Alum seems to be the mordant of choice for most dyers - in every resource I have read it so far it is either the first or only one mentioned. Vejar only uses alum mordants in The Modern Natural Dyer as "they are nontoxic, safe for the environment, accessible, and produce bright, long lasting colour" (2015, 58). The fact that it produces bright colour and does not sadden or darken the colour is a real positive.

Alum is referred to as safe in many other reliable sources and so I can assume it is. Alum is a chemical, and Casselman argues in Praxis and Paradox that: "there is no such entity as an entirely 'natural' dye. Many natural ingredients contain chemical ingredients such as mordants" (2009, 9)

The mineral has been mined for centuries, and Cardon covers the history of this in Natural Dyes. Most of the alum mined in Britain, until the 18th century, was sourced from the North Yorkshire coast, around Whitby. It is a finite resource, but as the third most abundant element in the earth's crust (AZOM, 2001, online) perhaps it is the best option to produce naturally dyes textiles that are colourfast. 

References:

• AZOM. (2001) Aluminium: The Resource. [Online] Available from: https://www.azom.com/article.aspx?ArticleID=309 [Accessed 3 December 2017].
• Cardon, D. (2007) Natural Dyes. London: Archetype.
• Casselman, K. (2009) ‘Praxis and Paradox: The Culture of Natural Dyes in Britain 1750-1900’. Textile: The Journal of Cloth and Culture., 1750-1900. 7 (1): 6-26. Available from: http://eds.a.ebscohost.com/eds/pdfviewer/pdfviewer?vid=6&sid=ebd42fd4-94bd-4e1e-b7c2-d59b6345707f%40sessionmgr4010 [Accessed 3 December 2017].
• Vejar, K. (2015) The Modern Natural Dyer. New York: Abrams.