Mercury Darkening

Types of Tube Stains and what causes them

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

	Darkening Around & Next to the Electrodes
	Darkening Starting a Short Distance Away from the Electrode
	Dark Bends & Splices
	Darkening Covering the Whole Tube
	Irregular Dark Patches

I 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 are old. It will be assumed in the following that only one defect is present in each instance.

Darkening Around and Next to the Electrodes:

There are at least three different kinds of darkening occurring at the electrodes. They can easily differentiated by their color when the tubes are new and, with more difficulty, by color and extent when they are older.

The first kind is called sputter, and it appears either 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 also extend back toward the electrode press, covering all of the electrode glass. The end of the dark area away from the shell is usually fairly well defined.

Sputter may appear during or after bombing. It occurs during bombing, it is caused by over bombing 9by heating the electrodes too hot, for too long a time, at too low a pressure, at too high a current.) If it occurs after bombing, the conditions above may still have been the cause, but, in addition, there are other possibilities. The reason may be low filing pressure, operating the tube on a current higher than the design limit of the electrodes, the use of uncoated electrodes or of electrodes having an inadequate supply of emission coating. Old tubes normally show some electrode sputter, and this is to be expected. Ordinarily the well-processed tube should show a little or no sputter during the first few hundred hours of operation.

The second kind of darkening staring from the electrode gives the clear glass a greenish color 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 staining grows older, its color becomes darker. It creeps farther and farther away into the tube, covering the coating and preventing it from fluorescing. It can eventually cover the whole tube.

This stain is associated with the emission coating and never appears when uncoated electrodes are use. Electrodes that have uncontrolled weights of emission coating are particularly difficult to bomb and tend to produce this kind of stain. Emission coatings that ate completely and properly converted do not produce it unless they become contaminated.

Conditions, which cause the emission coating to produce this stain, are as follows

	Incomplete breakdown of the emission coating during bombardment
	Under heating and/ore incompletely degassing the emission coating and electrode shell (similar to above.)
	Poisoning of the emission coating or absorption of impurity gases resulting from:

An inadequate pumping system

	The presence of water vapor
	Inadequately heated glass on the pump
	Contaminants in the tube or pumping system (No other stain appears as frequently as this one. The cure lies primarily in bombing procedures.)

The third type of stain appearing at the electrodes is a grey cloudy formation extending varying distances into the tube. It is caused by vaporizing mercury out of or off of an electrode. The vaporized mercury condenses on the nearest cool surface, forming the grey cloud. For this reason, mercury should not come into contact with a hot electrode. Rolling mercury in and out of an electrode can sometimes produce a black streak of contaminated mercury on the tube wall. (See section V) The grey cloudy stain may disappear or over as the tube is operated.

II. Darkening Starting a Short Distance away from the electrode:

There are four kinds of stains, which take this location.
Three can be serious, but the fourth is generally unimportant.

The first kind is associated with ceramic type electrodes and take the form and color described in the first Section. It is different only in that there is a clean area between it and the electrode when the stain is new. As the tube grows older, the stain may extend itself in both directions and close the original gap between it and the electrode. The edge of the stain at the end away from the electrode is diffused. It has the same cause and remedy described in the first Section. Apparently, at the start, the ceramic focuses the stain a little farther away than does the non-ceramic electrode.

The second kind of stain, staring two or more inches away from the electrode, can cover an area of 6 to 8 inches in length or, when very bad, the whole length of the tube excepting near the electrodes. 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 this stain and the clear area adjacent to the electrode. The edge of the stain at its end away from the electrode is indeterminate. This stain is caused by moisture in the tubing or in the pump system.

The third kind of stain has an appearance something similar to the one above and is produced by small amounts of air in the tube. The air stain, however, has a fairly well defined edge at both ends. It may move from one section of the tube to anther and may finally even disappear altogether. If a moving stain appears, it would be wise to check the vacuum system for leaks. A large amount of air will cause the tube to run very hot. The whole tube is likely to darken.

The fourth kind of stain at this location is a dark ring, generally quite narrow. It appears about 1 to 1 � inches away from the electrode splice and may also appear on both sides of other splices an next to bends. (The stain, which locates right on the splice or bend, is described in Sec. III. In tubing containing calcium silicate (pink or Ca-Whites) the ring sometimes has a pink color. In green tubing, it may be a lighter green than the rest of the tube. Generally, however, it is dark grey. In itself this discoloration 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 the bends have completed moisture can be kept out of the tube, no ring will appear.

