Continental Trade Sp. z o. o.


Mica shields

Mica shields provides the best protection possible, to flat transparent, sight glasses in water and liquid level gauges and columns in high pressure steam boilers. The shields retain basic strength and increase the service life of sight glasses when used at increased steam pressures and high working temperatures. 

Mica shields also provide perfect visibility for monitoring the nature or degree of combustion in blast furnaces of steam boilers. Sight glasses in steam boilers are exposed to harsh operating conditions (thermal, mechanical and chemical). On the surface of the glass crystallized deposits of mineral substances interact aggressively on the surface of the glass, especially at higher temperatures and pressures, and depending on the pH of the liquid. This interaction not only leads to tarnishing glass, but also the erosion, manifested by decreasing the thickness of the sight glass, which reduces its strength and may lead to dangerous failure. Glass losses for unprotected sight and gauge glasses, depends mainly on the pH and the temperature of the boiler water, could reach millimeters per month (see. fig. below). 

Glass erosion without mica shield.



Mica shields should be MANDATORY applied where the steam pressure exceeds 40 bar and the temperature exceeds 200 ° C.

Natural mica has high heat resistance, is transparent; optically clear, flexible, flat and unbreakable. Transparency of mica is unaffected by temperature changes as well as the action of gases and vapors in an environment where it works. In addition, mica is resistant to shock, vibration or deformation and therefore is used as a protection of glass exposed to radiation, mechanical damage and the action of corrosive chemicals.



Chart of glass erosion without mica shield. Typically, mica shields are supplied as complete with gauge glass and seal (here's an example set), but can also be supplied separately. We can also supply plates for non-standard sizes and shapes (see  also pages describing Mica windows and Mica disks in the chapter Our offer/Mica/Natural mica/Mica sheets ).

Mica shields provide excellent protection against acids, steam, water, air and oil at high pressure and temperature, and eliminates the risk of cracks, fractures, shock, burning, rinse the glass.

Mica shields are made from the highest quality natural transparent muscovite mica (species Rubby Stained A) and supplied in various lengths, thicknesses and widths according to customer needs.

Mica as a natural mineral, has a layered structure, but you must not separate these layers ! Thanks to the phenomenon of adhesion, naturally created mica layers guarantee its tightness. Any attempt to interfere (delamination) may cause loss of tightness and a significant reduction in the effectiveness of the mica shields.

Before delivery, all mica shields are electronically tested for quality and reliability. They are supplied in individual packages to eliminate scratches and protect them from damage.

Operating and maintenance instructions are enclosed with each package.



NoType A/BType AType B/HType TA-28
D   S D   S D   S D   S
0 95 x 24 95 x 30 95 x 34      
I 115 x 24 115 x 30 115 x 34 133 x 47
II 140 x 24 140 x 30 140 x 34 158 x 47
III 165 x 24 165 x 30 165 x 34 183 x 47
IV 190 x 24 190 x 30 190 x 34 208 x 47
V 220 x 24 220 x 30 220 x 34 238 x 47
VI 250 x 24 250 x 30 250 x 34 268 x 47
VII 280 x 24 280 x 30 280 x 34 298 x 47
VIII 310 x 24 320 x 30 320 x 34 338 x 47
IX 320 x 24 340 x 30 340 x 34 358 x 47
X 340 x 24 370 x 30 370 x 34 378 x 47
XI 360 x 24 400 x 30 400 x 34      
XII 370 x 24 430 x 30 430 x 34      
XIII 400 x 24 460 x 30 460 x 34      
XIV 430 x 24 500 x 30 500 x 34      
Mica shields dimensions

D = length, S = width 

All dimensions are given in mm. 

Standard thickness G:

0,10 - 0,15 mm / 0,13 - 0,17 mm
0,15 - 0,20 mm / 0,18 - 0,20 mm
0,20 - 0,30 mm / 0,30 - 0,40 mm

The above table does not reflect the full range of shields. We also provide mica shields in different sizes according to customer requirements. Please contact us.

How to install mica shields ?

Designation of elements:

1.        Graphite gasket
2.        Mica shields
3.        Transparent glass
4.        Gasket
5.        Nut

6.     Body
7.     Screw
8.     Cover
9.      Protective pad
10.   Mica shield with a hole

Mica quality classification based on visual properties

Quality classification based on visual properties

Depending on quality, in D-351 ASTM standard, there are 12 categories of mica sheets (note: some companies use their own classifications, which may contain a different number of categories). Bellow are described the various categories (form V-1 to V-10A) in accordance with the specifications of ASTM, also used in our company. A lower number of category means, generally speaking, the better the quality of the mica plate.

