1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  2. ATTENTION FORUM MEMBERS!

    Guest, if you have not yet updated your forum bookmark to a secure log in connection, please delete your unsecure book and add the following secure bookmark: https://www.aawforum.org/community/index.php

    You can dismiss this notice by clicking the X in the upper right of the notice box.

    Dismiss Notice

Hollow Form Cracks

Discussion in 'Woodturning Discussion Forum' started by Randy Anderson, Oct 1, 2020.

  1. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    I'm new to hollow forms. I've done about 10 so far with good success. These two failed on me yesterday after a few days of drying in a paper bag - my standard process for drying green single turned items. Walls are 1/4", opening is 1 1/2" and they're about 8 1/2" tall. I wouldn't win any contests on wall thickness consistency all the way to the bottom but in looking at my previous ones that warped but didn't crack I made the neck thickness thinner, around the inside curve and further down the wall. In all I've read here (lots of good info) it seems 3/16 or even less is the magic thickness to shoot for when turning green. I'm assuming wall thickness and bad luck caught me on these two (pecan and elm). Going to let them finish their dance on splits and cracks and then decide if a colored epoxy fill is worth the effort. I'm not a fan of epoxy fills that are noticeable and only use it clear to fill in small knots where it blends in, but rarely. That said, on "artsy" pieces I know it's a different approach so might be something to consider.
     

    Attached Files:

  2. Dave Bunge

    Dave Bunge

    Joined:
    Nov 22, 2009
    Messages:
    108
    Location (City & State):
    Midland, MI
  3. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    No cracks while turning and for the first few days in the bag it was fine. Like bowls, one day fine, next day a mess. I sometimes spray/mist things when they look like they're drying out while turning but didn't here. I have a big roll of the stretch wrap I use when I have to stop on something I'm turning for a while. Maybe in the future I stretch wrap the open end to help slow/even out the drying process while in the bag? I know some do that with single turned green bowls. They wrap the rim.
     
  4. Chris Lawrence

    Chris Lawrence

    Joined:
    Jun 13, 2020
    Messages:
    25
    Location (City & State):
    Jackson, NJ
    If you can plastic wrap the outside good and tuck it in the opening leaving it open for air it may work better without the paper bag. When i dry rough turned hollow forms that are going to get turned a second time when dry i coat the outside with anchor seal and leave the inside unsealed. With a small opening that will restrict how much moisture is released from the inside.
     
  5. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    Thanks. Really don't want to start twice turning them but maybe I should consider. Getting the outside shape just right and then hollowing to match sounds much easier. For now I'll try the plastic wrap around the open end. Have another pecan just like the cracked one keyed up to turn so a good compare.
     
  6. Curtis Fuller

    Curtis Fuller

    Joined:
    Jul 27, 2005
    Messages:
    143
    Location (City & State):
    North Ogden, Utah
    Although I've had occasional success (actually luck) finish turning green wood with the pith down the center, I had so few successful attempts I basically quit trying. Hollowing is a lot of work so I'd suggest you concentrate on a piece of wood that doesn't include the pith. The odds are stacked against your success.
     
    charlie knighton and hockenbery like this.
  7. hockenbery

    hockenbery AAW Advisor Staff Member

    Joined:
    Apr 27, 2004
    Messages:
    6,492
    Location (City & State):
    Lakeland, Florida
    Home Page:
    For a typical HF Curve looks fine at the top to allow wood move to.
    if the wall were 3/16 from rim through the foot that allows wood movement.
    1/4 should be Ok. Leaving the tenon on is asking for trouble.

    Big problem I see is the concentric growth rings.
    Each growth ring wants to dry to a smaller diameter than the growth ring inside It.
    The radial cracks a inevitable

    Pith in is a problem both before turning too very hard to keep blanks from developing radial cracks.
     
    charlie knighton and Bill Boehme like this.
  8. Bill Boehme

    Bill Boehme Administrator Staff Member

    Joined:
    Jan 27, 2005
    Messages:
    11,278
    Location (City & State):
    Dalworthington Gardens, TX
    Home Page:
    Al said what I was thinking, but Al's explanation of the concentric growth rings when you turn a hollowform centered on the pith really makes it clear.
     
    charlie knighton likes this.
  9. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    Interesting. I thought (or assumed) that pith centered hollow forms were common. I've done a few others like this black cherry one with no problem but perhaps I was just lucky. Actually the bottom cracks have been minimal and contained. I leave a small tenon foot but use my parting tool to cut a deep groove between the tenon and the bottom of the vessel. Thinking, maybe wrong, that it creates a relief point for cracks in the tenon that won't spread to the walls of the vessel.
     

