Modern forgeries passed off as prehistoric artifacts have been detrimental to the field. Callahan spoke out against such practices, encouraging flint knappers around the world to sign and date all of their production. Callahan also championed authentic and scientific reconstructions, which he defined in his article "What is Experimental Archaeology?," as reconstructions which are successful, functional units undertaken with the correct period tools, materials, and procedures and which are scientifically monitored. In this statement, Callahan urged other flint knappers and experimental archaeologists against using modern replica tools such as copper billets to reproduce stone tools instead of the traditional bone and stone hammers used throughout prehistory. Callahan also said in the statement that without proper documentation of the techniques and processes there is no real experiment. With Callahan at the forefront of experimental archaeology, the field of replication studies gained acceptance throughout the academic community. In 1972 Errett received a call from Hans-Ole Hansen, founder of the former Lejre Experimental Centre, nowadays Land of legends (Sagnlandet Lejre), in Lejre, Denmark. Hansen had founded the center in the 1960s as an experiment in Denmark’s past. Until Hansen contacted Callahan, the center had dealt mainly with Denmark’s Iron Age. With Callahan’s help, the center pushed back the time periods represented to the Neolithic and Mesolithic. Callahan first traveled to Denmark in 1979, where he conducted a conference that dealt specifically with the archaeology of the area. At the conference he set up a small knapping area where he engaged Danish archaeologists and raised awareness in experimental lithic technology. He returned to Denmark in 1981, this time spending seven months at the Lejre Experimental Centre conducting research into the techniques used to produce the stone tool kit of the Neolithic Danes, including the Neolithic Danish dagger, a highly advanced stone knife originally made to copy the forms of the Bronze daggers being produced in northern and central Europe at that time. Through his research Callahan was the first experimental archaeologist able to reproduce the daggers using traditional techniques. He later compiled his work done in Denmark to be published in a book. Along with his work in Denmark, Callahan was part of ongoing research into the Swedish Mesolithic and Neolithic as well. His replication study of Middle Sweden’s Mesolithic and Neolithic quartz and quartzite technology and usage, published in "An Evaluation of the Lithic Technology in Middle Sweden During the Mesolithic and Neolithic," shed new light on several long standing questions, including population migration, microblade technology, and knapping efficiency vs. waste, faced by Swedish archaeologists who studied the lithic technologies of the area.