A primary source is any record contemporary to an event or time period. Primary sources may be written, oral, visual or physical. Some of these sources were produced with the intent of being preserved for the future. Such intentional sources include government documents, church records, autobiographies or memoirs. On the other hand, many primary sources were produced without any intent of future use. Such unintentional sources may include private correspondence not originally meant for posterity but which later are deposited in archives and libraries. Physical evidence such as buildings, clothing, tools, and landscapes may also be labeled as unintentional sources.
--Galgano, Michael J., J. Christopher Arndt, and Raymond M. Hyser. Doing History: Research and Writing in the Digital Age. Cengage Learning, 2007, p. 57.
There are certain words that appear in the subject headings of items in MERLIN that constitute primary sources. The most important of them is sources, but there are others.
In the simple keyword search box, you can put such words after SU: in order to specify the SUBJECT field.
ex.: british east india and SU:sources
In the advanced keyword search, you can put your search term(s) in the top box, and in the second box, put the word(s) you're using to locate primary sources in the next box, and change the drop-down menu to Subject:
Keywords that will help you find primary sources:
Depending on the period being studied, it can also be helpful to limit your search by publication date.
More keywords identifying primary sources in Library of Congress Subject Headings