Yellow discoloration in oak

      Causes of yellow staining in kiln-dried oak are discussed. October 2, 2001

We are lumber importers specializing in oak (European oak and US white oak). All of the European lumber arrives fresh sawn and we dry it ourselves. Thicker stuff (6/4" and up) goes into classical low temperature (40-50 C) convection kilns. 4/4 and 5/4 goes into higher temperature (80 C) kilns with forced ventilation (Bollman - Germany).

On several occasions we've had problems with a pronounced yellow discoloration that goes deep into the grain and does not disappear with planing. We cannot find a constant factor in the different cases. All thicknesses are involved. Although we find it mostly in European oak, I have seen it in American white oak, too (backing boards). It happens in both of the kiln types. The affected wood comes from different sawmills. There are no similarities to be found in the conditions before kiln drying (under covered shed; in open air) or during kiln drying. There are no problems during the drying cycles. Some bundles have it, others do not.

This is quite a mystery to us. Here is a picture of the discoloration.

Forum Responses
When I was at Virginia Tech, we had this stain analyzed and it turned out to be a precursor involved with lignin. So, it seems to be related to the tree and something to do with its growth, growth stress, or whatever. As such, control is beyond the kiln operator.

Gene Wengert, forum technical advisor

Yellow discolouration of European and American white oak might be caused by a fungi infection (paecilomyces variottii). This was shown by Professor Bauch in Germany in 1991. The discolouration only occurs in heartwood, though the fungi also live in sapwood. The actual colouring substance is assumed to be the result of some kind of reaction between the fungi and extractives naturally occurring in the heartwood.

The fungi like high surface MC and it commonly occurs around stickers. The problem increases with low air speed and the use of thin stickers (less then 15 mm or 3/5). The fungi survive in temperatures up to 50C (122F) and may thereby survive the early stages of conventional kiln drying. It sometimes occurs during air-drying and might thereby develop during storage and transportation prior to the kiln drying.

A way to diminish the problem is to increase the air speed and ensure an even distribution of the airflow through the kiln load. It is also possible, but not recommendable to use chemical treatments. In some cases, e.g. thick European oak, it might even be economical to shift technology in to vacuum drying to avoid this and other problems.

Would you like to add information to this article?
Interested in writing or submitting an article?
Have a question about this article?

Have you reviewed the related Knowledge Base areas below?
  • KnowledgeBase: Primary Processing

  • KnowledgeBase: Primary Processing: Kiln Operation

  • KnowledgeBase: Primary Processing: Lumber Grading

  • KnowledgeBase: Wood Engineering

  • KnowledgeBase: Wood Engineering: Wood Properties

  • KnowledgeBase: Knowledge Base

    Would you like to add information to this article? ... Click Here

    If you have a question regarding a Knowledge Base article, your best chance at uncovering an answer is to search the entire Knowledge Base for related articles or to post your question at the appropriate WOODWEB Forum. Before posting your message, be sure to
    review our Forum Guidelines.

    Questions entered in the Knowledge Base Article comment form will not generate responses! A list of WOODWEB Forums can be found at WOODWEB's Site Map.

    When you post your question at the Forum, be sure to include references to the Knowledge Base article that inspired your question. The more information you provide with your question, the better your chances are of receiving responses.

    Return to beginning of article.

    Refer a Friend || Read This Important Information || Site Map || Privacy Policy || Site User Agreement

    Letters, questions or comments? E-Mail us and let us know what you think. Be sure to review our Frequently Asked Questions page.

    Contact us to discuss advertising or to report problems with this site.

    To report a problem, send an e-mail to our Webmaster

    Copyright © 1996-2016 - WOODWEB ® Inc.
    All rights reserved. No part of this publication may be reproduced in any manner without permission of the Editor.
    Review WOODWEB's Copyright Policy.

    The editors, writers, and staff at WOODWEB try to promote safe practices. What is safe for one woodworker under certain conditions may not be safe for others in different circumstances. Readers should undertake the use of materials and methods discussed at WOODWEB after considerate evaluation, and at their own risk.

    WOODWEB, Inc.
    335 Bedell Road
    Montrose, PA 18801

    Contact WOODWEB

  • WOODWEB - the leading resource for professional woodworkers

      Home » Knowledge Base » Knowledge Base Article