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Using Chemicals to Visualize Prints

Silver Nitrate:
A substantial portion of the solid latent print residue consists of sodium chloride, In the presence of silver nitrate; the chloride ion will react to form silver chloride. Although silver chloride is colorless it can be transformed by ultra violet light into silver. Silver shows up the latent print as black or reddish-brown in color,

1. Using a cotton swab, and working on a paper towel, brush a 3% solution of silver nitrate in water onto a card on which you have placed a print from your right thumb.
2. Air-dry the card for several minutes and then expose it to the UV light. (Do not get the silver nitrate on your skin… do not look into the UV lamp!)
3. Allow the print to develop and retain for your lab record.
4. An alternate method is to pipette a few drops of the silver nitrate solution directly onto the print and them proceed as above.

Iodine Fuming:
Iodine crystals, upon being heated, sublimate into a vapor, These vapors combine with the constituents of the latent print to make it visible.

1. Place your right thumbprint on a piece of filter paper and then quickly tape the paper to the top of the covered glass jar containing iodine crystals.
2. These jars are placed on a warming tray to hasten the fuming process,
3. After the print develops, remove the paper (use tweezers) and quickly re-cover the glass jar.
4. Your print is not permanent, but perhaps will last long enough to be handed in with the lab results. Have instructor initial the developed print.

Ninhydrin:
The development of a latent print with ninhydrin depends on its chemical reaction with amino acids and proteins present in trace amounts in perspiration to form a pink or purplish compound. Generally, prints begin to appear within an hour or so after application; however, the development can be hastened if the treated specimen is heated on a hotplate or in hot air oven at a temperature of 80 to 100 C. Ninhydrin is a suspected carcinogen and therefore you must adhere strictly to the directions!

1. Place your right thumbprint on a piece of filter paper and write your initials on it in pencil.
2. When a number of prints have been assembled the teacher will tape them to the back wall of the hood in room 327P. 
3. The teacher will then spray the samples with ninhydrin while the hood fan is in operation.
4. You will obtain your print (handle with tweezers) and allow it to first dry, if not already so, and then develop it on one of the hotplates provided in the lab.
5. The paper will be safe to handle without tweezers once it is dry.
6. Retain the developed print for your lab report.

Crazy Glue Fuming:
Observe the demonstration and examine the developed prints when they are available.