Commercial Kiln Drying

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Commercial Kiln drying

7/26/13       
tommac Member

We are trying to put up a bright white soft maple product for our customers. The sap content is note an issue as we put it up 95% and better sap. The concern comes up that some of the stock has pink cast and some a grey cast. When you try and call this bright white it doesn't work very well. Is this pink or grey cast a product of the log sub-species or is it the age of the log or the drying process? Our graders state that they see this color coming off the log in the mill on fresh cut logs and also gatewood.

7/26/13       #2: Commercial Kiln drying ...
Josh Member

To start at the log I would ask,
1.What season is this most notable for this pink/grey casting?
2.How long are the logs stored in the yard prior to sawing?
3.Are you sawing through all the logs in one run, if not are your logs being rotated for the next run?
4. Also where your logs are coming from geographically? I know this question may be something that is not possible to change, but nonetheless could be a factor.
Good luck, I hope this was a help.

8/5/13       #3: Commercial Kiln drying ...
GeneWengert-WoodDoc

#2 on Josh's list is a key item. The oxidation that turns the wood grey or pink begins in the log, although the bad color will not show up when sawn.

Pinking is often enhanced by slow drying. In fact grey colors are also.

8/5/13       #4: Commercial Kiln drying ...
Josh Member

In my experience greying can also be a factor of jumping your schedule to fast for the MC. I'm sure everyone uses a similar "Bright White" Schedule, just may have their own tricks on how to use it in their kilns.

8/5/13       #5: Commercial Kiln drying ...
Gene Wengert-WoodDoc

Greying occurs above 40% MC, although the color may not show up until later at a lower MC. Speeding the drying by using a higher temperature too soon can make the grey darker, but in most cases does not cause the grey.

8/6/13       #6: Commercial Kiln drying ...
Josh Member

In a perfect world we try to get Maples in the kilns ASAP, but we run into the trouble between sawing to get it out of the log yard and scheduling of kilns. I have read some studies from Madison about timelines in the intensity of greying in a controlled environment but they seem impractical in comparison to the industry with how much time Maples can spend in the yard. I guess my question is what you think are ideal conditions and time for air drying without sacrificing color?

8/8/13       #7: Commercial Kiln drying ...
GeneWengert-WoodDoc

The time on warm weather is a day or two. Perhaps more important is the initial humidity and air flow in the kiln. Errors in heating up to 100 F can prove fatal, as well as the wrong RH after the DB is reached. Remember, it is the actual conditions and not the desired or schedule that determines the color.

8/15/13       #8: Commercial Kiln drying ...
tommac Member

Thanks Josh and Gene for your inputs.
You've given us some good information to work with to solve our problem.

8/15/13       #9: Commercial Kiln drying ...
tommac Member

Wwith regards to this discoloration happening in the logs, in the beautiful wet summer we have had, how long can the logs sit before we see these color changes?

8/15/13       #10: Commercial Kiln drying ...
Gene Wengert-WoodDoc

A week or two of warm weather is probably the limit. It depends on how hot, etc.


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