In the last half-century the reputation of alum as an ingredient of paper has never been lower. Is this condemnation justified? In some classes of paper the presence of alum is undoubtedly harmful; but in others the role of alum has been misunderstood as indeed the form in which it is present. Appendix I of this book gives a brief account of the history of true alum (as distinct from hydrated aluminium sulphate, which took its place during the last decades of the 19th C.). This is based on Professor Singer's monumental study of the chemical together with an account of its extraction and purification as practised in North Yorkshire from the 17th-19th Cs., a process described by members of the Cleveland Industrial Archaeology Society.
The last part of Appendix I of The Whatmans and Wove paper is devoted to the subject of alum in paper and the essential role it has played for centuries in bonding the derived protein, gelatine, to the cellulose matrix. What has been overlooked is the all-important form of the gelatine when it is mixed and reacted with alum to form insoluble aluminium hydroxide, a precipitate stable between pH 4-9, into which the hydroxyl groups of the cellulose and gelatine are attracted and cross-link.
In The Extracts section, you can read the significant investigation which appears in Appendix I. This has four sections, spread over three pages: