(2 grrn.) Citric acid . . 18 gr.
grm.) Water, to make . . 4 Oz.
(100 c.c.) As the outcome of an experimental study of this trouble, Lumiere and Seyewetz (1903) recommended bathing the negative for 5 minutes in a 0.1 per cent neutral solution of potassium permanganate ; after rinsing, the negative is placed in a solution of 5 per cent to io per cent bisulphite of soda, in which the silver oxide formed by the action of the permanganate and also the brown colour of manganese dioxide formed in the gelatine disappear. The process is finished by washing in two or three changes of water.
434. Defects Occurring During Washing. Reti culation. The reticulation of gelatine, giving it the appearance of grained leather or crocodile skin, is due to the excessive swelling which may arise from various causes. One is transfer from a very concentrated or warm fixing bath into cold water, especially if the fixing bath contains no alum. Another is a considerable difference in temperature, one way or the other, between the developer and the fixing bath. In general, reticulation is liable to be caused by any circumstance tending to cause very rapid swelling or shrinkage of the gelatine, such as transfer from a very alkaline bath to one which is strongly acid, or inversely. When the reticu lation is not very marked, it is sometimes possible to remedy it by placing the negative in alcohol (with films, at least 20 per cent of water must be added to the alcohol), and then hardening it with alum, if necessary, before proceeding to any other operation.
Frilling of the Gelatine at the Edges. This defect, which starts as a kind of curling along the edges of the support, is due to the same causes as reticulation ; the same methods may be tried in order to prevent it.
Transparent or Clear Spots of Irregular Shape. Local attack of the gelatine by liquefying bacteria during prolonged washing, particularly in warm weather.
435. Defects Appearing During or After Drying. Partial Melting of the Gelatine leading to Irregular Deformation of the Image. The
negatives have been subjected to a temperature above the melting point of gelatine whilst in a wet and insufficiently hardened condition. They may have been placed too near a fire or in the direct rays of the sun.
Patches of Uneven Density. Local variations in the rate of drying due to changes of tem perature, humidity, or speed of air currents. These patches may sometimes be got rid of by converting the image into silver chloride and re-developing as described in § 432 for removing developer stains.
Light Spots or Marks with Dark Edges. These marks are caused by drops of water left on the face of the gelatine during drying, or which have been splashed on to its surface after it has already dried. In course of drying these splashes dry first at the edges of the moist region and causes the particles of silver to be dragged from the centre towards the edge. Re-wetting of the negative, followed by normal drying, does not always provide a remedy for this.
Clear Spots of Bare Glass or Film. Local liquefaction of the gelatine by colonies of bacteria during very slow drying in a warm, moist atmosphere. Clear, irregular spots may be due to the attack of various insects on the gelatine.
Metallic Stains on the Edges of the Negative. Opaque fog of a lustrous metallic appearance, generally seen only on the edges of the negative, may be due to the use of an exhausted developer with old emulsions, or to a superficial sulphiding of the silver by sulphuretted hydrogen (often present in the atmosphere of industrial towns). It may usually be removed by dry rubbing with chamois leather; in obstinate cases, moisten the leather with methylated spirit in order to increase the friction, but in any case rub very lightly.
Traces of Foreign Embedded in the Gelatine. Dust, particles of fibre, etc., deposited by the washing water and not removed by a final rinse in filtered water, or deposited during drying.