National Treasure

Benjamin Franklin Gates is an American historian, amateur cryptologist, former U.S Navy Diver and the youngest descendant of a long line of treasure hunters. As a child, Ben is discouraged from following the family line by his father, Patrick Henry Gates. Young Ben is driven on by a story told by his grandfather, John Adams Gates. According to John, a clueÂ â the phrase "The secret lies with Charlotte"Â â was entrusted to the family by Charles Carroll of Carrollton in 1832, and will lead to the fabled "national treasure", a wealth of artifacts and valuable items dating from Ancient Egypt that was collected from all over the world by the Knights Templar who believed that such a treasure was too great to be held by a single king or ruler, and secretly hid it throughout the centuries until the Founding Fathers and the Freemasons (who took over custody of the treasure after the disbandment of the Templars) placed it in a new location during the American Revolutionary War.
Thirty years later, Ben leads an expedition along with Ian Howe, computer expert Riley Poole, and Ian's accomplice Shaw to find the Charlotte, a ship trapped in Arctic ice, which is believed to be referenced in the clue. The group discovers a meerschaum pipe hidden in a barrel of gunpowder in the cargo hold. Ben discovers a clue is engraved on the stem of the pipe, which he concludes indicates the next clue to be on the back of the Declaration of Independence. When Ian suggests they steal the Declaration, a fight ensues, during which spilled gunpowder is ignited. Ian and Shaw escape the ship, trapping Ben and Riley, who escape through a smuggler's hold.
In Washington, D.C., Ben and Riley report Ian's plan to the FBI, and to Dr. Abigail Chase at the National Archives, where the Declaration is stored. Their claims are dismissed as stealing the Declaration is considered impossible, and if there had been a secret clue, it would have already been discovered. Ben decides to steal the Declaration at an upcoming gala event before Ian can. Ben sends Abigail one of George Washington's campaign buttons dipped in invisible ink, enabling him to access her keycodes and enter the preservation room, with Riley disabling the surveillance systems. Ben grabs the Declaration moments before Ian and his men show up, and quickly leaves the gala to join Riley. Abigail, suspicious of Ben's presence, follows him, but is captured by Ian, believing she is working with them. Ben and Riley rescue Abigail and are forced to bring her up to speed. Meanwhile, FBI Agent Sadusky, investigating the theft of the Declaration, finds evidence tying Ben to the theft and starts tracking him.
Ben, Riley, and Abigail arrive at Patrick's home. Not knowing what the actual document is Patrick replies quite nonchalantly that in order to view any invisible ink they need a reagent or heat and says, "Throw it in the oven." Anxiously declaring they can't do that Ben and Abigail review the animal skin the Declaration is written on, finding an Ottendorf cipher written in invisible ink on its back. The encrypted message references the Silence Dogood letters written by Benjamin Franklin kept at the Franklin Institute. Ben, Abigail, and Riley travel to Philadelphia and use schoolchildren to acquire the key words from the letters, with a message pointing them to the bell tower of Independence Hall where the Liberty Bell was originally located. They find a hidden cache with a pair of spectacles with multiple colored lenses, which, when used to read the back of the Declaration, reveal the clue Heere at the Wall. They split up when they realize that Ian and his team are following them. Ben is arrested and interrogated by the FBI, while Abigail and Riley lose the Declaration to Ian. Abigail is able to convince Ian to help them rescue Ben in exchange for the next clue, meeting at New York City's USS Intrepid museum, where they engineer Ben's escape.
When Ben remains coy about the next clue, Ian reveals he has kidnapped Patrick, coercing Ben to identify the clue as a street intersection in New York, near Trinity Church. Inside, they find a passage that leads deep underground (during which Shaw falls to his death), ending in a room lit by a single lantern, which Patrick asserts refers to Paul Revere's Ride, pointing to the Old North Church in Boston. Ian and his men ditch Ben's group and race to Boston, unaware that the clue was fake. Ben opens a secret door, behind which they find a notch which the pipe from the Charlotte fits, opening onto a large chamber containing the national treasure.The chamber is vast and has an exit stairs at the far end, thus eliminating the need to use the collapsing passage they entered by. Ben contacts Agent Sadusky, and learns he is a Freemason; Ben returns the Declaration and the location of the treasure in exchange for their names being cleared of stealing the Declaration. Ben also informs Sadusky of his bluff on Ian, who along with his men are successfully caught by the FBI and charged with abduction, attempted murder, and trespassing on multiple government properties.
Ben and Abigail have started a relationship, while Riley is somewhat upset that Ben turned down the 10% finder's fee for the treasure for a much smaller amount that still has netted them all significant wealth.

Question: What does Ben send Abigail dipped in invisible ink?
Answer: One of George Washington's campaign buttons