    Attached Files:

  10. Dennis Weiner

    Dennis Weiner

    Joined:
    Jan 3, 2012
    Messages:
    179
    Location (City & State):
    New City, NY
    I don’t believe your luck was random. Moisture content, species, tree growth rate, turning Temperature and humidity environment are some of the factors that reduce/increase the likelihood of cracking. Knowing as many factors possible about the green wood you are turning may reduce the risk of cracking. One suggestion is never use reaction wood. I only attempt these pieces from the main trunk. The pith located in the center may be slightly better. Some may even drill out the pith and later plug the bottom.
    I wish that I knew more details that can increase your chances. I think Species to turn and avoid in this situation is critical. Can anyone recommend the best and worst end grain green wood To turn for hollow forms? Is there a table somewhere?
     
  11. hockenbery

    hockenbery AAW Advisor Staff Member

    Joined:
    Apr 27, 2004
    Messages:
    6,492
    Location (City & State):
    Lakeland, Florida
    Home Page:
    Perhaps the best wood for pith in hollow-forms is Norfolk Island. It has a hollow pith. One of its features is the ring of knots that radiate outward. Using the ring of knots means having the pith in The piece. The thousands of NIP hollow forms turned each month testify to its ease of use with the pith in.

    hollow forms made from cylindrical blanks an inch or more off the pith can be turned from any hardwood and dried crack free as long as you
    1 have an even walk thickness of 3/16” ( madrone you need to be an 1/8” for increased success)
    2 have curves that allow the wood to move - no flats or sharp angles
    3 control the drying
    4 have no pre existing cracks.

    if you want to ignore those guidelines or stretch them Soft maple like red maple can take a lot of guideline abuse without cracking. Camphor is another wood that is more tolerant of guideline abuse.
     
    charlie knighton likes this.
  12. Dave Landers

    Dave Landers

    Joined:
    Dec 1, 2014
    Messages:
    158
    Location (City & State):
    Estes Park, CO
    Home Page:
    I've had reasonable success turning Aspen around the pith.

    I try to remove the pith in the top opening, and keep the pith near the center of the foot. I generally aim for around 1/8" wall thickness, or a bit more. Smooth inside walls (not sanded-smooth, but get rid of the ridges where stress will concentrate).
    Turn it as promptly as I can (not hurry, but no dawdling).
    I will often spray mist the piece with water as I go if I feel it's drying out or wants to start cracking. And plastic wrap is also helpful.
    I finish the piece fully (finish the foot, get rid of the tenon) and try to keep the bottom thickness consistent with the sides.
    If the wood is too wet, I don't bother gumming up the sandpaper - wait and sand off the lathe, after it's dry.
    Usually they go in a paper bag for a while to slow down the initial drying.

    I do expect some failures, but those pieces still look good in the fireplace.
     
    charlie knighton likes this.
  13. robo hippy

    robo hippy

    Joined:
    Aug 14, 2007
    Messages:
    2,874
    Location (City & State):
    Eugene, OR
    The stretch film works fairly well to seal the outside of the form so it can dry from inside out, which supposedly causes it to shrink inwards, which was the theory behind the DNA soaks and wrapping the outside with newspaper. The problem here, might have been in part caused by the paper bag, and centering your pieces on the pith. I can't remember ever getting a log section that didn't already have cracks coming off of the pith. If you try to wrap maple with the stretch film, it will mold under the film.

    I did hear of a technique once for sealing those types of cracks, but never tried it. Soak the piece in a mix of 1/2 water and 1/2 standard wood glue. The piece swells up again which closes the cracks. When drying, the glue holds the cracks together. It was a while ago that I heard it, and never tried it. It didn't get to be very popular, so, I don't know...