Name Description
V-1 Clear  Hard, of uniform color, nearly flat, free of all stains, foreign inclusion, cracks, and other similar defects.
V-2 Clear and Slightly Stained Hard, of uniform color, nearly flat and may contain slight crystallographic discoloration, and very slight air inclusions and not more than one fourth of the usable area.
V-3 Fair Stained Hard, of uniform color, may contain slight waves, slight crystallographic discoloration, and slight air inclusions and not more than one-half of the usable area.
V-4 Good Stained Hard, of uniform color, may contain medium waves slight crystallographic discoloration, and medium air inclusion in not more than two-third of the usable area.
V-5 Stained A Quality Hard, may contain medium air inclusions, uniformly distributed in the usable area; slight green vegetable stains, medium waviness, and heavy waves if specified.
V-6 Stained B Quality
Hard, may contain heavy air inclusions and heavy waves, medium green vegetable stains, slight black and red dots (mineral) and clay stains.
V-7 Heavy Stained Hard, and may contain heavy air inclusions and waves, slight light black and red dots (mineral), medium cloudy stains, clay stains and green stains (vegetable). Soft, buckles, ridges, and sand blast acceptable if specified.
V-7A Densely Stained Hard and soft. May contain heavy waves and air inclusions, cloudy stains. High black and red dots (mineral). Medium black and red stained (mineral), buckles, and ridges. Also, green stain (vegetable type), clay stains, herringbones, and sand blast.
V-8 Black Dotted Hard, may contain medium waves, heavy air inclusions, cloudy stains, light black and red dots (mineral), and green stains (vegetable).
V-9 Black Spotted Hard, may contain medium waves, heavy air inclusions, cloudy stains, light black and red dots (mineral), and green stains (vegetable type), slight black stains (mineral), and sand blast.
V-10 Black Stained Hard, may contain medium waves, heavy air inclusions, cloudy stains, light black and red dots (mineral), green stains (vegetable type), and sand blast, medium black stains (mineral), slight red stains (mineral), and clay stains.
V-10A Densely Black and Red Stained  Hard, may contain heavy waves, air inclusion, cloudy stains, light black and red dots (mineral), red stains (mineral), black and red stains (mineral), green stains (vegetable type), and sand blast, very dense black and red stains (mineral), and slight clay stains. Soft if specified.

Detailed specification of each category is contained in the following table, which as a PDF file is available here for download.

Standard qualities of mica (acc. ASTM Classification D 351)

Standard qualities of mica (acc. ASTM Classification D 351)

  V-1 V-2 V-3 V-4 V-5 V-6 V-7 V-7A V-8 V-9 V-10 V-10A
Crystallographic discoloration X *D *D *D * * * * * * * *
Air inclusions Very slight X * * * * * * * * * * *
Slight X X * * * * * * * * * *
Medium X X X *E *F * * * * * * *
Heavy X X X X X * * * * * * *
Cloudy stains X X X X X X *G *H * * * *
Light black and red dots (mineral) X X X X X *D *D *H * * * *
Black stains (mineral) X X X X X X X *G X *D *G *H
Red stains (mineral) X X X X X X X *G X X *D *
Black and red stains (mineral) X X X X X X X X X X X *
Green stains (vegetable type) X X X X *D *G *G * * * * *
Clay stains * * * * * *D *G * X X *D *D
Waviness Nearly flat * * * * * * * * * * * *
Slight X X * * * * * * * * * *
Medium X X X * * * * * * * * *
Heavy X X X X X * * * X X X *
Hardness Hard * * * * * * * * * * * *
Soft X X X X X X S * X X X S
Stones and holes X X X X X X X X X X X X
Buckles X X X X X X S *G X X X X
Reeves X X X X X X X X X X X X
Ridges X X X X X X S *G X X X X
Tears X X X X X X X X X X X X
Cracks X X X X X X X X X X X X
Hairline cracks X X X X X X X X X X X X
Wedge X X X X X X X X X X X X
Tangle sheet X X X X X X X X X X X X
Herring bones X X X X X X X * X X X X
Sand blast X X X X X X S * X * * *


Permissible *
Not permissible X
Permissible only if specified S
Few and tiny in 1/4 of usable area A
Few in one half usable area B
Very dense C
Slight D
Few in two-thirds usable area E
Uniformly distributed F
Medium G
Heavy H

While every attempt has been made to verify the source of the information, no responsibility is accepted for accuracy of data.

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