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--- projects:blinker:work_logs:2_window [2022/01/25 03:42] – [The Doing] tjhowse
+++ projects:blinker:work_logs:2_window [2022/02/17 02:24] (current) – tjhowse
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 ====== The window ======
 ===== The Plan =====
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 Second firing profile.
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+After the second firing. It definitely appears to have merged together more thoroughly. Some of the fine internal cracks have become invisible. I'm not sure whether this is because of the higher peak temperature or the longer time spend at temperature.
+I read that it was possible to harden glass by impregnating the surface with potassium. This is done in industry by immersion in a bath of molten salts. I have some potassium nitrate, so I filled a terracotta pot and buried the window inside and heated it up to 400°C for an hour.
+{{:projects:blinker:work_logs:pasted:20220131-035920.png}}
+The potassium nitrate bath.
+The bath turned a foamy grotty brown. Molten potassium nitrate should be clear. I suspect the stuff I have is impure, or it attacked the glaze on the terracotta pot. I let the bath cool down and harden and then chipped out the window. The potassium nitrate was a filthy grey-brown and full of bubbles. After the glass cooled down I was gently hitting it with a hammer, to enjoy the noise. After a fairly gentle donk, no harder than I'd hit it before, it developed a small, barely visible circular fracture. I'm not sure if the potassium bath weakened it, or whether the temperature profile was a factor, or whether I just hit it in a different way. I wanted to empirically test the strength of this window, so I will re-fire it to fix the fracture and test it afterwards.
+Sources disagree on the proper temperature profile for potassium-toughening glass, but the general consensus seems to be over 400°C for many hours. If I try this again I will try to get some higher-purity potassium nitrate, and soak it at 425°C for 30 hours.
+===== The Video =====
+{{ youtube>HKjkeqgr-0Q?large }}
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