glass annealing schedules guide, and it sticks to figures the makers themselves publish. If you need the controller side, see kiln controller programming.

What the makers publish

Sources: Northstar annealing chart; Glass Alchemy, Peacock working notes; Schott DURAN datasheet; Corning 7740 product information. Fahrenheit conversions of the Celsius figures are ours, rounded.

Editor’s note: these figures disagree, and the disagreement is expected. The Schott and Corning numbers are the glass’s annealing and strain points as datasheets define them, measured on the glass those companies make (Schott’s one-page sheet gives no strain point at all); the color makers publish the soak temperature they recommend for their own rod, frit, and tube in a studio kiln. When you work Northstar or Glass Alchemy glass, their schedule is the one to follow; when you work clear tubing from a specific maker, check that maker’s sheet. Clear boro quality and Pyrex, Duran and Simax cover who makes the clear.

Soak time by thickness: Northstar’s rule

Only one of these sources publishes a soak time scaled to the thickness of lampworked pieces, and it is Northstar. Its chart sets the time at the annealing temperature at roughly one hour per 1/4 inch of thickness, then steps the kiln down in stages rather than switching off:

Northstar notes that each listed time includes the ramp down to the new temperature as well as the time spent there. Its own worked example is a piece one inch thick: about four hours at the annealing temperature, then down 125 °F for another four hours, then shorter holds as the kiln steps lower.

Three practical points follow from the chart. Thickness means the thickest part, not the average, so a thin vessel with a solid foot is annealed for the foot. The step-down is part of the anneal, not an optional extra; the slow descent through the range below the soak is where new stress would otherwise form. And the time grows fast: a sculpture with a two-inch section is an overnight cycle and more, which is why solid boro sculpture treats the kiln as non-negotiable.

Editor’s note: Northstar’s chart is the only thickness-scaled boro soak rule we found published by a glass maker. Shorter community rules of thumb circulate; we do not reproduce them, because the makers do not. Kiln uniformity, thermocouple placement, and piece geometry all affect what your kiln actually delivers, so test a schedule on a clear piece before trusting it with finished work.

Garage, then anneal

Garaging is holding finished work at temperature during a session so nothing cools into the stress range before the anneal runs. Glass Alchemy’s color notes, such as its Peacock page, give 975 °F as the garage, below its 1050 °F anneal and above Northstar’s 960 °F strain point. Boro worker Mike Aurelius puts the distinction plainly: garaging keeps pieces safe while they wait, and it does not anneal them (Chaotic Glass). The full soak and step-down still have to run once the last piece is in. A piece that cracked between the torch and the kiln is a flame-side problem, covered in why is my borosilicate cracking in the flame.

When color changes the schedule

Boro colors do not all behave like clear. Northstar notes that colors with matching expansion can still have different annealing characteristics, which is one reason a mixed color piece can check even on a correct clear schedule. So the fix for a cracking color piece is the right schedule for every glass in it, not simply a hotter or longer one.

Striking colors add a twist. Some boro strikers are kiln struck at temperatures above the anneal; Northstar, for instance, publishes a kiln strike for its amber/purple family at 1125 to 1150 °F. A piece struck that hot still has to come down through a normal annealing cycle afterward. Our silver striking guide covers the striking side, and boro color brands explained covers the makers.

A schedule you can program

Using nothing but Northstar’s published rule, a boro cycle is: garage during the session; soak at 1050 °F for about an hour per 1/4 inch of the thickest section; step down 125 °F and hold for the chart’s share of that time; take the three further steps at 200, 350, and 550 °F below the anneal for a quarter of the anneal time each; then let the closed kiln cool to room temperature. Each line becomes a ramp and hold segment on a digital controller. The glass maker’s schedule for your specific glass, and your kiln maker’s instructions, take precedence over this outline.

Related guides

• Glass annealing schedules: anneal points and soak times
• Kiln controller programming for glass annealing
• COE 104 annealing schedule
• Clear boro quality: Simax vs Schott Duran vs import clear
• Sculpting solid boro basics
• Boro color brands explained
• Why is my borosilicate cracking in the flame?

Sources

• Northstar Glassworks, Borosilicate Glass Annealing Chart
• Glass Alchemy, Peacock 546 (garage and anneal temperatures)
• Schott, DURAN tubing datasheet (borosilicate 3.3)
• Corning, 7740 product information (annealing and strain points)
• Northstar Glassworks, Amber/Purple family (kiln strike)
• Chaotic Glass (Mike Aurelius), “Garaging and annealing” (community knowledge)