    Since they are 'art' pieces, you may want to play with the epoxy resins.

    robo hippy
     
  14. charlie knighton

    charlie knighton

    Joined:
    Jan 20, 2006
    Messages:
    1,828
    Location (City & State):
    virginia
    get u some mesquite
     
  15. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    Thanks for the tips - my next round I'll go for thinner walls (3/16-1/8), keep wet while turning and remove tenon with bottom as thin as I can get it to match walls. Like with bowls, the wood always gets a big unpredictable vote in the process so best can do is minimize the risk. Agree, one of the downsides with the plastic wrap is it can accelerate mold growth almost overnight sometimes. My next piece of pecan had some small pith cracks already in motion so set it aside. Maybe the failure rate for these is just going to be higher than what I've been used to for bowls.

    I actually have a lot of mesquite - my favorite wood of all to work with. All I have left is in thick plank pieces so would mean glue up to get something suitable for a hollow form but, still an option.
     
  16. Michael Nathal

    Michael Nathal

    Joined:
    Jul 5, 2015
    Messages:
    122
    Location (City & State):
    Strongsville, Ohio
    Those cracks would definitely be a design opportunity for me. With dark wood like walnut, turquoise chips are my favorite. (not turquoise dust).
     
  17. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    Michael, maybe you could help me. I've used 2 part 5 minute epoxy sparingly on my bowls and really only when it blends into a knot and will give a smooth feel to the inside. I really never fill in the outside - their natural wood bowls, part of it. I want to use these cracked hollow forms (and the cracks are bigger and more of them now) to learn a bit about colored epoxy. I think they could look very nice if done right. I have a small bottle of black epoxy color. I tested it yesterday and yes - it's deep shiny black. I want to use turquoise or other colors and went to amazon - too many options and variations. Made my head spin. What do I order? Liquid or powder? Translucent? I read some reviews of complaints of it fading? Do you buy turquoise or mix it yourself? Thx.
     
  18. Chris Lawrence

    Chris Lawrence

    Joined:
    Jun 13, 2020
    Messages:
    25
    Location (City & State):
    Jackson, NJ
    I get my turquoise from tuckersturnings.com. Its not natural in the sense that it was taken from the ground its made in a lab from the same minerals that the real stuff has in it and looks almost identical. I get the blend that has a couple different size pieces mixed together. I just pack as much in the crack as i can leaving it a little proud then soak it with thin ca glue. Sand it smooth its hard stuff and will take the edge right off tools.
     
  19. Don Wattenhofer

    Don Wattenhofer

    Joined:
    Jul 19, 2018
    Messages:
    645
    Location (City & State):
    Ponsford, MN
    I can honestly say that I have successfully turned more than a hundred hollow forms with the pith and of course the concentric rings are a part of that. The form that Randy has used should work if the wall thickness is consistent all the way down to the small base. I always turn to about 1/2 to 3/4" walls then coat the end grain areas with anchor seal, which only leaves a narrow band of uncoated wood at the largest diameter, then weigh and record the weight and date. The splotchy look of those two forms would seam to indicate that the blanks had been cur for some time and drying has occurred. The end checking happens when the end grain rapidly dries may appear to go away but it does not re-bond and that is what it looks like to me.
    URN111a2.jpg This lidded hollow form in black walnut is about 7" at the larger diameter, about 3" at the base and 16" overall height. The concentric rings have less tendency to go oval during seasonal movement and therefore the threaded lid, which I apply on all similar forms, has less chance of locking up.
     
    Dennis Weiner likes this.
  20. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    The pecan was sealed but well aged as you can tell from the color. Pecan is normally much lighter in color. The black cherry success piece, and others, are much more recent cuts. Net is that perhaps very small pith cracks, wall thickness, etc all came together. The opening was also smaller than some of the others so the pith cracks likely stayed in the opening rim and were turned out in the ones with larger openings. Who knows. I really like the option of making hollow forms from the smaller logs I gather when I get wood and I like the pith center line look they have so will work to get my process to an acceptable fail rate with the advice here.

    Don, I'm assuming you twice turn or do you sand off the sealer?
     
