Balloon Varnishes

The following is taken directly from Chapter II of How to Build and Fly Hydrogen & Hot Air Balloons (1984) which is a reprint of the original book A System of Aeronautics by John Wise (1850)

Chapter II

Of Oils-- Tests-- Table of specific gravity of oils--Preparation of author's invented balloon varnish-Varnish for recoating balloons-Bird-Lime Varnish-To make Bird Lime-Caoutchoue, or gum-elastic-Caoutchoucine.

     The greatest difficulty I had to overcome in the practice of ballooning, was the invention of a good varnish for coating the material of the balloon.  I have before stated the vexations and dangers that attended the use of gum-elastic varnish, and, although I shall not recommend it as a substance well adapted to balloon varnish, I will not condemn it.  I have my doubts, however, whether it contains in any form or preparation as used for a varnish, the unchangeable elastic properties which is found to exist in linseed oil when properly prepared.  At all events, I have never found gun-elastic varnish to attain a drying property without the addition of metallic oxides in its composition.  These oxides have a strong affinity for heat (oxygen), thereby being liable to fermentation and spontaneous combustion--the very plague-spot in aeronautics.  There is perhaps nothing so commonly uses as linseed oil, about which there is, chemically or experimentally, so little known.  Even the oil itself is not very well known by those who use it. Other oils are frequently sold for linseed oil, and as often used for it, by those who are in the habit of its daily use.

    It will, therefore, be well to lay down the tests for the various kinds of oil first.  By this the student will know how to avoid, in the beginning, that which might cause him perplexity and failure in the end.

Oil Test

    Doctor Ure Says: M. Heidenreich has found, in the application of a few drops of sulphuric acid to a film of oil, upon a glass plate, a means of ascertaining its purity.  The glass plate should be laid upon a sheet of white paper, and a drop of the acid let fall on the middle of ten drops of the oil to be tried.

With the oil of rape-seed and turnip-seed, a greenish-blue ring is gradually formed at a certain distance from the acid, and some yellowish-brown bands proceed from the centre.

With the oil of black mustard, in double the above quantity, also a bluish-green color.

With whale and cod oil, a peculiar centrifugal motion, then a red color, increasing gradually in intensity; and after some time, it becomes violet on the edges.

With oil of cameline, a red color, passing into bright yellow. Olive oil, pale yellow, into yellowish green.

Oil of poppies and sweet almonds, canary yellow, passing into an opaque yellow.

Oil of Linseed, a brown magna, becoming black.  Of tallow or oleine, a brown color.

     In testing oils, a sample of the oil imagined to be present should be placed along side of the actual oil, and both be compared in their reaction with the acid.  A good way of approximating to the knowledge of an oil is by heating it, when its peculiar odor becomes more sensible. 

     Specific gravity is also a good criterion.  The following table is give by M. Heidenreich:--

               Oleine, or Tallow Oil                                        0.9003 sp.gr.

               Oil of Turnip-Seed                                           0.0128 sp.gr.

               Rape Oil                                                           0.9136 sp.gr.

               Olive Oil                                                            0.9176 sp. gr.              

               Purified Whale Oil                                           0.9231 sp.gr.

              Oil of poppies                                                   0.9243 sp.gr.

              Oil of camelina                                                 0.9252 sp.gr.

              Linseed Oil                                                       0.9347 sp.gr.

              Castor Oil                                                         0.9611 sp gr.

     It will be seen that linseed oil is the heaviest in the list except the Castor Oil.

Preparation of Balloon Varnish

   Take pure linseed oil, as much as will fill half the vessel, and not over, and put over a fire.  Let it heat gradually up to a degree that will char wood.  Before it arrives at that heat, it will show symptoms of boiling, which is, however, nothing more than the boiling of the small quantity of water that is generally in solution with the oil.  Before the oil begins to boil, it must be brought to a much greater heat, and, as before stated, to that degree at which a piece of wood immersed in it will be quickly carbonized.  At this point, it will have to be closely watched, for soon it will commence an intensely heated reaction, beginning to foam and emit dense vapor, which if not well secured from the air, will soon burst out in aflame.  By using a boiler with a close cover, which has a very small air-hole in it, the operation may be continued briskly for at least an hour, and if not kept up so briskly, for two hours or more.  The best way to tell when it is boiled enough, is to take out a little with a spatula  occasionally, and putting it on a piece of glass or tin to cool,  it will be found thick and very stringy, when sufficiently done.  It will also be of a deep reddish color when done.  If the oil is of good quality and well boiled, or we might rather say decomposed in this way, it gets very thick when cooled, and in appearance very much resembles Indian rubber, and in elasticity superior to it. Its nature becomes entirely changed, as the fatty adhesive property of it seems to be destroyed in it.  After being prepared in this way, it should be stood in the light for a while, when it will settle, separating the pure portion by its floating above the black carbonaceous matter at the bottom.  Before using it, it must be thinned with spirits of turpentine, and this should be highly rectified in order to make the varnish dry readily.  This varnish will dry in the sun in five or six hours, and requires no driers of metallic oxides to facilitate it progress.

