How do you distil a foaming liquid?

This advice was posted long ago on the subject of bumping and foaming:

For reasons which are probably not fully understood it occasionally happens that the boiling of a liquid momentarily ceases. In consequence a portion of the liquid becomes somewhat superheated and this unstable condition is then relieved by the sudden formation of a correspondingly large volume of vapor, which causes the liquid to boil violently.
Ordinarily a liquid will boil without bumping, for the rising bubbles of vapor keep it moving and thus prevent superheating. Evidently we shall have an infallible method for the avoidance of bumping if we can keep the liquid moving. For example, when the liquid is being boiled in an open beaker, it is sufficient to make use of a mechanical stirrer. But when we are distilling, mechanical stirring is not conveniently provided. Here we can use the method of distillation of by drawing through it a continuous current of dry air. This technique has become common practice for distillations under reduced pressure, but it cannot be used when the objective is accurate measurement of the boiling point of the liquid, or when some of the compounds are decomposed by air. Another good method employs uses the capillary tube. They are best made from glass tubing having a thin wall and a bore of 2 - 3 mm. A suitable length of such tubing, supported at both ends and rotated, is heated at the in the edge of a flame until the walls melt and and then quickly drawn apart. One end of the tube is then cut off. The over-all length of the tube is then tube should be such that, when the lower end is resting on the bottom of the flask in which it is being used, the upper end will be supported by the neck of that vessel. The upper end of the tube may be closed by a rubber tube and a pinchcock clamp. The liquid in the flask must be deep enough to keep the lower end of the tube continously submerged. It is advisable to shield the equipment against the cooling effect of a possible current of air. And if several of these tubes are used simultaneously, trouble that might arise from the failure of a single tube may he avoided. This technique also is limited by exposure of the distilland to air. Other methods for the prevention of bumping succeed by providing conditions favoring the formation of bubbles of vapor at one or more points where the liquid is in contact with the heated surface of the vessel containing it. Fragments of unglazed porcelain or a small piece of pumice recently tone that has been weighted with platinum wire. Bumping can also be prevented by the introduction of 7-mm. discs of Vapoglas (an air-conditioning fiberglass filter material) cut from 3/16-inch sheets with a cork borer. The discs were cleaned with hot dichromate cleaning solution and rinsed with distilled water before being added to the hot digestion mixture. The porosity of the discs promotes a vigorous boiling and, because of their lightness, they move about rapidly in the solution and prevent development of localized hot spots. The discs are easily pulverized by pressure from a stirring rod thus assuring extraction of absorbed material. A small sliver or shred of polytetrafluoroethylene, makes an excellent boiling chip for the prevention of bumping during evaporation operations in the laboratory. This material is chemically inert to nearly known reagents (including fluorine and hydrogen fluoride) and is nonporous in nalure. Gas bubbles form very readily on its surface, and this property does not disappear on continued use and reuse in aqueous solutions or organic solvents. The numerous devices proposed for the alleviation of bumping during vacuum distillation may be classified generally as ebullators, packings, external capillary leaks, or mechanical agitators. Most ebullators become ineffective with time or on momentary interruption of boiling. Their use also involves the risk of contamination. Packings interfere with quantitative work. The use of an external capillary leak is generally quite satisfactory in prevent bumping. However, this results in a false (low) boiling point and in seriously in the loss of material by entraininent. The loss has been found be appreciable over the period of time required for a precision vacuum fractionation. Agitation is the most satisfactory method, except for the mechanical difficulties involved. The external electrically or air driven magnet will spin the magnet in the flask over a distance of about an inch through an electric heating mantle, an oil bath, or any other heating arrangement free of magnetic interference. The device has been used successfully in this laboratory for fractionations and distillations of a number of high boiling, moderately viscous compounds in the range of I - 5 mm. The dimensions of the stirring magnet(length and bulge) should be such as to provide fairly vigorous agitation.

If magnetic stirring didn't reduce the bumping adequately then the agitation wasn't enough to prevent superheating, you may need a more powerful magnetic stirrer and/or magnetic stir bar In the case of air sensitive materials one can connect a tank of argon or perhaps nitrogen to the capillary tube

My trick