Mercury Tube Darkening
Types of Tube Stains and What Causes Them
By John Tomasko

Mercury tube darkening is preventable, and each type of darkening is associated with certain specific causes. The ability to recognize the different types of stains and a knowledge of their causes, make it possible
to correct imperfect and to incorporate procedures which ensure best results. This article will describe in detail each type of darkening and its causes and remedies. It will also discuss sign tubing components and equipment as they effect darkening.

Darkening

It must be pointed out that a tube may have several things wrong with it at one time. Sometimes these faults can be recognized individually and sometimes not. Most types of darkening are stains which start out light in
color and gradually darken and spread with age. Older stains of certain types tend to look more and more alike. Therefore, identification of the various types is much easier when tubes are new then when they have aged. It will be assumed in the following that only one defect is present in each instance.

(1.0) Darkening Around and Next to the Electrode:
There are at least three different types of darkening occurring at the electrode. These can be differentiated by their color, especially when tubes are new

(1.1) The first type of darkening is called "sputter," and it appears as a black spot or ring just in front of the electrode or anywhere on the electrode glass. It is mainly a metallic throw-off from the electrode shell. During the life of the tube it may extend a distance of two to three inches from the electrode into the tube. It may extend back towards the electrode press, covering all of the electrode glass. The end of the dark area away from the electrode shell is usually well defined.

Sputter may appear during or after bombarding. If it occurs during bombarding it is caused by overbombarding (by heating the electrodes too hot, for too long a period of time, at too low a pressure, at too high a current). If it occurs after bombarding, the conditions above may still be the cause, but there are other possibilities. The cause may be operating the tube at too low a fill pressure, operating the tube at a current higher then the electrodes are rated, or electrodes with poor emission coating.

Older tubes normally show some electrode sputter over time. This is a normal operating condition. Ceramic collar electrodes show much less effect of normal sputtering over time.

(1.2) The second type of darkening starting from the electrode gives the clear glass a greenish tint when the tube is lit. It may be yellowish or brownish when the tube is not lit. It starts just at the open end of the electrode shell and the edge of the stain, at the end away from the electrode, is generally somewhat diffused. As the stain ages, its color becomes darker. It creeps farther into the tube, covering the coating and preventing it from fluorescing.

This stain is associated with the emission coating inside the electrode. Electrodes with uneven emission coating weights are difficult to heat uniformly and will produce this type of stain.

Conditions which cause the emission coating to produce this staining are:

(a) Incomplete conversion of emission coating during bombarding. Not getting electrodes hot enough.
(b) Using unmatched pairs of electrodes (which causes one to heat and convert much sooner then the other).
(c) Inadequate pumping/evacuation system.
(d) Presence of water vapor in processing system.

(1.3) The third type of stain appearing at the electrodes is a grey cloud extending varying distances into the tube. It is caused by vaporizing mercury out or off of an electrode. The vaporized mercury condenses on the
nearest cool surface, forming a grey cloud. For this reason mercury should never come in contact with a hot electrode. This type of stain can be dissipated by gently heating the area with a tipping torch. Use caution with
this technique.

(2.0) Darkening Starting a Short Distance From the Electrode

(2.1) The first type of darkening is associated with ceramic collar electrodes and takes the form and color describes in (1.2.) It is different only in that there is a clean area between it and the electrode when the
stain is new. As the tube ages, the stain may extend in both directions and close the original gap between it and the electrode. It has the same cause and remedy described in (1.2).

(2.2) The second type of stain, starting two or more inches away from the electrode, can cover an area 6 to 8 inches long, or when very bad, the whole length of the tube. It may start out as a light discoloration and may, but not always, become completely black with age. There is a definite line of demarcation between the stain and the clear area adjacent to the electrode. This stains caused by moisture in the tubing or processing system.

(2.3) The third type of stain has an appearance somewhat similar to the one in (2.2) and is produced by small amounts of air in the tube. The air stain has a fairly well defined edge at both ends. It may move from one section of the tube to another and may even disappear over time. If a moving stain appears it is wise to check the integrity of the pumping system for leaks. Tubes with air in them generally run very hot.

(2.4) The fourth type of stain at this location is a dark ring, generally quite narrow. It appears about 1 to 1 1/2 inches away from the electrode splice and may also appear on both sides of other splices and next to bends.
(A stain which locates right on a splice is detailed in 3.0). In itself, this stain is quite innocuous, and it does not become worse with age. These rings are produced by locally heating moist tubing and by allowing combustion
products from the fires to enter the tubing. The number of rings appearing can be reduced greatly by making bends only on tubing that is completely dry and by making splices only after bends have been completed. Be sure to store all tubing in a dry environment, in sealed bags to prevent introduction of moisture.

(3.0) Dark Bends and Splices:
Bends and splices may develop a discoloration which starts out grey in color and later turns black. In extreme cases, this darkening may be seen before pumping but usually not until after pumping and operating the tube. Only the heated areas of bends and splices and up to
half an inch on eight side is affected. It occurs mainly in tubing containing tungstates (blue), or halophosphates (whites). This means, for practical purposes, commercially available blue and standard white glass.

The discoloration is caused by overheating the glass while working it in the fire. Gentle heating prevents this.

(4.0) Darkening Covering the Entire Tube:
The whole tube may appear grey, brown, yellow, or black. The discoloration may be uniform over the whole length of the tube or there may be an area of full brightness. This darkening sometimes looks like the dimming that is due to lack of mercury vapor. One can determine which it is by heating a small section of the tube with a hand torch. If only the heated section brightens, the tube is stained. If a brightness develops over a wider area, it is lack of mercury.

Older tubes will show decreased brightness due to degradation of phosphor output. Tubes run on overloaded transformers will also show lower output then normal.

If the condition actually is a darkening or staining over the whole length of the tube, it can be caused by a number of different things. These causes are listed below in order of decreasing probability:

  • Poor vacuum conditions. Inadequate pumping capacity. Leaking manifold system.
  • Inadequate electrode bombarding.
  • Glass not heated sufficiently during bombarding.
  • Contaminated mercury.
  • Contaminated tubing.

(5.0) Irregular Dark Patches

Darkening of this kind takes the form of stains of irregular sizes and shapes, colors and locations. Stains such as these are usually caused by foreign matter in the tube including:

  • Bits of rubber, from hose or swivels, cause small roundish stains with a definite dark center, Usually these are yellow in color.
  • Oil from diffusion pumps, stopcock grease, or manometers produce a light yellow stain going around the tube, extending a great distance.
  • Mercury collects around "cold spots" such as tube supports, and makes small unsightly dark spots.
  • Moisture in the tube can produce bands and streaks, usually quite narrow.
  • Mercury run across hot electrodes produces thin black streaks for up to several inches. Avoid contacting mercury with hot electrodes.
  • Stains shaped like half moons and located close to the electrode are caused by overbombarding. Follow electrode processing instructions.

Please use this article as a reference to diagnose any staining problems. Refer to our recommend solutions to make your life easier.

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