The annealing range that I work with has a very broad range, from 900F to 1000F. Higher annealing temperatures can cause surface luster oxidation (burn-off) in reduction colors, and color shift (over-striking) in striking colors. Annealing is a time/temperature relationship. Typically, the lower the temperature, the longer the soak time. Most small glass objects have had their strain relieved within 30 minutes at 950ish. Slow cooling then prevents the accumulation of additional thermal stress. Kilns vary to a certain degree in the temperature shown on the controller vs the heat-work at the location of the bead. Additionally, kilns do not hold a steady constant temperature. Almost all are relay-fired, turning on at 100%, then OFF, then on 100% in sequence to produce an average temperature equal to the setpoint. The actual temperature may spike to 1000, shut off, and fire again when the temp drops to 950, yielding and average temperature of 975. Better controllers, or those that have been tuned properly, will experience a more limited overshoot/undershoot, and therefore hold a more true setpoint. The best course of action is to estimate the appropriate temperature and time, note your results/issues, and adjust accordingly. I have never personally experienced luster oxidation in the kiln, and therefore do not set my annealer to a lower temperature. I typically anneal at 950, and shut the kiln off after an hour or two. If I am intentionally trying to kiln strike a color, such as Pandora2 or RH446, I might set my annealer at 1000F and leave it there for hours on end. If your beads are losing their luster, turn down your kiln. Some people place a small stainless steel dish of activated charcoal in the annealer to consume excess oxygen, preventing luster burn-off. When the Italian companies list their annealing temperature as 968F, I believe they are listing the technical annealing temperature which indicates strain relief in a 15 minute time period. Since many glass artists work for several hours at a time, a lower temperature can be used without fear of breakage. In short, if your beads are losing their luster, over striking, or if you are getting kiln-floor imprints on your beads, lower the temperature and/or shorten the total soak time.
If your beads are breaking, and they have been held at 900something or higher for at least 30 minutes, I would look into bead construction, thermal strain, chill marks, or incompatibility as the culprit.
I prefer to make a small (10mm) ball on the end of a clear punty to test and practice with. This conserves glass, and also relieves me of the emotional attachment and time invested in a bead or sculpture. In this way I am able to test or practice striking a glass without undue loss of time or material.
