Measuring how to anneal glass.

If I've got a bit of glass, and a scrap of the same glass, can I tell
by measuring the viscosity (say how much it drapes over a form in
a few minutes) at a given temperature how much cooler it needs to be
to anneal?

The simple test for annealing temp is to measure the sag temp. To do this
take a long slender piece of the glass and either prop it on supports at
both ends or support one end between two fire brick. Bring the temp up
slowly until it just begins to sag. [In reality, pretest to get a rough
idea of the sag point, then put in the test piece, start about 20-40F below
that temp, raise the temp 5 degrees and let it settle. If the glass does
not move, raise it 5 more and hold, etc.] The sag and annealing temps are
related to specific viscosities which are very hard to measure.
The annealing temp is taken to be 90F or 50C below the sag temp. You
should be able to hold at the annealing temp without sagging. All the thin
glass should be held at that temp for an hour before starting down. Thick
glass, like paperweights, should be held longer. See schedules in books.
The idea is to work out the strains as quickly as possible without sagging
the glass and then cool slowly enough not to introduce new strains.

--
Mike Firth
Hot Glass Bits Furnace Working Website
http://users.ticnet.com/mikefirth/hotbit46.htm Latest notes

"Ian Stirling" wrote in message
...
If I've got a bit of glass, and a scrap of the same glass, can I tell
by measuring the viscosity (say how much it drapes over a form in
a few minutes) at a given temperature how much cooler it needs to be
to anneal?

Mike Firth wrote:
The simple test for annealing temp is to measure the sag temp. To do this
take a long slender piece of the glass and either prop it on supports at
both ends or support one end between two fire brick. Bring the temp up
slowly until it just begins to sag. [In reality, pretest to get a rough
idea of the sag point, then put in the test piece, start about 20-40F below
that temp, raise the temp 5 degrees and let it settle. If the glass does
not move, raise it 5 more and hold, etc.] The sag and annealing temps are
related to specific viscosities which are very hard to measure.
The annealing temp is taken to be 90F or 50C below the sag temp. You
should be able to hold at the annealing temp without sagging. All the thin
glass should be held at that temp for an hour before starting down. Thick
glass, like paperweights, should be held longer. See schedules in books.
The idea is to work out the strains as quickly as possible without sagging
the glass and then cool slowly enough not to introduce new strains.

Many Thanks.
Don't have books, have kiln

Do you have a book title that I can check out of the library?

Probably not at the library. Take a look at
http://users.ticnet.com/mikefirth/biblio.htm and check the titles. Henry
Halem's Glass Notes covers the topic, among others.

--
Mike Firth
Hot Glass Bits Furnace Working Website
http://users.ticnet.com/mikefirth/hotbit46.htm Latest notes

"Ian Stirling" wrote in message
...
Mike Firth wrote:
The simple test for annealing temp is to measure the sag temp. To do
this
take a long slender piece of the glass and either prop it on supports at
both ends or support one end between two fire brick. Bring the temp up
slowly until it just begins to sag. [In reality, pretest to get a rough
idea of the sag point, then put in the test piece, start about 20-40F
below
that temp, raise the temp 5 degrees and let it settle. If the glass does
not move, raise it 5 more and hold, etc.] The sag and annealing temps
are
related to specific viscosities which are very hard to measure.
The annealing temp is taken to be 90F or 50C below the sag temp. You
should be able to hold at the annealing temp without sagging. All the
thin
glass should be held at that temp for an hour before starting down.
Thick
glass, like paperweights, should be held longer. See schedules in books.
The idea is to work out the strains as quickly as possible without
sagging
the glass and then cool slowly enough not to introduce new strains.

Many Thanks.
Don't have books, have kiln

Do you have a book title that I can check out of the library?

Mike Firth wrote:
Probably not at the library. Take a look at
http://users.ticnet.com/mikefirth/biblio.htm and check the titles. Henry
Halem's Glass Notes covers the topic, among others.

Thanks again.

I suspect I probably could - last book checked out
"Huzel & Huang - Design of liquid propellant rocket engines".
Interlibrary loan is fun.