  21. Don Wattenhofer

    Don Wattenhofer

    Joined:
    Jul 19, 2018
    Messages:
    645
    Location (City & State):
    Ponsford, MN
    Yes I rough turn (or as is being said more recently twice turned) I would never consider trying to sand off that wax. The drying process can take a year and I do a regular weighing starting every day at the beginning then two days then 3 then a week then a month etc. The weight will stabilize at some point when there is no change between weighing's, which is the best indication that the piece is dry. Note when I say dry it would be more accurate to say reach equilibrium with the environment which will never be 0%. I do a lot of goblet forms or very thin walled side grain bowls in once turned but I have never tried a hollow form once turned cause I don't know how I would cut the threads. As long as I am venting I have never seen the need to have a tiny opening since most of the time it is out of proportion to the design and just a lot of trouble for nothing.
     
  22. Bill Boehme

    Bill Boehme Administrator Staff Member

    Joined:
    Jan 27, 2005
    Messages:
    11,278
    Location (City & State):
    Dalworthington Gardens, TX
    Home Page:
    Mesquite is probably the most stable wood for pith centered hollowforms. The higher the tangential to radial shrinkage ratio is in drying wood the more likely you are to encounter cracking problems. The ratio for mesquite is very close to one. Here is some information about wood shrinkage from the Wood Database.
     
  23. Michael Nathal

    Michael Nathal

    Joined:
    Jul 5, 2015
    Messages:
    122
    Location (City & State):
    Strongsville, Ohio
    I also purchased turquoise chips from Tucker's turnings. He calls it "manmade turquoise" I am pretty sure it is NOT made from minerals but from resin. Very similar to Inlace nuggets. The Tucker product is much more blue in tint than the green-ish Inlace. The most intense color, also on the green side, I have used came from manmade turquoise that was actual minerals, I bought a necklace on Ebay for about $10 many years ago. I prefer the look of distinct chips rather than powder, more like grains of sand up to about an eight inch chunk. To me the powder tends to look like a colored epoxy fill. The attached photo is with Inlace nuggets, you can see the tint is on the green side. I typically use CA glue more often than epoxy. The Inlace chips and the Ebay necklace have to be crushed to get my preferred chip size. For woods that are lighter in color than walnut, I try to get a black background for better contrast with the turquoise. I usually paint the crack surfaces with a black acrylic paint before gluing in the chips, but black CA works well also.
     

    Attached Files:

  24. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    So how do you keep the CA glue for bleeding into the surrounding area and messing up the area for applying finish or oil? I've seen guys use sanding sealer, tape, etc but not tried. I typically avoid using CA, especially thin, where it's likely to leach out from where applied and create a snail trail look. Some of my cracks are very thin but would like streaks of the color in them if possible. They sell Mica powder online for coloring epoxy. Wonder if that works.
     
  25. Don Wattenhofer

    Don Wattenhofer

    Joined:
    Jul 19, 2018
    Messages:
    645
    Location (City & State):
    Ponsford, MN
    At the 2011 AAW symposium I attended Stephen Hatcher's rotation and I still have the handout book where he went into great detail on how he creates his stone inlay designs. The paragraph titled "SEALING THE WOOD" goes into detail on preventing the problem you describe by spraying the area adjacent to area to be filled with thinned out dewaxed shellac. The book has everything you could want to know about inlaying minerals and one of the main features is a mineral chart with name, color, color depth, clarity, hardness and image. I don't know if the book is available from AAW but you could check and or contact Stephen.
     
  26. Bill Boehme

    Bill Boehme Administrator Staff Member

    Joined:
    Jan 27, 2005
    Messages:
    11,278
    Location (City & State):
    Dalworthington Gardens, TX
    Home Page:
    My experience is that the darkening caused by thin CA is indistinguishable from the darkening caused by oil based polyurethane varnish or by solvent based nitrocellulose lacquer. I use thin CA a lot on mesquite which varies in color from light tan to dark chocolate brown.
     
  27. Tim Connell

    Tim Connell

    Joined:
    Jan 22, 2018
    Messages:
    148
    Location (City & State):
    Cameron, Illinois
    Ditto Bill's comments. I use a lot of tung oil on different woods and don't worry about staining from CA. It disappears after the first coat of oil.
     
