Chemical exposure, decoding the limits

When talking about exposure to hazardous chemicals, the safety industry has developed some acronyms that make it difficult to understand what the actual exposure limitations are.

ppm: parts per million

OSHA: Occupational Safety and Health Administration http://www.osha.gov/

NIOSH: National Institute for Occupational Safety and Health http://www.cdc.gov/niosh/

ACGIH: American Conference of Governmental Industrial Hygenists http://www.acgih.org/home.htm

TWA: Time Weighted Average. The averaged exposure to a chemical over certain period of time (usually 8 hours for OSHA).

PEL: Permissible Exposure Limit. Usually provided in either ppm or in a concentration such as mg/M³ (micrograms per cubic meter) over an 8 hour TWA.

STEL: Short Term Exposure Limit. Provided in either ppm or in a concentration such as mg/M³ (micrograms per cubic meter) over a short term time period such as 15 minutes. This is also a TWA (time weighted average).

REL: Recommended Exposure Limit. This is purely a NIOSH term that takes into account OSHA data as well as “real life”, and sets a limit at which health and life may be affected for exposures beyond the REL.

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Glass is a (pick one)

Amorphous solid
Liquid
Huh?

Like many materials, glass has different forms, depending on temperature. It can be solid, it can be liquid, and at high enough temperatures, it will break down to its constituent components and turn into a gaseous state.

I like to think of glass as being similar to water. Because of its lower temperature transitions, we can easily see water in all three states: solid, liquid, and gas. At temperatures of 32 F and below, water is a solid. At temperatures between 32 and 212 it is liquid. At temperatures above 212 it is in a gaseous state (vapor).

I’ve never understood why people have to think glass is any different.

And no, glass does not move, or at least in any time frame that humans can see. Perhaps molecular movement may occur over tens of millions of years, but if anyone ever tells you that the reason old glass is thicker at the bottom is because the glass flowed that way is full of crap. Old glass was usually poured out in molds, and the installers always put the thickest part of the pane at the bottom simply because it was easier to handle that way.

You gotta love these little gems…

On WetCanvas there is a thread from back in 2005 about flame annealing, a topic which was dispensed with in several of my blog entries. From out of nowhere, this little gem showed up a couple of days ago:

I make miniature marbles and beads, everything I do miniature is flame annealed. All it really mean is heating the glass to a uniform temperature, once it is heated to a uniform temperature it is annealed, uniform cooling is a seperate issue really regardless of if it’s annealed in a kiln or in the flame. That’s my two cents but I’ve been flame annealing for ten years.

Continue reading

Garaging and annealing, two different aspects of the same beast

You already know that your kiln is a multipurpose device. You can use it (depending on the model and capabilities) for annealing, slumping, fusing, maybe making PMC.

Garaging is the practice of keeping your work hot, above the strain point, but below the annealing point. Any time you put a finished bead or pendant in the kiln, but before you start the annealing cycle, you are ‘garaging’.

Garaging is also the practice of keeping parts hot prior to assembling a finished piece from those parts. Continue reading

POOP or POPO – does it really matter?

The short answer is NO, it really doesn’t matter whether you turn on and turn off your torch using Propane – Oxygen – Oxygen – Propane or Propane – Oxygen – Propane – Oxygen, as long as you do it the same way all the time.

Some people will undoubtedly be horrified by this — but honestly, it really doesn’t matter, as long as both the propane and oxygen are turned off at the end of the session. I want you to just get into a habit of doing it the same way all the time, hence the mnemonic phrase POOP or POPO. And because of the scatological reference of the mnemonic it can easily be remembered (not to mention bringing a smile and chuckle when you are teaching it to a new student).

Ms. Nomers, revisited

“Flame annealing”, another favorite of mine.

Folks, there’s no such thing. And for my proof, let’s look at a typical kiln annealing cycle for soft glass:

  1. Hold at 960 F for 1 hour
  2. Drop to 850 F over 30 minutes, hold for 10 minutes
  3. Drop to 500 F over 2 hours
  4. Drop to room temperature over 2 hours

I don’t know anyone who can do that on a torch. Some folks do what they call “flame annealing”, but in reality, they are equalizing the temperature of the piece before allowing it to cool, so that all the parts are of an equal temperature to reduce the possibility of heat stress fracturing.