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Compo and castings

Discussion in 'The Grumble' started by jamesnkr, Jun 18, 2019.

  1. jamesnkr

    jamesnkr Grumbler in Training


    I spent some time at the weekend playing with compo. It worked quite well, casting it in some old boxwood moulds (molds for you Americans) I have a few questions and wondered whether anybody can help.

    (For those who don't know, to make compo: 1. melt rabbit skin glue, add Zinc White and glycerine; 2. melt rosin, add Venice turpentine and linseed oil, and add all this to the RSG mix from '1.'; 3. add this mixture to whiting and knead it until smooth. Whilst still warm, press into moulds, put into a book press for ten minutes and then take it out.)

    Soooo, now onto my questions:

    1. Can you clean the compo off the bowl? I've been buying metal bowls for £1 each from the pound shop, so it doesn't matter if it's not possible. But compo is notorious for disintegrating in water so I'm surprised that soaking the bowl hasn't cleaned it up.
    2. What do you use for cutting the compo off the backing once it's come out of the mould?
    3. Watching the videos on Youtube of people cutting the compo off the backing, they have very thick backs. Do they put the compo back and reuse it? It seems very expensive not to. But I didn't find that remelting the compo worked very well, I guess it dried out too much?
    4. I want to cast some moulds - for making the running patterns on Hogarth frames. The moulds obviously need to be pretty resilient if they are to be used repeatedly. What do you recommend? I've seen recommendations of silicone/PU, but I'm not sure I can see how these would be used in the book press; it would press the moulds flat!
    5. Do you add the linseed oil to the rosin before melting it, or after? Melting the rosin was rather an adventure...

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  2. JFeig

    JFeig SGF, Supreme Grumble Framer

    1. Use a wood bowl saturated with mineral oil. Mineral oil will not cross link like linseed oil.
    2. I use a thin meat carving knife.
    3. Yes it can be reused. I do not let the compo harden prior to cutting from the mold. NOTE: To remove the compo, have the mold well oiled and powdered with talc for easy release.
    4. For modern technology, I would use a rigid casting compound. See note in #3
    5. The rosin is melted in the linseed oil to form an emulsion which is then added to the whiting.
    Finest Fabric likes this.
  3. jamesnkr

    jamesnkr Grumbler in Training

    Jerome, thank you for that.

    1. I had not realised that linseed oil polymerised/'dried'. Everyday's a school day. https://wiki2.org/en/Drying_oil+Newton That has given me a whole new level of understanding to the process, thank you.
    2. What a good idea.
    3. I sense my problems here related to the polymerisation of the linseed oil; next time I shall work more quickly and exclude oxygen.
    4. What do you use? I do also want to experiment with mounting rubber/silicone moulds into wooden blocks as reportedly that works quite well.
    5. Thought as much, my instructions suggested adding the oil and turps to the melted rosin; that was quite a struggle.
    6. Venice turpentine is either £21 for 60ml - Cornelissen; or it is £35 for 16 oz if you buy the stuff intended for use on horses. Is this latter the same stuff, as it's a tiny fraction of the cost of the Cornelissen.

  4. JFeig

    JFeig SGF, Supreme Grumble Framer

    Here are some Rx that I collected over the years.


    1. 145gm. hide glue (#4380) to 16oz. distilled water - let stand 60 minutes

    2. heat in dbl. boiler... 8.5oz. rosin & 8oz. boiled linseed oil (pine oil)(lard oil)

    3. mix 2 parts glue; 2 parts oil mixture; 1 part molasses

    4. add whiting until still just sticky, not dry, store in ziplock freezer bags


    1. 30gm. R. S. G.; 150gm. hide glue; - in 220ml. water - standard preparation

    2. 90gm. crushed rosin; 2gm. balsam pitch, 2-4 drops Venice terp. - in 80ml. boiled linseed oil - heat slowly in double boiler until all melted

    3. 1kg. whiting - add warmed glue mixture & mix - then add oil mixture & mix


    1. 15gm. R. S. G. ; 75gm. hide glue; 40gm. raw linseed oil

    2. 130ml. water

    3. 500gm. whiting


    300 gm. hide glue; 246 gm rosin; 118 ml linseed oil; 5 ml glycerin; 1516 gm whiting

    soften in water

    melt rosin in linseed oil - double boiler

    mix wet ingredients with whiting


    10 parts kerosene; 1 part paraffin oil
    Finest Fabric likes this.
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