  28. Don Wattenhofer

    Don Wattenhofer

    Joined:
    Jul 19, 2018
    Messages:
    645
    Location (City & State):
    Ponsford, MN
    I have to disagree I have used lacquer often and if I didn't seal with shellac before applying CA there would be a noticeable difference between the CA stained wood and no CA.
     
  29. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    If I plan to do some aggressive sanding or second turn then I will use thin CA on cracks. Even then I sometimes can spot the slight trail along the edges of the crack but depends on the wood and tone. Some soaks it up like a sponge and it will bleed out well beyond the crack. Others not so much. I'm sure moisture content plays a role. I use walnut oil and I guess it doesn't hide it near as well as some of what you guys are using. If I can get med or thick CA into the crack then I will but it doesn't penetrate as well and I wonder if I really did anything other than a surface layer. I'm thinking of giving some of the epoxy clay stuff a try. Not expensive and might be worth a shot to be able to squeeze into small cracks, smooth a bit and put where I would not have time or patience with 2 part liquid stuff.
     
  30. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    Sometimes what you think is a ruined piece can turn out to surprise you. I let the hollow forms posted at the start of this thread dry in open air. Do what you're gonna do and get it over with was my attitude. The pecan cracked a bit more but the neck cracks actually closed up a bit for me and left the opening in good shape. I was surprised. Mixed some gold mica powder (wife's choice) in with some epoxy, filled all the cracks and finished up this morning. I'm pleased with it and glad I didn't pitch it on the burn pile.
     

    Attached Files:

  31. Glenn Lefley

    Glenn Lefley

    Joined:
    Jul 10, 2017
    Messages:
    488
    Location (City & State):
    Invermere, British Columbia
    Here is link to Stephen Hatcher’s booklet https://s3.amazonaws.com/media.guidebook.com/upload/121751/831EhbE0fRC68qQr4eRy70aoRKpn6nEq9CVs.pdf
     
  32. Leo Van Der Loo

    Leo Van Der Loo

    Joined:
    Oct 13, 2016
    Messages:
    356
    Location (City & State):
    Rainy River District Ontario Canada
    Drying a hollow form has it's challenges, if we have a look at this, turning away the wood on the ends and the wall, we end op with thin engrain that will dry much quicker than the sides, so as it dries it shrinks but the sides are not as much because that side grain dries slower, if we can slow the endgrain drying that will help a lot to prevent or at least minimize the splitting.
    Of course that is not the only thing agains it, very often there is also sapwood and heartwood we have working against us, where sapwood will shrink more than the heartwood, also the outside will dry quicker than the inside even though it is thin or should be, and again this will give us splitting.
    Slowing down all the drying does help as it gives the wood time to slowly have the wood fibers move by all the tension there builds up in it, boiling wood is one of the ways to soften the lignin in the wood and release the tension, as it is the lignin that glues the woonfibers together, soften that up and the wood fibers are able to move more easily.

    Hollow forms.jpg
     
    Last edited: Oct 10, 2020
  33. Randy Anderson

    Randy Anderson

    Joined:
    May 25, 2019
    Messages:
    167
    Location (City & State):
    Eads, TN
    Home Page:
    Leo, from my limited experience with these I agree. The neck, if turned thin like the walls, dries very quick while the bottom not near as fast. I spray it with water while I'm hollowing to keep it from drying too fast before I can finish. The last few I've turned I wrapped the top half of the form and over the opening with plastic wrap and cut a hole for some air to get in and out. I then put it in a paper bag. So far so good.
     
  34. John Tisdale

    John Tisdale

    Joined:
    Apr 11, 2014
    Messages:
    233
    Location (City & State):
    Dallas, TX
    Randy,
    I also use plastic wrap - the stuff is cheap if you get the thin stuff like the meat market might use. After roughing, the plastic wrap goes on and stays on until hollowing is complete. I then boil and start the drying.
    Insuring that the inside dries at a comparable rate, a computer fan glued on a piece of pvc that goes midway into the vessel and then the vessel in a sealed box until under 20% MC - the fan is always in an exhaust mode inside the box.
     

Share This Page