III 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 area of bends and splices and up to half an inch or so on either side is affected. It occurs mainly on tubing containing tungstates (blue power) or halophosphates (white powder). This means, for practical purposes, any blue or white tubing (whites may contain either tungstates or halophosphates or both). For dark rings near the bends and splices see section II, fourth kind above.

This discoloration is caused by overheating the glass when working it in the fire. Gentle heating prevents it. The glass should be heated only enough to make a strong splice without much "melting in, " just enough to make bends without kinking.

The discoloration on bends and splices in glass that has been heated too much can be reduced greatly by running clean water through the tubing before the electrodes are attached. Few minutes of full flow suffices. (A water flow that is too forceful my wash the powder out of the tube). The tubes must then be IMMEDIATELY and THOROUGHLY dried by flowing air through them before attaching the electrodes. Oven drying is helpful, but it should not be attempted before drying with the air. If tube is still moist when it is put into the oven, a number of small spots will appear. Water washing will reduce the brightness of the halophosphates tubing to some extent and is therefore recommended only for those cases where it is known that too much heat has been applied.

IV Darkening Covering the Whole Tube:

The whole tube may look grey, brown, a depressed yellow or black. The discoloration may be uniform over the whole length of the tube, or there may be an area of normal brightness located near the middle. (See Sec. II, third kind).

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 with a torch and watching the change in brightness. If only this section brightens, the tube is stained. If brightness develops which extends some distance beyond the heated area, it is lack of mercury.

Darkening of the entire tube may also be confused with other low light output conditions. If the tube has a uniform low brightness without any evidence of stains, it is probably just old. Tubes operated on an over-loaded transformer also may look dim and stained when they may be merely operating at a reduced current.

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 caused are listed below in order of decreasing probability:

Poor vacuum conditions. Either a slow pump or a large air leak in the manifold can cause this kind of staining. Wet tubing can also do it by slowing down the pump and/ or by contaminating the emission coating.

Inadequate electrode bombarding.

Glass not heating sufficiently during pumping. This can be particularly troublesome when using no-bake tubing.

Too short a pumping and bombarding period

Contaminated mercury

Contaminated tubing. Again, no-bake is difficult to process

V Irregular Dark Patches:

Darkening of this kind takes the form of stains of irregular sizes and shapes, color and locations. This group includes those stains, which do not ordinarily cover the entire circumference of the particular area in which they are located. Foreign matter in the tube usually causes stains such as these.

	Poor vacuum conditions. Either a slow pump or a large
	Bits of rubber give small roundish stains with a definite darker center. They are frequently yellow
	Small pieces of emissions coating make small irregular blotches. They are generally yellow or grey, but pieces, which fall into very hot fluorescent coating, may turn blue.
	Oil may give regular or irregular shapes and colors. It can also produce a light yellow stain going all the way around the tube and extending for a large distance. It is likely to turn black. (The oil may come from the splicing fires, the diffusion pump, stopcock grease, oil manometers or "gremlins").
	Mercury collects around cold spots in a tube; such as near tube supports, and makes small but unsightly dark areas. Its effect on the fluorescent coating may be the same as described in below.
	Moisture in a tube can produce some strange looking bands/ streaks. These re usually quite narrow and are done essentially the same as described in Sec. II, fourth kind. If narrow, they apparently cause great harm.
	Mercury, contaminated by contact with an over bombarded electrode or hot electrode, may produce a number of very thin black streaks up to several inches long which are generally harmless. Sometimes the streaks may be as wide as one-quarter inch, in which case there can be a serious appearance defect.
	Stains shaped like half-moons and located close to the electrode are caused by over bombing. They seldom become serious.
	Heavy clusters of mercury droplets, sometimes in the shape of a large teardrop, have also appeared in connection with over bombing. The teardrops are usually a minor appearance defect unless the concentration of mercury is heavy. In such cases they may lift the fluorescent coating off of the glass, leaving bare spots.

This completes the list of important stains. Other types are occasionally seen. These, like the types discussed above, can be prevented. In fact, the cure for darkening is almost entirely a matter of bending, splicing and pumping technique.

(Voltarc is not responsible for errors in any printed material. Such errors are subject to correction.)