The story...

Potassium discovered on metal samples at the WTC may indicate the use of thermate.

Our take...

Professor Steven Jones explains that his tests on a metal sample revealed spots of Aluminum, Sulfur and Potassium, along with Manganese and Fluorine, and that these compounds are often present in thermite residue:

Fluorine is present in an oxidizer using Polytetrafluoroethylene as its base, used in thermite charges.
http://www.osti.gov/bridge/servlets/purl/1344-QDsu9M/webviewable/1344.pdf

Potassium Permanganate (KMnO4) is also commonly used as an oxidizer in thermite-reactants (aluminothermics).

Hence, K, Mn and F are often present in thermite residue, suggesting they are part of a “thermite fingerprint” at WTC.
http://worldtradecentertruth.com/JonesAnswersQuestionsWorldTradeCenter.pdf

We look at possible sources of the other chemicals elsewhere, but what about Potassium? Another dust study found it, too, but has a simple explanation as to the cause:

The total element compositions of the dust samples reflect the chemical makeup of materials such as: ... concrete and aggregate (containing calcium and aluminum hydroxides, and a variety of silicate minerals containing silicon, calcium, potassium, sodium, and magnesium)...
http://pubs.usgs.gov/of/2001/ofr-01-0429/chem1/

There was certainly no shortage of concrete dust around the WTC site, and it’s no surprise to find Potassium in the dust. Could this be the source for Potassium in the metal samples? Are there other possible, alternative sources? Here’s what Encarta tells us about the uses of Potassium. Most don’t look relevant here, however perhaps you’ll spot some that are:

Potassium metal is used in photoelectric cells. Potassium forms many compounds resembling corresponding sodium compounds, based on a valence of 1. A few of the element's most important compounds follow. Potassium bromide (KBr), a white solid formed by the reaction of potassium hydroxide and bromine, is used in photography, engraving, and lithography, and in medicine as a sedative. Potassium chromate (K2CrO4), a yellow crystalline solid, and potassium bichromate, or potassium dichromate (K2Cr2O7), a red crystalline solid, are powerful oxidizing agents used in matches and fireworks, in textile dyeing, and in leather tanning. Potassium iodide (KI), a white crystalline compound that is very soluble in water, is used in photography for preparing gelatin emulsions and in medicine for the treatment of rheumatism and overactivity of the thyroid gland. Potassium nitrate (KNO3), a white solid prepared by fractional crystallization of sodium nitrate and potassium chloride solutions, is used in matches, explosives, and fireworks, and in pickling meat. It occurs naturally as saltpeter. Potassium permanganate (KMnO4), a purple crystalline solid, is used as a disinfectant and germicide and as an oxidizing agent in many important chemical reactions. Potassium sulfate (K2SO4), a white crystalline solid, is an important potassium fertilizer and is also used in the preparation of potassium alum. Potassium hydrogen tartrate (KHC4H4O6), commonly known as cream of tartar, is a white solid used in baking powder and in medicine.

The term potash originally designated potassium carbonate, obtained by leaching wood ashes, but is now applied in general to a number of potassium compounds. Potassium carbonate (K2CO3), a white solid, also called potash or pearl ash, is obtained from the ash of wood or other burned plant materials, and by reacting potassium hydroxide with carbon dioxide. It is used in making glass and soft soap. Potassium chlorate (KClO3), called chlorate of potash, a white crystalline compound, is formed by the electrolysis of potassium chloride solution. It is a powerful oxidizing agent and is used in matches, fireworks, and explosives, as a disinfectant, and as a source of oxygen. Potassium chloride (KCl), a white crystalline compound commonly called chloride of potash or muriate of potash, is a common constituent of potassium salt minerals, from which it is obtained by volatilization. It is an important potassium fertilizer and is also used in making other potassium compounds. Potassium hydroxide (KOH), called caustic potash, a white solid that is dissolved by the moisture in the air, is prepared by the electrolysis of potassium chloride or by the reaction of potassium carbonate and calcium hydroxide; it is used in the manufacture of soap and is an important chemical reagent. It dissolves in less than its own weight of water, liberating heat and forming a strongly alkaline solution.
http://encarta.msn.com/encnet/refpages/RefArticle.aspx?refid=761558818

It seems the idea that Potassium = Thermate is just an assumption, then. There are plenty of other sources around, and no reason given to explain why thermate charges are the best or most plausible explanation.