b. The physical properties of Collodion affected by the proportions and purity of the Solvents.—Pyroxyline of the varieties termed B and C dissolves freely in a mixture of Ether and Alcohol; but the characters of the resulting solution vary with the relative proportions of the two solvents.
When the Ether is in large excess, the film is inclined to be strong and tough, so that it can often be raised by one corner and lifted completely off the plate without tearing. It is also very contractile, so that a portion of the Collodion poured on the hand draws together and puckers the skin as it dries. If spread upon a glass plate in the usual way, the same property of contractility causes it to retract and separate from the sides of the glass.
These properties, produced by Ether in large proportion, disappear entirely on the addition of more Alcohol. The transparent layer is now soft and easily torn, possessing but little coherency. It adheres to the surface of the glass more firmly, and exhibits no tendency to contract and separate from the sides.
From these remarks it will be gathered that an excess of Ether, and a low temperature in preparing the Pyroxyline, both favour the production of a contractile Collodion; whilst on the other hand an abundance of Alcohol, and a hot Nitro-Sulphuric Acid, tend to produce a short and non-contractile Collodion.
The physical properties of Collodion are affected by another cause, viz. by the strength and purity of the solvents, or, in other words, their freedom from dilution with water. If a few drops of water be purposely added to a sample of Collodion, the effect is seen to be to precipitate the Pyroxyline in flakes to the bottom of the bottle. There are many substances known in chemistry which are soluble in spirituous liquids, but behave in the same manner as Pyroxyline in this respect.
The manner in which water gains entrance into the Photographic Collodion is usually by the employment of Alcohol or Spirit of Wine which has not been highly rectified. In that case the Collodion is thicker, and flows less readily than if the Alcohol were stronger. Sometimes the texture of the film left upon evaporation is injured; it is no longer homogeneous and transparent, but semi-opaque, reticulated, or honeycombed, and so rotten that a stream of water projected upon the plate washes it away.
These effects are to be attributed not to the Alcohol, but to the water introduced with it; and the remedy will be to procure a stronger spirit, or, if that cannot be done, to increase the amount of Ether. Collodion prepared with a large proportion of Ether, and water, but a small quantity of Alcohol, is often very fluid and structureless at first, adhering to the glass with some tenacity and having a short texture; but it tends to become rotten when used to coat many plates successively, the water on account of its lesser volatility accumulating in injurious quantity in the last portions.
THE COLORATION OF IODIZED COLLODION EXPLAINED.
Collodion iodized with the Iodides of Potassium, Ammonium, or Zinc, soon assumes a yellow tint, which in the course of a few days or weeks, according to the temperature of the atmosphere, deepens to a full brown. This gradual coloration, due to a development of Iodine, is caused partly by the Ether and partly by the Pyroxyline.
Ether may, with proper precautions, be preserved for a long time in a pure state, but on exposure to the joint action of air and light it undergoes a slow process of oxidation, attended with formation of Acetic Acid and a peculiar principle resembling in properties ozone, or Oxygen in an allotropic and active condition. Iodide of Potassium or Ammonium is decomposed by Ether in this state. Acetate of the Alkali, and Hydriodic Acid (HI), being first produced. The ozonized substance then removes Hydrogen from the latter compound, and liberates Iodine, which dissolves and tinges the liquid yellow.