    Silk or Muslin, when coated with this valuable preparation, posses the peculiar property of being a non-conductor of heat, instead of the dangerous property pertaining to most oil varnishes of contracting heat (affinity for oxygen).  Balloons prepared with this varnish have been suffered to remain packed up in a chest for months with out the slightest development of heat or adhesiveness-- a difficulty which cost me more perplexity the first five years of ballooning than all other obstacles put together.

    This varnish improves by keeping, and if kept in a glass vessel it will improve and become clear faster than when kept in a vessel where the light is shut from it. By keeping it for three or four hours in an intense heat when preparing it in the boiler, it will upon cooling be almost as solid as Indian rubber.  It may be diluted with Turpentine as soon as sufficiently cooled down to bear it when first prepared, or it may be partly warmed, when it has been suffered to cool and settle after being prepared without having been thinned at this time.

    This varnish does not dry so well in the shade, and, as it becomes necessary to varnish a balloon sometimes after it is made, which is generally done in a room, the following preparations will be found to answer the purpose very well:--

   Take umber make fine, two ounces, common yellow ochre four ounces, and litharge one ounce, to half a gallon of the above mentioned varnish, and boil them well together for half an hour.  This will dry very rapidly in the shade or sun, and should be use very thin for re-coating.  A half gallon of it can be diluted with turpentine so as to be sufficient to coat over a balloon of twenty-five feet diameter, if made of silk.  Cotton ones take a little more.

    Having now given what I deem to be the best composition for coating hydrogen balloons, it will not be improper to mention such other as may be used.

Bird-Lime Varnish

   This makes a very good balloon varnish, and is composed in the following manner:  Take bird-lime four pounds; boiled oil four pounds.  Boil these together until they are perfectly incorporated, then add six pounds more of boiled oil, and one pound of litharage (Oxide of Lead); boil again until the whole mass becomes well intermixed and stringy:  this can be ascertained by occasionally taking a little out with a spatula and putting it on glass or tin and trying it.  When done and partly cooled off, add to this quantity about three quarts of turpentine and let it settle, when it is ready for use.  This varnish dries very readily in the sun, and should be applied lukewarm.

    As bird-lime is an article rarely found in this country, it will be proper to state how it is made:  Take the middle bark of the white holly in any quantity; boil it for seven or eight hours in water, or until it gets soft; then drain off the water and place it in a pit  three or four feet deep in the ground, and surround it with stones; let it remain in this place until it passes by fermentation into a mucilaginous state; then take it out and pound it, or run it through a crushing mill; after this wash it in several waters, then take it out and let it ferment four or five days to purify itself.

    As gum-elastic has been recommended for balloon varnish, as well as the first hydrogen balloon having been coated with a solution of it, we will here give what knowledge we have of it.

Caoutchouc or Gum Elastic.

   This substance exudes from certain plants when incisions are made into them, into a milky-like fluid state, and congeals when left exposed long to the atmosphere, and when hardened in this manner is of a lightish-yellow color.  It is generally imported from South America, and that which we get in the shape of shoes and bottles is of a blackish-brown color, which it acquired from being dried in Smokey places.  It is remarkable for its elasticity, from whence proceeds its popular name.  It is soluble in various oils, such as cajeput, sassafras, and naptha, but it most common solvent is spirits of turpentine.  When dissolved in any of these substances and used as a varnish, it never regains it former elasticity, but is said to recover its elasticity when precipitated from cajeput oil by alcohol.

    I believe it would dissolve in very high pressure steam, as it has been so softened by steam of four atmospheres, as readily to yield under the palate knife.  By proper distillation it yields a volatile liquid of the specific gravity of .0.64 which is extremely light! To this substance chemistry has assigned the name of caoutchoucine.  It is perfectly limpid, and it is said to be a perfect solvent for the gum it is made of.  And it is also stated, that Indian rubber, or as it is also called, gum-elastic, dissolved in this liquid and applied as a varnish, will recover its original elasticity.  With this I have not yet tried experiments in coating balloons, because the linseed oil preparations answers the purpose so well, but I have not doubt this caoutchoucine preparation would be very good, if it would not be too expensive.

    Therefore are many other preparations recommended in receipt books for coating balloons with, but none that I would recommend the experimentalist in aerial machinery to have anything to do with. The application of metal leaf, such as Prince's Metal, Dutch gold, and others, I have found to do very well in making aerial vessels tight, when applied before the varnish has been quite dry; but these are too expensive for large balloons.  They answer  an admirable purpose for small ornamental balloons, or any kind of aerostatic figures.