Photographic emulsion - Shashin Nyuuzai

It is a dispersion of fine silver halide crystals in an aqueous gelatin solution. The size of the silver halide crystals varies depending on the application, but is generally around 0.1 μm to several μm. In special cases, crystals less than 0.1 μm or around 10 μm are used. Low-speed emulsions use silver chloride or mixed crystals of silver chloride and silver bromide, while high-speed emulsions use silver iodobromide. Both silver chloride and silver bromide are NaCl-type crystals and can form mixed crystals in any ratio, but in the case of silver iodobromide, the crystal structure of silver iodide is different, so in practice the proportion of silver iodide is only a few percent. To manufacture an emulsion,
(1) Silver halide crystals are precipitated by mixing an aqueous silver nitrate solution with an aqueous alkali halide solution in the presence of gelatin. There are various mixing methods, such as the single jet method and the double jet method. Gelatin acts as a protective colloid and disperses the precipitated crystals.
(2) The crystals are grown to the aforementioned size by heat treatment at several tens of degrees Celsius in the presence of an excess of alkali halide or ammonia (physical ripening).
(3) To remove excess halides and water-soluble salts,
(a) Add gelatin, cool it to gel, then shred it and wash it with running water, or
(b) Gelatin is added to entrap the silver halide crystals and allow them to settle, the supernatant liquid is discarded and the mixture is washed several times with water.
(4) Gelatin, water, etc. are added to the desalted emulsion, and a small amount of chemical sensitizer is added and heat-treated again. During this time, no crystal growth occurs, but photosensitive nuclei are formed and the sensitivity increases (chemical ripening).
The emulsion prepared in this way contains an equal or lesser weight amount of gelatin as the silver halide, and is coated onto a support such as film, glass, or paper. The film thickness after drying is usually 15-25 μm for normal negative materials and about 5 μm for photographic paper.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

微細なハロゲン化銀結晶をゼラチン水溶液中に分散したものをいう．ハロゲン化銀結晶の大きさは用途によって異なるが，一般的には，0.1 μm から数 μm 程度である．特殊な場合には0.1 μm 以下あるいは10 μm 程度の結晶も用いられる．低感度乳剤には塩化銀または塩化銀と臭化銀の混晶，高感度乳剤にはヨウ臭化銀を用いる．塩化銀と臭化銀はともにNaCl型結晶であり，任意の割合で混晶をつくるが，ヨウ臭化銀の場合はヨウ化銀の結晶構造が異なるため，実用的にはヨウ化銀の割合は数％ である．乳剤を製造するためには，
(1)ゼラチンの存在下で硝酸銀水溶液とハロゲン化アルカリ水溶液とを混合してハロゲン化銀結晶を析出させる．混合の方法にはシングル・ジェット法，ダブル・ジェット法など各種の方法がある．ゼラチンは保護コロイドとしてはたらき，析出結晶を分散させる．
(2)過剰のハロゲン化アルカリ液またはアンモニアの存在下，数十 ℃ で熱処理して結晶を前述の大きさまで成長させる(物理熟成)．
(3)過剰のハロゲン化物・水溶性塩類などを除去するため，
(a)ゼラチンを加えて冷却ゲル化後細断し，流水などで水洗する，あるいは，
(b)ゼラチンを加えてハロゲン化銀結晶を包含させて沈降させ，上澄み液を捨て数回水洗する．
(4)脱塩した乳剤にゼラチン・水などを加え，化学増感剤を微量添加してふたたび熱処理する．この間には結晶の成長は起こらず，感光核が生成し，感度が上昇する(化学熟成)．
このようにして調整された乳剤は，ハロゲン化銀と等重量またはそれ以下のゼラチンを含み，フィルム，ガラス，紙などの支持体上に塗布される．乾燥後の膜厚は，通常のネガ用材料で15～25 μm，印画紙で5 μm 程度である．

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