What is Pyrex made of

How to recognize Pyrex mirrors

Hello astronomical friends

In connection with Far East and Taiwan Dobsonians has recently been
occasionally discussed the properties of the various types of glass
and how to recognize the materials.

In this post we would like to explain how to use common materials
recognizes. Specifically, it's about how to get a pyrex mirror from others
Can distinguish mirror carriers.

It's easy to use Pyrex from other types of glass like BK7 or window glass
to distinguish. when a mirror has a density (specific gravity) of
around 2.5, it is definitely not a Pyrex. If a 25 millimeter
thick mirror with 200mm diameter weighs more than 1800 grams, it is with
Security no Pyrex.

Pyrex is pale greenish-yellow-brownish.
BK7 is crystal clear and completely colorless.
Window glass is bright green.

Color: BK7 is crystal clear and completely transparent - BK7 is also called
Front element used in Fraunhofer lenses! Pyrex is pale greenish
with a tinge towards yellowish-brownish. Window glass (float glass) is
also green, but much stronger than Pyrex and with one
Stitch direction bluish. In direct comparison, the coloration is Pyrex
significantly warmer than the cool, blue-green window glass. Another one,
Occasionally used material - sometimes as plate glass, or sodium
Designated UltraPlex - at first glance it clearly looks like BK7
off, but on closer inspection it does not have complete transparency but rather
acts like a light gray filter with a 20cm glass path.

It is important to hold the mirror against a white surface to see the colors
to be able to distinguish better.

You can find photos at http://www.intercon-spacetec.de/ratgeber/spiegelsorten.html

The density ("specific gravity") of the mirror material is practical
safe way to distinguish Pyrex from other types of glass. The
Density of Pyrex is 2.23; which is from BK7, Fensterglas and Plate Glass
2.50. That is a difference of 12% - a mix-up is therefore
practically impossible. You only need a ruler and a scale to get around
determine the density of a mirror.

With the Taiwan mirrors we know, the mirror (not the
mirrored surface!) almost always a diameter of exactly 200 mm; the
most are exactly 25 mm thick. A Pyrex mirror with these dimensions weighs
weighs about 1680 grams, a mirror made of BK7, window glass or plate glass
about 1890 grams.

More information, and a calculator that lets you check the density of a
You can find out how exactly you can calculate the mirror on our advisory pages
The advantages of Pyrex over BK7 and cheap mirror mounts can also be found there.

Stefan Schuchhardt
Intercon Spacetec
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